Emergency Radiology

, Volume 24, Issue 6, pp 619–633 | Cite as

Infectious and inflammatory diseases of the central nervous system—the spectrum of imaging findings and differential diagnosis

  • Joseph M. Rozell
  • Edward Mtui
  • Yu-Ning Pan
  • Shan Li
Review Article


The infectious and inflammatory diseases of the central nervous system (CNS) including the brain and spine can present with a wide spectrum of clinical symptoms, locations, and appearance. The purpose of this exhibit is to review the different patterns of their presentations, to illustrate their imaging characteristics and techniques, and to discuss their clinical features and pathology so that the correct diagnosis can be made and prompt intervention can be initiated on a timely fashion.


Infectious diseases Inflammatory diseases CNS Brain and spine 


Neurological emergencies constitute a significant portion of patient visits to the emergency room. Although transient ischemic attacks or acute ischemic strokes are some of the more common reasons for admission, acute infectious and inflammatory diseases of the central nervous system (CNS) are not infrequent causes of acute neurological catastrophes. Infectious and inflammatory diseases of the CNS include a wide spectrum of diseases affecting the brain and spine, some of which are potentially life-threatening. Accurate identification and diagnosis of the underlying etiology are critical as prompt treatment can be life-saving.

In this article, we review 10 disease entities of acute infectious and inflammatory diseases of the brain and spine commonly encountered in the emergency room. Ten case scenarios are presented, which include the patient’s clinical symptoms, pertinent physical findings, and laboratory results as well as the related imaging findings. The characteristic imaging patterns of the disease entities and their differential diagnoses, the patients’ clinical courses, and the final diagnoses will be analyzed and discussed.

Acute bacterial meningitis

Acute meningitis is an infectious disease of the leptomeninges and pachymeninges, which are the thin layers of tissues surrounding the brain and spinal cord. The etiology of meningitis can be bacterial, viral, fungal, etc. Meningitis caused by bacteria, however, is both more common and more severe and can be deadly, requiring immediate medical intervention. The classic triad of clinical presentations of bacterial meningitis consists of fever, headache, and neck stiffness. Other common presentations include seizures and focal neurologic deficits [1]. Lumbar puncture with detection of positive CSF gram stain/culture is the single most important diagnostic study. Early imaging is pivotal in aiding prompt diagnosis and in evaluation for associated complications.
Fig. 1

Acute bacterial meningitis. Non-enhanced axial CT of the head (a) shows no definite abnormal attenuation. There may be subtle effacement of the sulci of the left parietal lobe (arrow). On MR, axial T1 (b) and coronal T2 (c) images show no abnormal signal of the brain and meninges. DWI (d) shows no restricted diffusion. Post-contrast axial FLAIR (e) and coronal T1 (f) images demonstrate hyperintense signal within the subarachnoid spaces and diffuse enhancement along the pia and cerebral veins but no abnormal signal of the adjacent brain parenchyma. The patient later developed a subdural empyema. CSF culture and surgical drainage showed Streptococcus pneumoniae infection

Initial computed tomography (CT) of the brain could be normal or show mild sulci effacement and mild ventriculomegaly, but CT is usually the initial imaging modality to exclude contraindications to lumbar puncture [2]. Contrast-enhanced MRI has higher sensitivity and is performed to confirm suspected meningitis, rule out meningitis mimics, detect the etiology and complications of meningitis, and monitor treatment response. On MRI, acute bacterial meningitis shows either focal or diffuse meningeal enhancement, which could be leptomeningeal, pachymeningeal, or a combination of both. There are also increased subarachnoid signals on fluid-attenuation inversion recovery (FLAIR) MRI. [3] (case 1). Complications of meningitis include hydrocephalus, venous thrombosis, small vessel infarct/ischemia, cerebritis, subdural/epidural empyema, and ventriculitis.

Case 1

A 6-month-old male presented to the ER with right-sided seizure. He was diagnosed with a left ear infection and prescribed antibiotics by his pediatrician earlier the same day. His seizure began in the car on the way to the pharmacy. Upon arrival to ED, the patient had eye deviation to the right and jerking movements of his right arm only. Temperature was 100.0 (°F). CBC and lumbar puncture showed elevated WBC. Non-enhanced head CT at ED showed left otitis media and mastoiditis but otherwise an unremarkable brain. Neurology was consulted and recommended stat MRI brain (Fig. 1).

Common differential diagnosis of meningeal enhancement includes meningeal carcinomatosis and intracranial hypotension.

Meningeal carcinomatosis refers to tumor cells spreading to the surfaces of the brain—mostly leptomeningeal but may also be pachymeningeal. This occurs in approximately 5% of patients with cancer. Breast and lung cancers are the most common tumors related to leptomeningeal spread [4]. CNS symptoms are dependent on areas of involvement with the cerebral, cranial, or spinal nerves. Positive CSF cytology is found on the initial lumbar puncture in 50–70% and in nearly all cases after three attempts [5]. Increased CSF pressure and elevated CSF protein are also commonly found. Gadolinium-enhanced MRI is the preferred imaging modality over CT because of its sensitivity and specificity. Enhanced MRI shows either diffuse “sugar coating” or thicker, lumpy, multi-nodular enhancement along the affected meninges [6].

Diffuse dural thickening and enhancement with dural venous engorgement and downward sagging of the brain are the main features of intracranial hypertension. Leptomeninges are not involved [7]. Intracranial hypotension commonly results from a CSF leak somewhere along the membrane surrounding the brain or spinal cord. It can be confirmed by measuring the opening pressure on a lumbar puncture (less than 7 cm H2O). The patients usually present with postural headaches, nausea, vomiting, and disturbances in vision and hearing. Fever and neck stiffness typically seen in patients with meningitis are not present. Classically, intracranial hypotension occurs in middle-aged women [8].

Herpes simplex (HSV-1) encephalitis

Herpes simplex type I virus (HSV-1) encephalitis is the most common cause of sporadic fatal encephalitis worldwide, accounting for 10–20% of the roughly 20,000 annual viral encephalitis cases. All age groups are affected, with a third of all cases occurring in children and adolescents. HSV-1 encephalitis is a fulminant necrotizing encephalitis that usually manifests due to reactivation of its latent virus in the CNS. Once acquired, HSV-1 remains in the body for life, typically living dormant within the trigeminal ganglia. Thus, reactivation of the virus typically has a propensity to asymmetrically affect the limbic system. [9]. Clinical symptoms are nonspecific, consisting of fever, headache, seizures, focal neurologic deficits, and impaired consciousness, which in some instances may result in low or even no suspicion towards the disease.

The initial appearance of herpes encephalitis on CT is frequently normal or subtle low density within the anterior medial temporal cortices. Suspicion for herpes encephalitis should never be excluded on the basis of a normal CT. IV acyclovir therapy should be initiated immediately with the suspicion of the disease even without confirmation by Herpes PCR test. MRI is far more sensitive for the detection of this disease. Findings on MRI show increased T2/FLAIR signals with some restricted diffusion on DWI, correlating histologically with cytotoxic edema. In more advanced stages of the disease, the affected areas show significant swelling, focal hemorrhages, necrosis, and heterogeneous enhancement [10] (case 2).

Case 2

A 55-year-old male was transported to the ER by EMS for confusion, garbled speech, and incontinence of urine and stool. Initial assessment reported the patient was hypotensive and tachycardic with a fever of 101.9 (°F). Labs revealed leukocytosis. Stat CT and MR showed a large right temporal lobe lesion (Fig. 2). Neurosurgeon was consulted.
Fig. 2

Herpes simplex (HSV-1) encephalitis. Non-enhanced axial CT of the head (a) shows a large irregular hypodense lesion involving the right temporal lobe. It contains several foci of acute hemorrhages (arrows). On MR, axial T2 (b) image shows heterogeneous hyperintense signals in the same region with associated significant vasogenic edema and mass effect. Right cerebral peduncle is compressed by swollen right medial temporal lobe (arrow head). Sagittal T1 (c) and axial GRE (d) images show multifocal areas of hyperintense T1 and hypointense GRE signals within the lesion, consistent with acute hemorrhages (arrows). Post-contrast axial (e) and coronal (f) T1 images show heterogeneous enhancement of the lesion. Brain biopsy confirmed HSV-1 encephalitis

Other common abnormalities involving the temporal lobe that are worth considering are limbic encephalitis and temporal lobe neoplasms, both of which are less acute and less fulminant than HSV-1 encephalitis.

Limbic encephalitis is an autoimmune disorder characterized by inflammation of the brain, focused on but not limited to the “limbic” system. Many of the cases are associated with a tumor, in which case, it is called paraneoplastic syndrome [11]. Hyperintense T2/FLAIR signals are seen in the temporal lobes, often bilaterally. Hemorrhage and necrosis are not present. Other key imaging features that help to differentiate herpes encephalitis from limbic encephalitis is that the former will almost always spare the basal ganglia and is usually only unilaterally involved, whereas the latter will usually involve the basal ganglia and is more often than not bilateral. Limbic encephalitis is characterized by rapid progressive short-term memory loss, seizures, and other psychiatric symptoms [12].

Common primary tumors of the temporal lobe include astrocytoma, ganglioglioma, pleomorphic xanthoastrocytoma (PXA), and dysembryoplastic neuroepithelial tumor (DNET). The patients with these tumors have much more gradual and indolent onset of symptoms compared to those of HSV-1 encephalitis, of which there is a rapid progressive and fulminant clinical course. On CT or MR, the tumors often present as a focal- and cortical-based mass while HSV encephalitis often appears infiltrative with diffuse involvement of the temporal lobe. Hemorrhages are not commonly associated with the tumors unless they are of a very high grade; restricted diffusion is usually absent; calcifications may be present (astrocytoma, ganglioglioma, and DNET); and enhancement varies—some contain cystic components (ganglioglioma and PXA) [13]. Antiviral treatment is not effective.

Brain abscess

Brain abscess is a focal collection of pus in the brain due to necrosis of infected brain tissues. It is a potentially life-threatening condition that requires emergent diagnosis and treatment. The etiology can originate from infection of contiguous structures (e.g. sinusitis, dental or ear infections, mastoiditis) or following skull trauma or surgery. It can also be secondary to hematogenous spread from other parts of the body (heart, osteomyelitis, etc). In at least 15% of cases, no source can be identified. The predominant causative bacteria are anaerobic and microaerophilic cocci and gram-negative/positive anaerobic bacilli. Patients’ typical clinical symptoms include high fever, drowsiness, and fatigue (due to infection); headaches, vomiting, and confusion (due to increased intracranial pressure); seizures, hemiparesis, and speech difficulties (due to focal brain tissue damage), together with a rapidly progressive course. Brain abscess begins with focal cerebritis. It often goes through four stages: early cerebritis, late cerebritis, early capsule abscess, and late capsule abscess [14]. Abscess is the final stage of the cerebritis resulting in a necrotic cavity in the brain parenchyma which appears as an irregular-shaped rim-enhancing collection of fluid and debris.

MRI is far more sensitive for the diagnosis of brain abscess than CT, particularly when attempting to distinguish it from other ring-enhancing lesions. The abscess will demonstrate central T1 heterogeneous hypointensity and marked T2/FLAIR hyperintensity with significant surrounding vasogenic edema. If a rim or capsule is present, this will usually demonstrate enhancement which appears thinner near the ventricle and thicker away from the ventricle. Most importantly, the core necrotic tissues demonstrate restricted diffusion on the DWI sequence [15] (case 3). Prognosis of this disease is dramatically worsened if the abscess ruptures into the ventricular system causing ventriculitis.

Case 3

A 46-year-old female with history of mental retardation, blindness, deafness, and mutism presented to the ED for new-onset seizure and decline of baseline neurological functions. The temperature was 98.3 (°F); heart rate of 78 bpm; normal blood pressure. Labs revealed leukocytosis. CT and MR of the head showed a fluid-filled left parietal lesion (Fig. 3).
Fig. 3

Brain abscess. Non-enhanced axial CT of the head (a) shows a large irregular-shaped hypo- to iso-dense lesion with hyperdense rim in the left parietal lobe suggesting cystic morphology (arrow). On MR, the fluid within the cystic lesion appears hypointense on axial T1 (b) and hyperintense on axial T2 (c) images, and demonstrates restricted diffusion on DWI (d) and ADC (e) images. Post-contrast axial T1 (f) image shows irregular asymmetrical rim enhancement of the cystic lesion (arrow head) which appears thinner near the ventricle and thicker away from the ventricle. The surrounding white matters show hypodensity on CT (a), hypointense signal on T1 (b) image, and hyperintense signal on T2 (c) image without associated restricted diffusion (d, e), consistent with vasogenic edema. Craniotomy and surgical drainage showed a “fibrinopurulent exudate” and 4+ gram-positive cocc, consistent with an abscess

Although many lesions in the brain can present as ring-enhancing lesions, very few will demonstrate central-restricted diffusion. Most cystic neoplasms of the brain will not demonstrate restricted diffusion aside from epidermoid cyst and CNS lymphoma [16]. Additionally, scrutiny of clinical history will become highly important with symptoms of fever, headache, and leukocytosis, now raising much higher concern for abscess as opposed to infarct or tumor.

Subacute infarct is another common entity included in the differential diagnoses. It differs from an abscess as it is usually not cystic and follows the vascular territory. The restricted diffusion persists; cortical necrosis occurs at 2 weeks; and hemorrhagic transformation is uncommon after 1 week [17]. Parenchymal contrast enhancement occurs from 1 to 8 weeks; however, the enhancement patterns are usually heterogeneous, not cystic or rim enhancement [18].

Subdural empyema

Subdural empyema is a collection of pus in the potential space between the inner layer of the dura mater and the arachnoid mater. It is a life-threatening disease, accounting for nearly 20% of focal intracranial infections. In the era of antibiotics, its mortality rate has been reduced to approximately 10%; this decrease has largely been due to more effective early diagnosis. In infants and young children, subdural empyema most often occurs as a complication of meningitis. In young adolescents and adults, the most frequent predisposed causes are sinusitis, otitis media, and mastoiditis, although head trauma, surgery, and hematogenous spread are also common causes. Headache, fever, focal neurological deficit, and nuchal rigidity are the most frequent clinical features at presentation [19]. The most common causative organisms are anaerobic and microaerophilic streptococci [20]. The frontal region is the most common location of subdural empyema (95%).

CT and MRI appearances of subdural empyema are of a crescent fluid collection overlying the cerebral convexity or within the interhemispheric fissure alongside the falx. The fluid is of slightly higher attenuation than CSF on CT. It is slightly hyperintense compared to CSF on T1 and hyperintense on T2 and FLAIR images [21]. Subdural empyema shows restricted diffusion, appearing hyperintense on DWI and hypointense on ADC maps [22 , 23]. Accordingly, when anteriorly located as a complication of frontal sinusitis, they do not cross the midline, in contrast to epidural empyema, which can. The margins of the collection may be irregular and scalloped as a result of loculation. Although contrast enhancement at the deep margin of a subdural empyema is a characteristic finding, it can be subtle or absent in the early stages of infection. The adjacent brain may show edema or cortical contrast enhancement (case 4).

Case 4

A 13-year-old male with chronic sinusitis presented to ER with fever, vomiting, and headaches for 2 days. On the morning of admission, he woke up with right leg weakness. Upon admission, he had a temperature of 102.9 (°F) with rigors, tachycardia, and vomited bilious liquids. Lab tests showed elevated WBC in blood and CSF. Neurologic exam showed right arm and leg weakness. CT and MRI of the head without and with contrast were obtained (Fig. 4).
Fig. 4

Subdural empyema. Non-enhanced axial CT of the head (a) shows low-density collection in the left interhemispheric fissure (arrowhead). On MRI, the collection appears hyperintense on the axial T2 (b) and isointense on axial FLAIR (c) images. It shows restricted diffusion on axial DWI (d) and ADC map (e). Post-contrast axial T1 (f) shows diffuse pachymeningeal enhancement along the periphery of the collection, consistent with a subdural empyema. On axial T2 (b) image, the left frontal sinusitis is also noted (short arrow). The left frontal lobe shows subtle hyperintense signals on T2 (b), FLAIR (c), and DWI (d) (long arrows). Heterogeneous hypointense signal on ADC map (e) and subtle meningeal enhancement of the related sulci and dura (f) are suggestive of focal cerebritis and meningitis. The patient underwent endoscopic sinus surgeries and a left frontal craniotomy. Culture of the drained pus from the left frontal sinus and the interhemispheric subdural fluid collection grew Streptococcus viridians

Subdural hematoma is a common differential diagnosis of the subdural empyema. Since the fluid collections are in the subdural space, they are deep to the falx and do not cross the midline. This is in contradistinction to epidural collections which can cross the midline. CT attenuation and MRI signal characteristics of subdural hematomas are different from that of subdural empyema. CT attenuation of the blood in a subdural hematoma slowly decreases: as a rule of thumb, it remains denser than the brain for 1 week and becomes less dense after 3 weeks. MRI signal characteristics of hemorrhages change over time as red cells break down and degrade. From acute to subacute stages, the hematoma changes from iso/hypo-intense to hyperintense on T1WI and iso/hyper-intense to hyperintense T2WI. From the subacute to chronic stages, it appears hyperintense on both T1WI and T2WI [24].

Another differential diagnosis of subdural collections is subdural effusion, which is non-infected. It may be a complication of meningitis or encephalitis [25]. On CT and MRI, subdural effusion has identical signals to that of CSF, i.e., hypodense on CT, hypointense on T1, and hyperintense on T2/FLAIR images on MRI. It shows no enhancement after intravenous contrast administration. On MR, the signals do not evolve with time as in subdural hematomas.

Cerebral toxoplasmosis

The most common cause of multiple ring-enhancing lesions in the brain in an immunocompromised patient is cerebral toxoplasmosis, occurring in 15–50% of patients with acquired immunodeficiency syndrome (AIDS). Toxoplasma gondii, an obligate intracellular protozoan, is the causing agent. Toxoplasma encephalitis ensues in the setting of advanced immunosuppression CD4 < 100 in one third of AIDS patients.

Cerebral toxoplasmosis typically manifests as multiple focal lesions with a predilection for the basal ganglia, thalami, and corticomedullary junctions. CT reveals multiple abnormal low attenuation lesions that demonstrate ring or nodular enhancement after contrast. [26]. On MR, toxoplasmosis abscesses are generally hypointense on T1, but may have peripheral hyperintensity which may help distinguish toxoplasmosis from lymphoma. They have mixed or hyperintense signals on T2. Post-contrast T1 images show multiple ring or nodular enhancing lesions. A small eccentric nodule abutting an enhancing ring is called a “target sign” and is highly suggestive of toxoplasmosis [27]. Unlike pyogenic abscesses, the core tissues of ring-enhancing toxoplasma abscesses show no or minimal restricted diffusion. The lesions may show peripheral hyperintensity on the DWI images in the presence of hemorrhage within their walls [28] (case 5).

Case 5

A 43-year-old female presented to ER with lethargy, slurred speech, and difficulty walking following a syncope episode. She had a history of HIV for 20 years and had not been on medication for months. Upon admission, her temperature was 99.1 (°F) with normal heart rate, respiration, and blood pressure. Labs were mostly normal, esp. WBC is 4 k/mm3. Neurological exams were intact. The initial CT of the head was markedly abnormal. MR was then ordered. Subsequent tests showed decreased CD4 lymphocytes and increased serum toxoplasma IgG Ab (Fig. 5). The LP showed positive toxoplasma IgG in CSF.
Fig. 5

Cerebral toxoplasmosis. Non-enhanced axial CT of the head (a) shows ill-defined heterogeneously and subtly hyperdense nodules within bilateral globus pallidi (arrow) surrounded by large low-attenuation areas in the basal ganglia, internal and external capsules, and the left thalamus. On MRI, the nodules appear hypointense on T1 (b) and T2 (c) images. The surrounding hypointense T1 and hyperintense T2 areas are seen corresponding to the low attenuation areas on CT, consistent with vasogenic edema. Restricted diffusion is seen in some of the nodules (d) (short arrow). Post-contrast axial T1 (e) and coronal T1 (f) sequences show rim-enhancing lesions containing eccentric enhancing nodules within the bilateral basal ganglia (target sign, arrowheads). There are also multiple smaller solid and rim-enhancing nodules throughout the visualized brain. Biopsy of the right temporal lobe lesion confirmed infection by toxoplasmosis

The differential diagnosis of multiple ring-enhancing lesions in the brain is extensive. The most common ones seen in non-immunocompromised patients are septic emboli and metastasis.

Septic emboli affect the brain by hematogenous spread of infection. The most common etiology is from endocarditis (20–40%). Staphylococcus aureus is the most common causative agent, followed by S. viridans. Septic emboli encephalitis (SEE) represents two insults to the brain—early emboli can cause a brain ischemic insult due to vascular occlusion, and an infectious insult from a deep-seated infection nidus, which is difficult to treat. In some 90% of all cases, SEE occurs in MCA distribution and rarely in posterior circulation. Emboli also cause acute infarcts in the associated vessel distribution/territory that is not seen in toxoplasmosis or metastatic disease. The infection can cause multiple abscesses of various sizes in the brain, i.e., multiple cystic lesions with rim enhancement associated with restricted diffusion in the central lumen.

Brain metastasis of a systemic cancer is the most common etiology of multiple ring-enhancing lesions in the brain. In descending order, lung, breast, melanoma, renal, and colon cancer account for the majority of cases in adults. Lymphoma or leukemia, neuroblastoma, and sarcoma occur more commonly in children. Contrast-enhanced MRI is more sensitive than CT in detecting brain metastasis. The images show multiple round well-circumscribed ring-enhancing lesions which are often similar in sizes and are typically located in or near gray-white matter junctions. They are usually associated with significant vasogenic edema but lack central restricted diffusion [29].

Progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a rare and fatal opportunistic infection that occurs in up to 5% of acquired immune deficiency syndrome (AIDS) patients. People on chronic immunosuppressive medications are also at increased risk. PML is caused by the JC virus, a ubiquitous DNA virus that infects oligodendrocytes of the brain in immunocompromised patients and leads to massive demyelination of white matter. Symptoms develop over several weeks, rapidly progressing in the last few months which usually results in permanent severe disability or death. Prognosis is poor even with treatment. If left untreated, the mortality rate is 30–50% within the first 3 months of diagnosis.

CT may reveal bilateral but asymmetrical multifocal hypodensities without mass effect and little contrast enhancement. MR with contrast was the imaging tool best suited to demonstrate multifocal hypointense T1 and hyperintense T2/FLAIR lesions, located in the subcortical and deep white matters, mostly sparing the cortical ribbon—the so-called “scalloping out” of the gray–white border. Gadolinium enhancement is rare and, if it occurs, is patchy [30]. Diagnosis is made by discovering JC virus DNA in spinal fluid together with consistent white matter lesions on brain MRI; alternatively, a brain biopsy is diagnostic (case 6).

Case 6

A-42-year-old male with no known past medical history was brought to the ER by his brother because progressive disorientation for 2 weeks. In the ER, the patient had no fever or focal neurological deficits. He was confused with expressive aphasia. Lab tests showed decreased WBC and increased ESR in blood. Drug screen was positive for cocaine and cannabinoid. Non-enhanced CT of the head and MRI of the brain with contrast were obtained the same day (Fig. 6).
Fig. 6

Progressive multifocal leukoencephalopathy. Non-enhanced axial CT of the head (a) shows bilateral but asymmetric hypodensities in the right parietal and left frontoparietal white matters, and the splenium of the corpus callosum. These appear hyperintense on axial FLAIR (b), GRE (c), and DWI (d) sequences. No restricted diffusion given T2 shine-through artifact on the ADC map (e). Most lesions do not enhance on post-contrast sagittal T1 (f), but there is minimal peripheral enhancement. HIV test came back as positive the next day. Serology tests later showed low CD4 and positive PCR test of JC virus

Common differential diagnoses include posterior reversible encephalopathy syndrome (PRES) and acute disseminated encephalomyelitis (ADEM).

Posterior reversible encephalopathy syndrome (PRES), also known as reversible posterior leukoencephalopathy syndrome (RPLS), is a syndrome characterized by headaches, confusion, seizures, and visual loss. Common causes include malignant hypertension, eclampsia, and some medical treatments [31]. CT images show hypodensities in supratentorial subcortical white matters bilaterally, especially in the parietal and occipital lobes without mass effect. On MRI of the brain, hypointense T1 and hyperintense T2 signals can be seen. Because it is a vasogenic cerebral edema, DWI displays equisignal or hypointense signal, and ADC image shows hyperintense signal. On post-contrast images, there is no definite enhancement [32]. Unlike PML, the clinical manifestations and imaging changes of PRES can be fully recovered after active treatment, generally leaving no sequel. Vasogenic cerebral edema on MRI can help differentiate PRES from other diseases.

Acute disseminated encephalomyelitis (ADEM) occurs more commonly in children. It is a severe immune-mediated inflammatory disorder of the CNS typically following a prodromal viral infection. The lesions of ADEM are often large, patchy, and poorly marginated on CT or MRI. There is usually asymmetrical involvement of the subcortical and central white matters and cortical gray-white junctions of cerebral hemispheres—namely, the cerebellum, brainstem, and spinal cord. The gray matter of the thalami and basal ganglia is frequently affected, particularly in children, usually in a symmetrical pattern. Contrast enhancement is not a common feature [33]. Onset age and pathogenesis of ADEM and PML are different, though there are some similarities on imaging.

CNS vasculitis

CNS vasculitis is the inflammation of blood vessels in the brain or spine. It can be caused by viral, bacterial, or parasitic infections and is often accompanied by other autoimmune disorders such as systemic lupus erythematosus or dermatomyositis, as well as systemic diseases such as Behcet’s syndrome or Wegner’s granulomatosis [34]. In the case that it occurs without any of the associated systemic diseases, it is designated primary angiitis of the CNS (PACNS). PACNS is a rare form of vasculitis whose cause is currently unknown. The mean onset age is 50 years, with men being affected twice as often as women. Headaches and encephalopathy are the most frequent initial symptoms. Less than 20% of patients develop strokes or focal symptoms at the primary onset of the disease, and are uncommon in the absence of initial symptoms. However, patients with PACNS who do develop strokes have usually suffered more than one and in different anatomic territories. Angiography had both a low sensitivity and specificity. The diagnosis of PACNS is established by brain biopsy and is usually treated with a combination of high-potency steroids and an immunosuppressant [35, 36] (case 7).

Case 7

A 43-year-old female with a past medical history of inflammatory polyarthritis and migraines presented to ER because of a far worse headache than usual—migraines and acute onset of paresthesia of her left upper and lower extremities, which had quickly progressed to complete numbness. ER assessment also showed weakness of her left side of the body. CT of the head and CTA were obtained immediately, followed by diagnostic cerebral angiogram 2 days later (Fig. 7). MR of the brain was obtained 2 month later for follow-up.
Fig. 7

CNS vasculitis. Non-enhanced axial CT of the head (a) shows an acute hematoma in right parietal lobe with surrounding edema and mass effect. There are subarachnoid hemorrhages within bilateral sulci. On MR, sagittal T1 (b) and axial T2 (c) show an acute/subacute hematoma in the right parietal lobe. Sagittal MIP images of CTA (d) shows multiple focal stenoses in the right MCA branches (arrowheads). Lateral views of the anterior (e) and posterior (f) arterial circulation of cerebral angiogram show multiple stenoses (arrowheads), consistent with vasculitis

The most common differential diagnosis is intracranial atherosclerotic disease (ICAD), which occurs due to the buildup of atherosclerotic plaques along intracranial arteries, causing stenosis or thromboembolism. Intracranial arterial stenosis is similar to coronary heart disease, in that both are part of the same systemic process. The risk factors are identical: age, hypertension, hyperlipidemia, smoking, and diabetes. ICAD is the most common cause of ischemic stroke in the world [37]. The order of emergent CTA has increased significantly due to the advance of intra-arterial thrombectomy for acute stroke management that was introduced with the publication of the MR CLEAN trial [38]. Conventional angiograms are usually unnecessary.

Another differential diagnosis is reversible cerebral vasoconstriction syndrome (RCVS). RCVS is a group of disorders characterized by acute onset thunderclap headaches and multifocal narrowing of the intracranial arteries. RCVS, as its name indicates, usually resolves symptoms within 3 months, and has a high probability of affecting women between 40 and 50 years of age. Although true occurrence rates are unknown, it has been reported with increasing frequency in recent years due to the widespread use of CTA or MRA. CSF analysis is usually normal. The hallmark is prolonged but reversible vasospasms documented on angiography. Its exact etiology is unknown, but transient brain edema has been suggested. It may be related to posterior reversible encephalopathy (PRES), as patients with PRES have higher rates of reversible intracranial arterial vasoconstrictions [39]. Treatment of RCVS does not require immunosuppressive medication.

Spinal epidural abscess

Spinal epidural abscess (SEA) is an infected fluid collection within the epidural space of the spinal canal. It has become more frequent due to IV drug abuse epidemic. It is usually concomitant with spinal osteomyelitis and discitis. SEA is a neurosurgical emergency, with delays in diagnosis leading to serious neurological deficits, paralysis, or even death. It is one of the most frequent reasons for stat MRI in the emergency department, especially with the clinical triad of back pain, fever, and neurological deficits involving the extremities. Laboratory studies often show leukocytosis and elevated ESR. Pathologically, the spread of bacterial infection to the epidural space is either through hematogenous dissemination from septicemia, or direct invasion from adjacent infected structures such as the vertebral bodies or disc spaces. Although CT may reveal evidence of discitis or osteomyelitis if present, MRI is the only true modality which can accurately diagnose SEA. The disease could present either as a phlegmonous or a liquid form. Phlegmonous forms demonstrate prominent heterogeneously enhancing soft tissues within the epidural spaces while liquid forms show a single or multiple fluid collections with peripheral rim-like enhancement [40]. Additionally, SEA is often associated with edema/inflammation or abscesses in adjacent paravertebral spaces, paraspinal muscles, and bony structures (case 8).

Case 8

A 66-year-old male with history of uncontrolled diabetes, pericarditis, and recently treated bacteremia presented to the ER with fever, neck pain, and difficulty swallowing. Vital signs showed no evidence of fever; however, labs revealed leukocytosis and elevated ESR. He was otherwise neurologically intact. Emergent MRI of the cervical spine without and with contrast was ordered (Fig. 8).
Fig. 8

Spinal epidural abscess. MR of the cervical spine shows prominent prevertebral and epidural soft tissues from lower clivus to the dens (arrowheads) which appear isointense on axial (a) and sagittal (d) T1 images, heterogeneously hyperintense on axial (b) and sagittal (e) T2 images, and demonstrate heterogeneous enhancement on post-contrast axial (c) and sagittal (f) T1 images, consistent with phlegmon. There are multiple non-enhancing foci with corresponding hypointense T1 (a, d) and hyperintense T2 (b, e) signals (arrows) within the prominent soft tissues of the prevertebral and epidural spaces, consistent with prevertebral and epidural abscesses

Acute epidural hematoma (EDH) of the spine is the main differential diagnosis of SEA. EDH is most commonly caused by spontaneous venous bleeds in the setting of coagulopathy or anticoagulation therapy. Fever is not the usual presenting feature beside back pain. Neurologic deficits depend on the location and size of the EDH. The most common location of EDH is in the cervicothoracic region, usually posterior to the thecal sac over 2–4 vertebral levels. On non-enhanced CT, EDH appears to be a hyperdense extradural mass. The key features to distinguish SEA from EDH are mostly based on signal characteristics of MR. The signals of the EDH change with time due to evolution of the blood products. It has better defined margins and do not demonstrate peripheral enhancement [41], though there are rare cases of infected EDH highly mimicking abscess.

Epidural metastases can also mimic SEA, though they usually are not associated with fever, leukocytosis, or elevated sedimentation rate. The most frequent causes of epidural metastasis in adults are breast, lung, and prostate cancers. In children, the most common causes are sarcoma, neuroblastoma, and lymphoma. Clinical course is also important to distinguish SEA from metastases, as tumors almost certainly have a more chronic clinical progression, whereas SEA presents more acutely and progresses rather rapidly.

Epidural metastasis is rarely an isolated phenomenon; it usually (86% of cases) accompanies adjacent vertebral body metastasis [42]. Multiple levels of involvement are common. The thoracic spine is most often (60%) involved, followed by lumbosacral region (20–35%), and cervical region (10%) [43]. Cord compression is the most dreaded complication which may require immediate radiation or surgery. Contrast enhancement is helpful in delineating the extent of tumor and may help in outlining regions of spinal cord compression.

Neuromyelitis optica

In the case of acute myelopathy, multiple sclerosis and acute transverse myelitis are far more commonly encountered diseases in the ER than the less-seen neuromyelitis optica (NMO).

Multiple sclerosis (MS) is a chronic demyelinating disease affecting the brain and spinal cord in which the myelinated axons were attacked by the patient’s own immune system, producing different degrees of damages and symptoms. Spinal MS is usually concomitant with brain lesions, although 20% of patients with spinal lesions do not have intracranial plaques. Contrary to the brain, both gray and white matter lesions can be seen in the cord. MS plaques appear hypointense on unenhanced T1WI and hyperintense on T2WI. Spinal plaques generally are less than two vertebral bodies in length. Enlargement of the cord is usually seen with active disease. Enhancement often indicates active demyelination causing acute breakdown of the blood-brain barrier. The enhancement may last 2–8 weeks. Steroids typically do not suppress the enhancement of the active plaques. Chronic lesions often demonstrate focal cord atrophy and do not enhance [44].

Neuromyelitis optica, also called Devic’s disease, is an autoimmune disease that predominantly affects the optic nerves and spinal cord. NMO was previously believed to be a variant of multiple sclerosis, but due to the discovery of the disease-specific serum anti-aquaporin-4 antibody (anti-AQP-4) [45] and mounting evidence of distinct clinical features, it is now considered an independent entity [46]. The prevalence of NMO in various studies ranges from 0.5 to 10 per 100,000. Its female predominance is 5:1 to 10:1 while 2–3:1 in MS. The onset varies with two peak periods of incidence, once during childhood and once during adulthood at 40–50 years of age (older than MS). MS is more prevalent in European populations worldwide, while NMO is higher represented in non-European populations, especially Asian countries.

Patients with NMO often have more abrupt and severe neurologic attacks such as weakness/paralysis of the extremities, sensory alterations, and loss of bladder/bowel control. On MRI, the spinal cord involvement extends over three or more vertebral bodies. The abnormal signals span the entire width of the cord in NMO rather than just partial or peripheral white matter tracts as in MS (case 9).

Case 9

A 46-year-old female presented with shortness of breath and ascending bilateral lower extremity numbness and weakness for a few days. Upon admission, she also had arm weakness and abnormal sensation of the bladder. Neurological exams showed multiple deficits and pathological reflexes, suggesting myelopathy. MR of the entire spine was ordered. No vision loss at the time of the admission although acute vision loss was developed 2 years later (Fig. 9). Lab tests later showed positive antibodies for neuromyelitis optica.
Fig. 9

Neuromyelitis optica. MR of the cervical spine shows an elongated area of hyperintense signals in the central cervical cord (arrow) on sagittal PD (a), sagittal T2 (b) and axial T2 (d) images, consistent with myelopathy. Post-contrast axial T1 (e) image shows heterogeneous enhancement within the central cord lesion. MRI of the brain obtained the same day shows no white matter abnormality on axial T2 (c) image. Post-contrast axial fat-sat T1 (f) image of the orbit (in later time) shows enlargement and diffuse enhancement of the left optic nerve (arrow head), consistent with optic neuritis

Individual optic neuritis in NMO is analogous to isolated optic neuritis or those related to MS, though visual loss is generally more severe in NMO. Sequential optic neuritis that occurs in rapid succession or bilateral simultaneous optic neuritis is both highly suggestive of NMO. In the past, the brain was not typically involved in NMO, but recent studies suggest that brain involvement can be present, although it differs from that seen in MS. On MRI, the distinct brain lesions typically spare the cortex, with focal hyperintense T2 lesions in the hypothalamus, and periaqueductal gray matter, or confluent hyperintense T2 lesions in the periventricular white matter (not Dawson’s fingers) [47].

Distinguishing NMO from MS flare up is important, as immunosuppressive therapy can be initialized in patients with the former but not the latter.

Transverse myelitis (TM) is often used as a non-discriminatory term to refer to a spectrum of diseases that affect the spinal cord, such as bacterial/viral infections, MS, NMO, and ADEM. In this article, acute transverse myelitis is considered an acquired disorder caused by an infectious process, such as viral/bacterial infections and Lyme’s disease. Patients with TM often present with rapid onset of weakness, sensory alterations, and bladder/bowel dysfunctions. MRI of the spine shows non-specific acute myelopathy across the entire width of the spinal cord which could either be single or multiple, depending on the state of enhancement [48]. The brain is commonly involved as well, showing non-specific bilateral white matter disease with or without enhancement. It is not associated with optic nerve pathology. In case of evaluating acute onset myelopathy, it is important to exclude any compressive or non-inflammatory causes as well as distinguish various types of TM.

Guillain-Barré syndrome

Guillain-Barré syndrome (GBS) is an autoimmune disorder in which the patient’s immune system attacks his own peripheral nerves and cause acute inflammatory demyelination. People of all ages can be affected, but it is found more frequently in adults. Both sexes are equally affected. It usually presents with lower extremity weakness and change of sensation, followed by rapid progression of symmetric ascending neuropathy, such as weakness and paralysis [49]. The disorder can be life-threatening, as 15% of patients develop involvement of the breathing muscles requiring mechanical ventilation.

Most GBS cases are preceded by upper respiratory tract infections or diarrhea 1–3 weeks before its onset. The most commonly inciting agent is Campylobacter jejuni (30%) [50]. GBS is diagnosed by combination of clinical presentations of ascending peripheral neuropathy, CSF abnormalities, and electrophysiological criteria. CSF abnormalities are non-specific, including increased protein without pleocytosis. Nerve conduction abnormalities include slow or blocked nerve conduction, prolongation of distal latency, and f-waves.

Characteristic imaging findings on contrast-enhanced T1WI in GBS is symmetric thickening and enhancement along the conus and nerve roots of the cauda equina. The predominance of inflammation involving the spinal roots and particularly in the ventral ones is an outstanding feature in GBS [51, 52] (case 10).

Case 10

A 23-year-old male presented with sudden onset of weakness of bilateral feet 4 days before admission which became progressively worse, ascending to bilateral leg weakness. At time of admission, he had difficulty lifting things with his arms and experienced tightness of the upper back and neck. No loss of bladder control. Two weeks prior to presentation, the patient had a bout of sore throat, cough, and malaise which resolved by itself. MRI of the brain and cervical spine was normal. Lumbar spine MRI revealed abnormalities as illustrated below (Fig. 10). Nerve conduction studies are consistent and strongly suggestive of a demyelinating neuropathy such as Guillain-Barré syndrome
Fig. 10

Guillain-Barré syndrome. Axial T2 (a) and non-enhanced T1 (c) MRI sequences show normal appearance of the conus medullaris (arrow) and associated ventral nerve roots (arrowhead). Post-contrast axial T1 (b, d) and sagittal T1 (e) of the lumbar spine show enhancement of ventral nerve roots (arrowheads)

The more common etiology of leptomeningeal enhancement along the nerve roots of the cauda equina includes spinal meningitis/arachnoiditis and drop-metastasis.

Acute spinal meningitis is rare, often accompanying cerebral meningitis. In addition to the high fever, headaches, and stiff neck associated with cranial findings, there are usually symptoms of acute cord dysfunction or nerve root involvement, such as bladder incontinence, leg weakness, and sensory abnormalities. On MRI, the nerve enhancement is diffuse without preference to the ventral nerve roots.

Arachnoiditis is the inflammation of the arachnoid membranes that surround the spinal nerves. It is commonly seen in patients with prior lumbar puncture or surgery. The most common symptom is back or leg pain. On CT myelogram and MRI, the nerve roots appear clumped with peripheral adhesions; often producing the so-called empty sac. The enhancement pattern on MR is variable, and can be thin, linear, or nodular.

Leptomeningeal metastasis accounts for 5% of spinal malignancies. A primary CNS tumor that spreads inferiorly along the CSF spaces is referred as drop-metastasis. This is more common than the systemic malignancies spread through hematogenous route. The most common causative CNS neoplasms in children are malignant astrocytomas, medulloblastomas, pineal cell tumor, ependymomas, etc. The most common systemic malignancies in adults include breast and lung carcinomas and melanomas [4, 5]. Enhanced MRI of the spine can show nerve root thickening, cord enlargement, intraparenchymal and subarachnoid nodules, or epidural compression.


Accurate diagnosis of the infectious and inflammatory diseases of the CNS can often be very challenging. Because the prompt treatment can be critical for the patients’ outcome, it is very important for the ER radiologists and physicians to have a thorough understanding of a wide spectrum of these disease entities, their clinical presentations, and imaging features so that accurate diagnosis can be achieved and timely intervention can be initiated. At the end of this article, the readers will:
  1. 1.

    Become familiar with the imaging patterns and characteristics of a wide spectrum of the infectious and inflammatory diseases the CNS and their differential diagnosis.

  2. 2.

    Develop a pattern approach method of these diseases and an organized search of their pathways.

  3. 3.

    Understand their etiology, clinical presentations, and courses as well as choices of treatment.



Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interests.


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Copyright information

© American Society of Emergency Radiology 2017

Authors and Affiliations

  1. 1.Department of Radiology, Baystate Medical CenterUniversity of Massachusetts School of MedicineSpringfieldUSA
  2. 2.Department of RadiologyNingbo First Hospital, Zhejiang UniversityNingboChina

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