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Interhemispheric epidermoids - An uncommon lesion in an uncommon location: A report of 15 cases
Correspondence Address: DOI: 10.4103/0028-3886.76874 PMID: 21339669
Of the intracranial epidermoids, interhemispheric epidermoids are extremely rare and only about 19 cases have been reported. This is a retrospective study of 15 patients with interhemispheric epidermoids surgically treated over a 13-year period. The age at the time of presentation varied between 17 and 45 years and there were 9 males. The presenting feature was seizures (focal with secondary generalization) in 12 patients, hemiparesis in 5 and features of raised intracranial pressure in 3. On computerized tomography scan the lesions were hypodense in the interhemispheric region. On magnetic resonance imaging, the lesions were located in the interhemispheric region with heterogenous signal intensities. Restricted diffusion was evident on diffusion-weighted images and apparent diffusion co-efficient images. All the lesions were predominantly located in the anterior interhemispheric region, with either basal or parietal extension along the interhemispheric fissure. Eleven patients underwent frontal or fronto-parietal craniotomies, 3 underwent bifrontal craniotomies and 1 patient underwent supra-orbital craniotomy and endoscopic procedure. Total excision could be achieved in 11 patients; near-total, in 3; and partial excision, in 1 patient. Follow-up was available in 10 patients. Three patients had recurrence of lesion at 5½, 8 and 10 years, respectively. Keywords: Anterior interhemispheric, congenital, epidermoids, parasagittal, seizures
Epidermoids are benign, slow-growing congenital lesions and are most commonly located in the cerebellopontine angle, followed by the suprasellar cisterns, other locations include Sylvian fissure, brainstem, intraventricular, pineal regions, intradiploic space of skull, and spinal cord. [1] Interhemispheric epidermoids are rare, and only 19 cases have been reported. [2] We report our experience with 15 patients, the largest series from a single center.
Fifteen cases of interhemispheric epidermoids were operated between 1997 and 2010 at the National Institute of Mental Health and Neurosciences, Bangalore, India. The case records were reviewed and the data analyzed included clinical characteristics, neuroimaging findings, operative notes and follow-up data.
Patient characteristics of the 15 cases are given in [Table 1]. The age ranged between 17 and 45 years with mean of 28.6 years and there were 9 males. Twelve patients had history of seizures, 8 presented with focal-onset seizures with or without secondary generalization and one had focal sensory seizures, which later progressed to focal motor seizures with secondary generalization (Case 3). Four patients presented with generalized seizures with no focal onset. The mean seizure duration was 4.5 years (range 6 months - 15 years). One patient presented with frontal- and temporal-lobe dysfunction, and 2 others presented with raised intracranial pressure. Neurologically, papilledema was seen in 3 patients; and hemiparesis, in 5. Nine patients had no neurological deficits.
Neuroimaging All patients were investigated with a cranial computerized tomography (CT) and 7 patients were also investigated with a magnetic resonance imaging (MRI). All the lesions were frontal interhemispheric in location with varying degrees of basal or parietal extension. The smallest and the largest lesions were 3 and 10 cm in its maximum diameter, respectively. On CT scan, the lesions were hypodense, well defined, with density similar to that of cerebrospinal fluid (CSF). Four lesions showed peripheral specks of calcification [Figure 1]. None of the lesion had perilesional edema or contrast enhancement. On MRI, the lesions were located in the interhemispheric region with heterogenous signal intensities. On T1-weighted images, they were hypointense to gray matter and slightly hyperintense to CSF. On T2-weighted images, they appeared hyperintense to both gray matter and CSF [Figure 2]. Restricted diffusion was evident on diffusion-weighted images (DWI) and apparent diffusion co-efficient (ADC) images. There was no contrast enhancement. Constructive interference in steady state (CISS) 3-D images showed the lesions to be heterointense with a good anatomical delineation from the surrounding tissues. In recurrent cases, there was minimal peripheral enhancement with contrast. The preoperative diagnosis of epidermoid was made in all the cases.
Operative details Eleven patients underwent appropriate frontal or fronto-parietal parasagittal craniotomies. Three patients underwent bifrontal craniotomies, while one basal epidermoid was approached endoscopically through a supra-orbital mini craniotomy. In 12 patients, an interhemispheric approach was used. However, in 2 patients, due to large draining cortical veins and severe adhesions between the dura and the cortex, a transcortical approach through the superior frontal gyrus had to be used (Cases 6 and 14). Total excision was possible in 11 patients. In 3 patients, near-total excision was done, and a small part of the capsule adherent to the major branches of the anterior cerebral artery or the cortex was left behind. In 1 patient only, a partial excision could be done (Case 6). Before closure of the craniotomy, the entire field was irrigated thoroughly with hydrocortisone-saline solution. Postoperative course All patients were on injectable steroids for a week; and then, on a tapering dose of oral steroids for a month. Immediate postoperative CT scan was done for all patients to rule out surgical complications. Immediate MRI or an MRI at the 3-month follow-up was not done to document the extent of excision. The patient in whom a partial excision was done, presented within 2 months of surgery in altered sensorium. CT showed a cystic recollection. Stereotactic aspiration revealed foul-smelling pus. He later underwent a ventriculoperitoneal shunt after the infection was treated. However, he remained in a state of decortication and was discharged against medical advice. None of the patients had postoperative chemical meningitis or fresh neurological deficits at the time of discharge. Histopathology Grossly, the tumors were soft, friable, with pearly-white flakes. Microscopic examination revealed stratified squamous epithelium on a collagenous base, consistent with epidermoid. Follow-up Follow-up data was available for 10 patients and the follow-up ranged from 3 months to 10 years. Six patients had 3 or more years of follow-up. Of the 8 patients with 1 year or more-than-one years of follow-up, only 2 patients were seizures free (1 patient did not have seizures even prior to surgery). The remaining 6 patients continued to have seizures with reduced frequency as compared to the preoperative state. Two patients with hemiparesis improved after surgery to normal power. Follow-up MRI had been done in 8 patients in the period between 1 and 10 years following surgery [Figure 3]. The surgical impression was total or near-total excision of the lesion in all the 8 patients. Recurrence of the lesion was seen in 3 patients (5½, 8 and 10 years, respectively, following surgery), 1 of whom was reoperated (Case 13). The patient with a recurrence at 10 years was symptomatic but refused surgery, while the other had an asymptomatic recurrence. One more patient (Case 7) had a residue at 1-year follow-up; however, as an immediate postoperative MRI was not done, it is difficult to say whether it was a residual lesion or a recurrence.
Epidermoids are benign congenital lesions and account for 0.5% to 1.8% of all intracranial brain tumors. [3],[4] During the third to fifth week of embryonic life, the ectoderm on the mid-dorsal region forms a neural plate, which then infolds to form the neural tube. During infolding, if the surrounding ectoderm (which later forms the cutaneous structures) does not separate from the neural ectoderm completely, nests of these cells may be entrapped along with the neural ectoderm. These nests of cells later grow within the central nervous system (CNS), resulting in the formation of a spectrum of lesions, viz., epidermoids, dermoids and dermal sinuses. [5],[6],[7] Epidermoids grow slowly and become symptomatic during the third to fifth decades of life (17 to 45 years, in our series), unlike dermoids, which are faster growing and which become symptomatic in the second decade. [5] The epidermoids have a growth pattern similar to that of the skin. Like skin cells, these epidermoid cells also desquamate but into a closed cavity. Slowly as the keratin, cholesterol and desquamated cells accumulate, the lesion grows and becomes symptomatic either due to the mass or rupture. [1],[8],[9] In general, there is no gender preponderance, [1],[10] but in our series the male-to-female ratio was 3:2. Epidermoids are mostly cerebellopontine angle in location followed by the suprasellar cistern, the other sites being the Sylvian fissure, brainstem, pineal region, petrous apex, intra-fourth ventricular. [1],[5],[7],[10],[11] Interhemispheric location is rare, and only 19 cases (including one reported from our institute previously) have been reported. [2],[12] As these lesions are slow growing and "soft," they tend to mould according to the surrounding structures and seep through the cisternal spaces, encasing rather than displacing the nerves and vessels. These lesions become symptomatic due to either the pressure effects on the surrounding neural elements or irritation of the nerves or cortex, presenting with ataxia, nystagmus, hemiparesis, hydrocephalus, neuralgia or seizures. Rupture of these cysts spontaneously or spillage of contents during surgery can cause aseptic chemical meningitis. [7],[11],[13] In our series, 80% of patients presented with seizures of long-duration due to cortical irritation; 33% with hemiparesis due to mass effect, and 20% with features of raised intracranial pressure. Radiologically, epidermoids may be seen as extra-axial lesions in the basal cisterns growing along the CSF spaces encasing vessels and nerves and causing a disproportionately less distortion of the surrounding brain. On CT scan, the lesions appear as hypodense lesions with attenuation similar to or lower than that of CSF. Occasional calcification of the saponified fat contents may be seen as hyperdensities in about 10 - 25% of the cases. There is usually no enhancement on contrast. [7],[14],[15],[16] Four of our cases showed calcification (27%), and none showed any contrast enhancement. In the interhemispheric region, the differential diagnosis include a low-grade or a nonenhancing high-grade glioma.With the advent of MRI and the newer sequences, preoperative diagnosis of an epidermoid can be made in almost all cases. On T1-weighted images, epidermoid appears hypointense to the gray matter and slightly hyperintense to CSF. On T2-weighted images, the lesion is hyperintense to gray matter and similar to that of CSF. However, heterogeneity of the signals is seen commonly. [17],[18],[19] Diffusion-weighted images show restricted diffusion making the lesion hyperintense. CSF and arachnoid cysts appear hypointense as there is no restricted diffusion. [20] Fluid-attenuated inversion recovery (FLAIR) images and constructive interference in steady state (CISS) 3-D images are also useful in diagnosing this condition. In the former sequence, the epidermoid appears heterogenous and hyperintense to CSF; while in the latter, the lesion appears heterogenous with hypointense and hyperintense areas. The extent of the lesion and its relation to the neurovascular structures can also be seen clearly. [21],[22] As in CT, there is usually no enhancement on administration of gadolinium contrast. The radiological features were similar in our series. The interhemispheric location of the lesion and its proximity to the anterior cerebral arteries, its branches, motor cortex and supplementary motor cortex make surgical excision technically challenging. Preservation of the bridging veins must be given utmost priority. Despite this, safe radical excision can be achieved in most of these cases. In 3 patienst, we had left a small residue due to its attachment to the anterior cerebral artery branches and surrounding cortex. In 2 patients, in order to preserve the bridging veins, a transcortical approach was used. Chemical meningitis is a well-known complication after epidermoid surgery. [7],[13] Up to 40% incidence of chemical meningitis following surgery has been reported. [11] In our series, none of the patients had chemical meningitis. Intraoperative hydrocortisone irrigation, postoperative steroids, and safe radical excision might have helped in preventing this complication. [10],[11] Since there is not much CSF in interhemispheric region, and there is no free communication with the CSF of the basal cisterns, intraoperative spread of the contents of epidermoid is less, hence incidence of chemical meningitis is also minimal. These slow-growing epidermoids are known to recur during the long-term, especially when partially removed. However, as the risk of recurrence is low for small residues, it is better to try to achieve total excision safely and when not possible leave behind small residues attached to vital structures. [7],[13],[23] As these lesions are congenital, the time to symptomatic recurrence following total excision may be estimated as, age at the time of diagnosis plus 9 months. It has been stated that due to the above reasons the chance of symptomatic recurrence during one's lifetime following total excision may be negligible or practically absent. [7],[8],[13],[24] However, an overall recurrence rate of 24% has been quoted in older studies. [24] On long-term follow-up, there were 3 recurrences in our study, though the surgical impression was near-total or total excision in these patients. This highlights the need for a long-term follow-up of these patients. On follow-up, all our patients improved symptomatically, and the neurologic deficits recovered. However, seizures still continued to occur in majority with reduced frequency.
[Figure 1], [Figure 2], [Figure 3]
[Table 1]
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