Multiple Unilateral Skull Base Defects in a Child With Conductive Hearing Loss
A computed tomography (CT) of the temporal bone demonstrated right-sided enlargement of the foramen ovale, fallopian canal, internal auditory canal (IAC), and an atypical cystic structure passing through the subarcuate tract (Fig. 1). There was no dehiscence of the carotid artery. The differential diagnosis of these expanded skull base structures is schwannoma, neuroma, venous malformation/hemangioma, meningioma, encephalocele, arachnoid cyst, and congenital malformation. A magnetic resonance imaging (MRI) with and without contrast was performed to better characterize the soft tissue signal within these enlarged structures. The MRI showed an encephalocele at the foramen ovale extending into the infratemporal fossa (Fig. 2, A and B). The expanded fallopian canal and subarcuate cyst both had a T2 signal characteristic of cerebrospinal fluid (CSF) (Fig. 2C). Additionally, the right cerebellar flocculus protruded into the IAC causing displacement of the seventh and eight cranial nerves. There were no skull base defects on the left. Bilateral optic disc elevation was also noted on the MRI and confirmed with 3+ disc edema on fundoscopic examination.
Herniation of dura mater (meningocele) or brain tissue (encephalocele) is an uncommon disorder with protean manifestations depending on the site involved. Temporal lobe encephaloceles with or without CSF leakage may cause CHL and a serous middle ear effusion. Persistent otorrhea following tympanostomy tube placement or a history of meningitis should raise suspicion of a skull base defect (1). Other associations include pulsatile tinnitus, seizures, chronic headache, rhinorrhea, and optic disc edema secondary to intracranial hypertension (Fig. 3).
Most lateral skull base encephaloceles are acquired (traumatic, chronic infection, cholesteatoma, iatrogenic) but they may also be congenital or idiopathic. Congenital encephaloceles are much less common occurring in only 1 to 5 cases per 10,000 live births and often present as an isolated defect in the posterior fossa (2).
Congenital defects through the fallopian and subarcuate canals are especially rare. Isolated defects have previously been reported but the combination of separate fallopian canal and subarcuate canal defects has not been described (3–5). The additional finding of an ipsilateral foramen ovale defect suggests some sort of developmental anomaly, possibly exacerbated by elevated intracranial pressure.
CSF is bounded by the subarachnoid space. A review of 162 adult temporal bones identified subarachnoid space extension along the fallopian canal into the perigeniculate region in 8% and the horizontal segment in 4%. Gacek theorized that increased intracranial pressure, either as an adult or in utero, displaces the lateral limit of the subarachnoid space causing CSF within the fallopian canal (4). Surgical approaches to the management of CSF leaks through the fallopian canal have been described but, carry the risk of injury to the facial nerve (4,5). Currently, this patient has no evidence of CSF leakage. Her family has been counseled on the risks of meningitis and a workup for idiopathic intracranial hypertension is underway. The CHL is being managed with the use of a hearing aid.