Epilepsy surgery for refractory epilepsy due to encephalocele: a case report and review of the literature

ARTICLE

Auteur(s) : Howard J Faulkner¹, David R Sandeman^2,4, Seth Love³, Marcus J Likeman⁶, Desnomd A Nunez⁷, Samden D Lhatoo^1,4,5

¹Department of Neurology
²Department of Neurosurgery
³Department of Neuropathology
⁴Bristol Complex Epilepsy Surgery Service
⁵Department of Neurophysiology
⁶Department of Radiology
⁷Department of Otolaryngology, Frenchay Hospital, North Bristol NHS Trust, Bristol, United Kingdom

Article reçu le 27 Octobre 2009, accepté le 22 F�vrier 2010

Encephaloceles are characterised by defects of the bony skull through which herniation of the intracranial contents can occur. Encephaloceles are classified according to their location and contents. Those containing only cerebrospinal fluid (CSF) and meninges are termed craniomeningoceles; if the lesion also contains neural tissue then it is termed a meningoencephalocele. Encephaloceles may be congenital, due to failure of neural tube closure, resulting in a bony defect through which herniation of neural tissue may occur from birth or at a later date (Naidich et al., 1992) They may also be acquired lesions following traumatic, neoplastic, metabolic, or infectious damage to the skull base (Mandl et al., 2007; Nager, 1987).

Patients usually present with direct neurological complications due to traction or herniation of involved cortex resulting in weakness, sensory disturbance (Fountas et al., 2005) or with seizures (Ruiz García, 1971). They may also present with CSF otorrhoea or rhinorrhoea, recurrent meningitis and symptoms related to encroachment on adjacent regions such as nasal obstruction (Kohrmann et al., 2007; Magliulo et al., 1998).

An encephalocele can be visualised on CT or MR imaging. The presence of a bony defect is best shown on skull base CT with MRI enabling differentiation of the contents and showing any connection with brain tissue (Schuknecht et al., 2008).

The imperative for surgical treatment is usually strong, given the potential for recurrent meningitis, brain damage from herniation and refractory seizures. The surgical approach and technique is dependent upon the position and size of both the defect and the encephalocele (Woodworth et al., 2004).

Case study

At age 27 there was a change in seizure type to one of complex partial seizures, without preceding aura, occurring twice per week. The episodes were typified by behavioural arrest and confused speech, lasting up to two minutes, followed by a two-hour period of amnesia during which she could perform complex tasks. Physical examination remained normal throughout. There was no response to carbamazepine, topiramate, levetiracetam, lamotrigine, phenytoin, tiagabine or zonisamide.

MRI scans 1.5T and subsequently 3T were performed as part of a pre-surgical workup with a view to resective surgery. These showed a 3 cm right nasal meningoencephalocele with a prolapsed right gyrus rectus through a defect in the cribriform plate (figure 1). Skull base CT confirmed a defect in the cribriform plate. A 18-FDG PET-scan showed no distinct areas of hypometabolism.

Prolonged video-EEG monitoring was carried out, using the 10-20 electrode system supplemented by bilateral sphenoidal electrodes, with partial drug withdrawal. Inter-ictal EEG showed regional right-sided slowing and bilateral synchronous as well as asynchronous sharp waves with an electrical maximum in the sphenoidal electrodes (figure 2A). During monitoring, we documented two of her habitual partial seizures, characterised by a right frontal ictal EEG onset, maximum at F8, Fp2, and F4 respectively, which spread within seconds to the whole of the right hemisphere and subsequently the left hemisphere (figure 2B).

Nasal endoscopy demonstrated a right intra-nasal mass consistent with an encephalocele. After discussion with the otorhinolaryngologist, we decided to address the treatment for the meningoencephalocele and refractory epilepsy in a two-staged approach. Firstly we planned to excise the meningoencephalocele and second, to carry out resective frontal cortex surgery through craniotomy.

At surgery, the meningoencephalocele was dissected off the walls of the right nasal cavity, decompressed and the dura and herniated brain tissue excised. An intra-cranial-free abdominal fat graft was placed through the skull base defect and attached to its edge, then sealed from below with harvested nasal septal cartilage. Histology of the excised meningoencephalocele confirmed meningeal structures and the presence of brain tissue (figure 3). Neurons within the tissue appeared haphazardly arranged, without consistent orientation or discernable lamination but the individual cells did not show significant dysmorphism. No ballooned cells were present.

Post-operative MRI scans showed complete excision of the meningoencephalocele and satisfactory repair to the right frontal skull base (figure 4). Following the procedure, there was dramatic seizure remission; no seizures were reported, compared to twice weekly before surgery, completely obviating the need for potentially hazardous further resective brain surgery. The patient has remained seizure free for more than 2 years, having elected to remain on medical treatment (200 mg lamotrigine and 100 mg zonisamide, twice daily).

Discussion

We postulate that the mechanism of epileptogenicity in this patient lay in an irritative, traction effect on orbitofrontal cortical tissue as it prolapsed through the bony defect into the upper nasal cavity. Additionally the neurons within the resected tissue appeared haphazardly arranged, without consistent orientation or discernable lamination. Although individual cells did not show significant dysmorphism and no ballooned cells were present, an element of dysplasia either secondary to the encephalocele or as part of the congenital abnormality, cannot be ruled out. What is remarkable about this case is the prolonged, terminal seizure freedom that occurred following trans-nasal endoscopic excision of the prolapsed cortex with the rest of the meningoencephalocoele. It is likely that the patient would otherwise have required craniotomy and possibly prolonged intracranial EEG monitoring in order to delineate the epileptogenic zone and guide resection. Having been seizure free for over two years, it is unlikely that this outcome is simply chance or part of the variability within the natural history of an individual's epilepsy. This case is illustrative of the fact that a multi-disciplinary approach involving the epileptologist, the epilepsy surgeon and the ENT surgeon can result in an effective solution for a refractory problem which minimises treatment morbidity.

Although rare, meningoencephaloceles are described in 33 previously reported cases in the literature (table 1). Of those cases where the data was reported: 82% were congenital, presentation was typically in middle age (mean age 34) and 78% were female. The female preponderance is seen with encephalocele of all types and may be genetic in origin. The remaining 18% were due to trauma, post-operative complications, neoplasms and osteopetrosis.

The location of encephaloceles associated with seizures is variable; 45% temporal, 23% occipital, 29% frontal (including 13% nasal) and 3% parietal. Associated cranial abnormalities are reported in only 15% of cases. These include band heterotopia, nodular heterotopia, diffuse cortical dysplasia and schizencephaly. The fact that the majority of cases show no other pathology strongly suggests that the encephalocele alone can act as an epileptic focus.

Further evidence for the encephalocele as the epileptic focus comes from the high levels of concordance with EEG localisation; 81% of cases showed EEG localisation to the site of the encephalocele. Of the remaining cases, one had a normal EEG and three had non-specific diffuse EEG abnormalities associated with other developmental structural pathology including cortical dysplasia and heterotopia.

In 18 cases surgical treatment strategies were reported. In total, 56% underwent local excision of the encephalocele contents and 44% underwent a wider excision with a lobectomy. Of the surgically managed encephaloceles, 33% were extra-temporal, of which 100% were managed with local excision of the encephalocele contents, and 67% were temporal. In contrast to other locations however, 67% of the temporal lobe encephaloceles were treated with lobectomy versus only 33% with local excision of the encephalocele contents. This may reflect a familiarity with temporal lobe resection.

Post-operatively, there was no difference in the seizure freedom rates between those patients who underwent a lobectomy and those who underwent a local excision of encephalocele contents. Of the 17 cases with post-operative data on seizure frequency and outcome, 100% were seizure free. Those who underwent a local excision thus had equal rates of seizure freedom to those who underwent more extensive resections, irrespective of the location of the encephalocele. The similar rate of seizure freedom suggests that wide resections may not be required in this situation. The current case, with a minimally invasive local excision via a nasal endoscopic approach, furthers this argument.
Table 1 Reported cases of meningoencephalocele associated with epileptic seizures.

Conclusion

Disclosure

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