Background

Meningoencephalocele (ME) is a herniation of brain parenchyma covered by meninges through a bone defect and could be malformative or secondary [1, 2]. The subsequent herniation of the brain with its meninges would weaken the dura with its rupture and a final cerebro-spinal fluid (CSF) leakage [2].

A bone defect can allow herniation of brain tissue through its meningeal covering, and frequently occurs in congenital forms of developmental anomalies in which arachnoid granulations can lead to direct contact with bone, resulting in its erosion. For the less frequent acquired forms, on the other hand, a chronic inflammatory status, iatrogenic injury, or rarely head trauma can result in herniation of the meningeal tissue. Chronic inflammations, previous surgery, neoplasms, and irradiation are well-known predisposing factors and can represent the most common causes of a secondary ME [1].

In particular, temporal MEs usually protrude into the middle ear by tegmen tympani. Except for rarer cases of spontaneous ME, it is usually secondary to infections, chronic inflammatory diseases, trauma, and surgery [3–5].

ME symptomatology is often mimicked by rino/otological issues, the diagnosis of spontaneous MEs relies on a high degree of suspicion in any case of unilateral clear leaking prolonged for many days, and rarely months. Consequently, delay in diagnosis can lead to neurological complications [6]. Rarely, forms of ME secondary to head trauma may occur at a late stage from the primary event making diagnosis and subsequent treatment. Such "delayed" forms often find anecdotal evidence in the literature making the correct choice of follow-up management, diagnosis and treatment complicated to manage.

With this review, we report all the cases described in the literature of delayed ME after head trauma analyzing the timing of clinical onset, treatment and complications of this rare event we also want to emphasize the importance of radiological follow-up in head trauma performed also with thin-layer computed-tomography (CT) scan with bone window, necessary to identify late secondary bone defects not visible at the time of diagnosis and eventual first treatment describing a representative case from our experience.

Methods

Eligibility criteria

We performed a review of the literature by analyzing all reported cases of diagnosed ME occurred after a head trauma reported from anamnesis with the aim of identifying the clinical features and a timing of the diagnosis in patients suffering from this uncommon complication.

Therefore, while screening the literature, we adopted the following inclusion and exclusion criteria:

Meta-analysis, case series, clinical study or clinical image reporting cases of patients who suffered from ME secondary to a head trauma;

Conversely, we excluded the following:

cases reported without detailed clinical features of patients; cases reported without description of radiological images; papers that report other pathologies (out of topic); and papers written in languages other than English.

Information sources and search

The English literature was systematically investigated using MEDLINE, the NIH Library, Pubmed and Google Scholar. The last search date was November 1, 2021. The following search terms were used: Meningoencephalocele and/or Meningoencephalic herniation with a research string of "Meningoencephalic herniation" OR "Meningoencephalocele" AND “traumatic".

For each case, we reported the number of patients analyzed, age, sex, initial diagnosis at the time of trauma, the time elapsed between the initial traumatic event and the radiological diagnosis of ME, the clinical onset at the time ME was suspected, the cranial region involved, the treatment performed, and the complications reported.

Results

The search returned a total of 92 papers, including radiological, clinical studies and technical note. To this initial cohort, the exclusion mentioned above criteria were applied to abstracts, eliminating 33 papers. The resulting 59 papers are included in our analysis. 37 Articles are subsequently excluded after complete revision of the paper. The list of articles is reassumed to the flowchart in Fig. 1.

Fig. 1
figure 1

A flowchart of research strategy in our literature review

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The total number of patients collected for this review was 61 including a case from our experience, with a mean age of 31.1 years. Details are reported in Table 1.

Table 1 Patient’s demographics
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The presence of ME is more reported in the male than the female sex (33/61, 54%) following the international epidemiology of head trauma in adults [7].

All reported cases have a history of initial diagnosis of mild to moderate head trauma in which CSF leak, rhinorrhea, otorrhea, or other signs of ME were not identified in the early stages (Table 2).

Table 2 Group analysis
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Only three patients were treated surgically at the time of the first access to the emergency room after the trauma in sites unrelated to the subsequent finding of ME (and therefore excluding iatrogenic formation of ME).

Symptoms onset timing and subsequent radiological diagnosis of ME (obtained by magnetic resonance imaging, MRI) is reported as high variable, varying between 24 h (reported in 2 cases) to 43 years. In 15 patients, onset occurred in the range of days (with a mean of about 12 days); in most cases, the diagnosis remained silent for months or years. Trauma is reported in the frontal site in 19/61 cases (31.1%), in the parietal or parieto-occipital area in 10/61 cases (16.4%), and temporal site in 32/61 cases (52.45%). Therefore, wide variability of ME onset symptoms follows, wherein the majority of cases, there are otological symptoms (27/61, 44.26%), CSF leak (10/61, 16.4%), ocular symptoms (such as ecchymosis, exophthalmos, ptosis, in 8/61 cases, 13.1%). In comparison, there are less frequent cases of new-onset neurological disorders (3/61 seizure 4.9%, neurological deficits 2/61 3.3%, weakness 1/61 1.6%, loss of consciousness 2/61, 3.3%), headache and swelling of the injured area (3/61, 4.9% and 4/61, 6.55% respectively). 4 patients started with symptoms referable to meningitis (fever and neurological disorders, nuchal rigidity).

The treatment of this condition involves various reported treatment strategies highly dependent on the location and extent of the defect: in the majority of cases (29/50 procedures reported, 58%), duroplasty by the heterologous dural patch was the procedure of choice, followed by bone reconstruction alone frequently performed in cases of intradiploic herniation (8/50 procedures reported, 16%), autologous flap reconstruction with galea flap was performed in 6/50 cases reported (12%), 4 patients were performed with endoscopic nasal repair (8%), and 3 subjects reported received conservative treatment.

From the perspective of outcome analysis, there is a low rate of complications due to delayed diagnosis of ME. The only 4 cases (6.5%) that presented problems in the follow-up (3 meningitis and one ocular pro-ptosis) were those for whom a bone-only or conservative reconstruction procedure was preferred.

Representative case

A 77-year-old male patient was subdued to a chronic subdural hematoma (CSDH) evacuation at our Neurosurgical Department. The postoperative course was uneventful, and the patient was discharged at home a week later. After four months, patient started to suffer a water-like leakage by the left ear. Anamnesis revealed a history of paroxistic ear leakage during youth after recurrent ear infections. Leakage’s chemical examination with high values of β2-transferrin revealed the CSF nature. A high-resolution CT scan (HRCT) evidenced the presence of phlogistic tissue in the left tympanic cavity and mastoid antrum with tegmen tympani erosion and a skull-base significant defect. Subsequently, a brain MRI was performed and confirmed the presence of a temporal ME through the skull base defect; moreover, signs of chronic flogosis were present, as tympanic cavity and mastoid cells obliteration were found. The aforementioned conditions necessitated a surgical repair with a subtemporal approach [8, 9], whereas the point of herniation was repaired by muscle, heterologous dural patch, and fibrin glue [10].

The postoperative course was uneventful, and the patient was discharged one week after. At one year follow-up, no signs of CSF leakage were present. (Fig. 2).

Fig. 2
figure 2

A The patient following cognitive impairment and right sensorimotor neurological deficit underwent an initial cranial CT scan documenting large CSDH and subsequently underwent evacuation surgery by craniotomy hole. After four months, patient started to suffer from otorrhea of the left ear. A high-resolution CT scan (HRCT) evidenced the presence of phlogistic tissue in the left tympanic cavity and mastoid antrum with tegmen tympani erosion and a skull-base significant defect (B, C, red and green dot). A brain MRI was performed and confirmed the presence of a temporal ME through the skull base defect (D). The patient underwent surgical repair with a subtemporal approach, and the herniation site was repaired with muscle, heterologous dural patch, and fibrin glue. Postoperative follow-up HCT documented a successful procedure without complications (E)

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Discussion

In Neurosurgery centers with a high turnover of traumatic pathology, it is not unusual to find cases of delayed post-traumatic complications. From the literature search we conducted, the occurrence of late ME long after trauma is an under-described and under-reported occurrence; however, we believe that it may be more common than what is normally believed and that the issue is related to the reduced follow-up of hospital centers and by the misdirection of follow-up radiological examinations. In predisposed patients, even mild head trauma can lead to extrinsic theca defects that can lead to the formation of an ME. CSF pulsation through the dural defect may cause progressive erosion of the bone and enlargement of the extradural collection. On the other side, osteogenesis promoted by the dural and periosteal layers would limit the collection to the diploic space and eventually remodel the skull profile in a slow process. These factors probably account for the missed evidence of fracture at the diagnosis many months after the initial head injury and the underlying defects can result in post-traumatic ME often with clinical presentation of otorrhoea or otorhinorrhoea as consequence of violation of meninges and relative cerebrospinal fluid (CSF) leak (in more than 50% of cases). In addition, the defects provide a route for the spread of infection into the intracranial cavity, resulting in recurrent episodes of meningitis and brain abscess. Symptoms in delayed ME may be absent, particularly if the herniated cortex is non-eloquent, or may be subtle and chronic due to irritated cortex. Acute clinical onset after physiological conditions with the elevation of intracranial pressure has been anecdotally reported [18], and our analysis shows that neurological symptoms are infrequent compared with local symptoms such as rhinorrhea or delayed otorrhea.

Clinical history and radiological investigation with high-resolution CT scan (HRCT) and MRI may be the only way to make a diagnosis [1], although even with a bone thin-layer scan is not easy to define a bone defect [11], especially after orbit roofs fractures [28, 52] and after surgical treatments resulting from the trauma [15, 29–32]. In fact, surgical treatment can promote this phenomenon even in a minor procedure in some predisposed patients. An example is ME with a CSF leak that can be found after the treatment of a CSDH-patient with mild head trauma [12] as is reported in our illustrative case. Neuro-radiological examinations, including a thin-layer bone CT scan, revealed that a temporal ME was the cause of leakage. CSDH is a typical condition among 65 years old or older patients, and it is usually related to a mild trauma [22, 23]. The entire process would take about 20 days. Consequently, the patient becomes symptomatic when CSDH exerts a local mass effect on surrounding parenchyma with a subsequent intracranial pressure elevation [23–26]. After the uncomplicated discharge, patients referred us with otorrhea. A more profound patient interview revealed episodes of chronic otitis during his youth. Since the most common cause of acquired temporal MEs actually is chronic otitis media [19] and are also the most frequent secondary form of delayed ME; we argued that a congenital thinner tegmen tympani broke because of intracranial hypertension with a subsequent herniation of the brain, thus excluding head injury as a direct cause of ME, but defining it as a contingent and triggering cause. Nevertheless, the initial dura covering was gradually damaged by a middle ear inflammatory process [33], leading to arachnoid disruption, and favored by the intracranial pressure variations, to the CSF otorrhea. Rarely a head trauma with CSDH can be caused by a spinal CSF leakage and intracranial hypotension, especially in younger patients, but in some case we believed that could be a predisposing factor for ME. Kenning et al. [13] suggested that patients with a CSF opening pressure > 20 cmH2O, BMI > 30 kg/m2, and other predisposing factors should be considered for ventriculoperitoneal shunting after temporal ME repairing. Although CSDH can cause intracranial hypertension, is not considered a typical risk factor for temporal ME, but in adult patients with recurrent CSH and prior presence of bilateral CSH, a survey for an underlying spinal CSF leak should be considered in the differential, especially on a remote trauma or inflammatory disease in clinical history [12].

Whether this is an unusual associated primary injury [14–17], an effect of the chosen therapy, or a combination of these factors, is unclear [20]. Repair of the dural tearing is the aim of surgery, along with decompression of the herniated cortex if incarcerated or strangulated.

Treating physicians should be aware of this unusual complication, especially in the development of new symptoms during therapy [21]. As a spontaneous CSF leakage, literature agrees that a greater than one-week duration should represent an indication for surgical repair [1, 8, 9, 27].

Formation to that of growing skull fractures suggested that brain pulsations played a significant role in pushing the cortex through the neo-membrane in the same way that the leptomeninges are forced through dural calvarial defects in patients with growing fractures. This proposal seems logical, especially given that we could not find a report suggesting that this particular complication has been reported in the adult population [15, 29].

Further study and limitation

Our study has several limitations: first, we pointed out that most of the reported cases are either part of larger studies or report only the salient aspects of treatment focusing little on the timing of onset and delayed clinical onset. Second, little consideration is given to predisposing aspects of ME formation such as a history of chronic sinusitis or chronic otitis that may be risk factors even in mild trauma.

Conclusion

Various factors play a role in the etiopathogenesis of delayed post-traumatic ME, and a bony defect alone is not sufficient for meningoencephalic herniation to develop. We would alert physicians caring for a patient with mild head trauma or CSDHs to be aware of this unusual complication. Although the development of new symptoms should be a reason for patients to undergo neuroimaging, we would also suggest that the identification of areas of cortical irregularity along the inner surface of subdural collections may be an indication for regular and relatively frequent follow-up imaging executed with thin-layer bone CT scan even if a bone defect was not revealed at first examinations.