Background

Cerebrospinal fluid (CSF) fistulas are classified based on their etiology into congenital and acquired fistulas. The acquired fistulas are further classified as nontraumatic, traumatic and spontaneous [1, 2].

Spontaneous CSF leaks may arise from defects in the skull base, in relation to the Tegmen tympani, Tegmen mastoideum, sigmoid sinus and posterior semicircular canal. Leaks in these locations cause indirect CSF rhinorrhoea implying a communication of the subarachnoid space with the middle ear cavity. A constellation of extensive pneumatization, arachnoid pits and empty sella plays a role in the pathogenesis of sphenoid sinus fistulae [3] apart from congenital skull base defects. Arachnoid granulations in relation to the temporal bone are another contributing factor for spontaneous CSF otorrhea and rhinorrhoea [1]. Detection of these defects is best done with computed tomography (CT) and CT Cisternography with an additional role of MRI in assessment of associated parenchymal herniation with utmost accuracy. In this study, we wish to highlight the existence of spontaneous CSF leak as an entity associated with hyper-pneumatized petrous bone and sphenoid sinus.

Case presentation

Case 1

A 26-year-old female patient presented with history of right sided rhinorrhoea for 1 month. There was no history suggestive of meningitis, trauma, visual blurring. Patient was conscious, oriented. No meningeal signs were seen. Fundoscopy was normal. Clinically patient appeared of appropriate built and nourishment for age and gender. Blood work-up revealed no significant abnormality. No previous similar complaints were noted in the past.

Plain CT study showed hyper-pneumatization of the petrous bones with associated soft tissue thickening within (Fig. 1). In addition, a bony defect of ~ 2 mm with adjacent bone scalloping involving the anterolateral aspect of petrous part of right temporal bone (Fig. 1) was demonstrated. CT cisternography showed contrast opacified CSF extending into the pneumatized petrous apex cells, Eustachian tube, middle ear cavity, aditus, antrum, mastoid air cells (Fig. 2). However, no contrast opacification of cochlea and vestibular apparatus was observed. Pooling of contrast in the right nasal cavity and ethmoidal air cells was also seen (Fig. 2). Corresponding MRI brain revealed features of right mastoiditis extending to involve the pneumatized petrous bone (Fig. 3). Diagnosis of right petrous defect with paradoxical CSF rhinorrhea was hence made.

Fig. 1
figure 1

A 26-year-old female presented with history of right sided rhinorrhoea. A Plain CT PNS and temporal bone axial section showing bilateral hyper-pneumatized petrous bones with overcrowding of the right mastoid air cells (red arrow), opacification of the hyper-pneumatized petrous bone (red arrow head) and mild right ethmoid sinusitis. B Plain CT temporal bone coronal section demonstrating the overcrowding of the right mastoid air cells by mucosal thickening (red arrow). C Plain CT PNS and temporal bone axial section demonstrating the scalloping along the antero-medial aspect of the right petrous bone (red arrow)

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Fig. 2
figure 2

A CT cisternography performed in prone position depicting the scalloping along the antero-medial aspect of the right temporal bone (red arrow) with contrast leak through this into the adjacent hyper-pneumatized petrous bone. B CT cisternography coronal section demonstrating the increased density corresponding to the contrast leak into the right mastoid air cells (red arrow) and middle ear and C CT cisternography axial section demonstrating contrast leak at the nasopharyngeal opening of the right Eustachian tube (red arrow). D CT cisternography coronal section demonstrating the contrast leak into the right nares (red arrow) with the nasal pledgets showing contrast soakage (red arrowhead)

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Fig. 3
figure 3

A T2W axial section showing T2 hyperintensity within the right mastoid air cells (red arrow) and middle ear cavity. It is further seen extending to involve the hyper-pneumatized right petrous body. B MRI CISS coronal section showing the T2 hyperintensity within the right mastoid air cells (red arrow)

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Follow up: Patient underwent right temporal craniotomy and CSF leak repair. Intra-operatively, the petrous bone was thin with cobble stoning and few areas of defect over the petrous ridge. A defect of 1 × 1 cm was seen toward the lateral aspect close to the anterior petrous ridge. A temporal muscular pedicle flap was used as a covering fascia. Patient tolerated the procedure well and was discharged with no CSF leak. Patient is currently asymptomatic.

Case 2

A 49-year-old female patient presented with history of right sided watery nasal discharge over the last 15 days. There was no history suggestive of meningitis, trauma or visual blurring. Patient had no other comorbidities with no similar prior complaints. Clinically patient appeared of appropriate built and nourishment for age and gender. Blood work-up revealed no significant abnormality.

On HRCT of the temporal bone, hyper-pneumatization of sphenoid bone involving right greater wing of sphenoid bone and bilateral pterygoid processes was noted (Fig. 4). Also, bony defect measuring 1.4 mm noted in the right greater wing of sphenoid bone (lateral to the foramen rotundum) (Fig. 5). Corroborative MRI brain CISS sequence revealed predominant downward herniation of meninges and adjacent minimal brain parenchyma through the above-mentioned defect into the hyper-pneumatized right greater wing of sphenoid bone and right pterygoid process (Fig. 6).

Fig. 4
figure 4

A 49-year-old female presented with history of right sided watery nasal discharge. A Plain CT PNS coronal section showing the hyper-pneumatized right sphenoid bone (vertical red arrow) with mucosal thickening within (oblique red arrow) and B axial section showing the hyper-pneumatized bilateral pterygoid bones (red arrows) with opacification on the right side by mucosal thickening

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Fig. 5
figure 5

CT cisternography coronal section in bone window demonstrating the bony defect in the right greater wing of sphenoid bone (red arrow) and resultant contrast opacification (red arrow)

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Fig. 6
figure 6

A T2W image coronal section showing the defect in the right greater wing of sphenoid bone (red arrow) with predominant downward herniation of meninges and adjacent minimal brain parenchyma (red arrowhead). MRI brain CISS sequence B Coronal and C sagittal sections showing the defect in the right greater wing of sphenoid bone (red arrow) with downward herniation of meninges and adjacent minimal brain parenchyma (red arrowhead)

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Follow up: Patient was lost to follow-up and hence information regarding further management could not be provided.

Discussion

The development of skull base defects is caused by the interaction of CSF pulsation with the bone. The anatomical predisposition differs between the anterior, middle and posterior cranial fossa. Site of CSF leaks in temporal bone in various studies are summarized in Table 1 according to the age of the patients [4,5,6,7,8].

Table 1 Site of CSF leaks in temporal bone in various studies—table adapted and modified from study by Nadaraja et al. [8]
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The novelty in the first case lies in the fact that none of these cases presented with a defect in the anterior ridge of the petrous bone. Unlike the thin tegmen tympani and tegmen mastoideum, the anterior ridge of the petrous bone is thicker and does not have any congenital dehiscences. Due to the presence of petrous apex hyper-pneumatization, the otherwise thicker anterior ridge could have given way in response to the raised intracranial pressure. The scalloping of the anterior ridge of petrous bone indicated a chronic process in the background of raised CSF pressure. Other factors for spontaneous CSF leak such as arachnoid granulations defects and idiopathic intracranial hypertension were ruled out in this patient [1]. Multiple defects were detected in the right petrous bone in this case. Difference between the previous cases and this case was that despite having a temporal bone defect, the patient presented with rhinorrhea instead of otorrhea. This indicated that the tympanic membrane was intact. The cause of rhinorrhea could be attributed to paradoxical leak into the nasopharynx via the eustachian tube and further into the ethmoid sinus and nasal cavity. It is very important to highlight here the identification of the middle fossa defect, which if left unrepaired may result in persistent leak. Also emphasizes the role of radiologists in accurate localization of site of leak which decides the appropriate surgical strategy [1, 4,5,6,7,8].

The second case highlighted the association of CSF rhinorrhea with presence of meningoencephaloceles and hyper-pneumatized paranasal sinuses. Literature has thrown light on the likely pathogenesis of development of meningoencephaloceles secondary to CSF fistulas. The presence of raised intracranial pressure has been strongly associated with the presence of spontaneous CSF fistulas-they are seen to promote formation of prominent arachnoid granulations [1, 2, 9]. Owing to the altered CSF dynamics within these, there is a path of reduced resistance created through which the meninges and cerebral parenchyma tend to easily herniate resulting in creation of a spontaneous CSF fistula [1]. However, the current accepted theory is that of a multifactorial process involving the combination of elevated intracranial pressure and anatomical deficiencies (viz. hyper-pneumatized paranasal sinuses) [10, 11] aiding in the development of meningoencephaloceles and spontaneous CSF fistulas [1].

High resolution computed tomography (HRCT) of the paranasal sinuses as well as the temporal bones is a baseline imaging modality which helps identify skull base defects irrespective of the presence of CSF leaks [2, 11]. However, the detection of dural defects and differentiation of mucus secretions from actual CSF is not possible. To aid in this, use of additional CT Cisternography and MR cisternography is beneficial. CT Cisternography utilizes intra-thecal injection of iodinated contrast in order to opacify the CSF and thus, it accurately demonstrates the site of leak with visualization of active leak through the defect. MR Cisternography (MRC) utilizes the property of high signal intensity of CSF on heavily T2 weighted images in the subarachnoid spaces with the added advantage of its excellent soft tissue resolution thereby helping detect the presence of an associated dural defect with super-added meningoencephaloceles [2, 10, 11]. However, bony details maybe sub-optimally evaluated on MRI and hence interpretation of the imaging findings must be performed along with CT. Additional modalities that have been proven to aid in the diagnosis are contrast enhanced MRC and Radionuclide cisternography [10]. At our institute, we follow a protocol consisting of HRCT of the paranasal sinuses and temporal bones followed by MR brain with MR Cisternography (CISS sequences—heavily T2 weighted images). This is further followed by a confirmatory CT Cisternography to demonstrate site of active leak with use of dynamic maneuvers facilitating the leak.

As demonstrated in these cases, in the presence of normal intracranial pressure, hyper-pneumatization is a significant risk factor for the development of spontaneous CSF fistulas. Such correlation has not been documented in previous studies. Hence, we tend to propose a novel hypothesis of hyper-pneumatization predisposing to formation of spontaneous CSF leaks. As a part of routine reporting protocol, documentation of presence of hyper-pneumatization is essential to estimate the risk of development of such fistulas.

Limitation

The second case was lost to follow-up and thus a confirmation regarding the radiological diagnosis by surgical correlation could not be made.

Conclusions

The recognition of spontaneous CSF leaks is vital and is to be made with accurate detection of the site, etiology and severity of the same as poorly managed cases result in deleterious complications. HRCT of the involved region with further CT cisternography and corroborative MRI is an essential and beneficial imaging protocol. Careful scrutiny of the skull base to look for hyper-pneumatized sinuses and anatomical dehiscences should be performed in a meticulous manner.