Mastoid Air Cell Hyperplasia Associated with Middle Fossa Arachnoid Cyst: Case Illustration

Intracranial arachnoid cysts are known to induce chronic changes in the adjacent skull. We report the case of a 2 year old girl referred to neurosurgery for a 4.5 × 5 x 5 cm middle fossa arachnoid cyst discovered during evaluation for precocious puberty. The patient was neurologically normal and did not demonstrate any interval change on imaging over the following 2 years. The patient was not seen again until age 16, when she presented to the Emergency Department with a 3 day history of sore throat, headache, and intermittent blurry vision. Neurologic and ophthalmologic examinations were normal. New brain imaging showed continued presence of the middle fossa arachnoid cyst along with significant interval enlargement of the adjacent mastoid air cells (Figs. 1 and 2). The patient responded symptomatically to migraine treatment. Pneumosinus dilatans (PD) is a rare condition in which any of the paranasal (frontal, ethmoid, sphenoid, and maxillary) sinuses undergo abnormal expansion and aeration, and can cause facial deformity. PD should be distinguished from two other similar conditions, hypersinus and pneumocele, in which the frontal sinus is also enlarged. A 2014 review found a total of 123 reported cases of PD in the literature, of which 19 cases (16%) involved 2 or more sinuses. At least one other concurrent pathology was identified in 83% of patients, including visual loss, exophthalmos, meningioma, or arachnoid cyst. Pneumosinus dilatans multiplex (PDM) is a far more rare entity which involves mastoid air cells in addition to multiple paranasal sinuses. Occurrence of PD or PDM in the presence of an intracranial arachnoid cyst has been reported as early as 1949 at autopsy and 1976 on CT imaging. A case report from 2001 identified 17 prior patients reported with these findings, although only 2 patients were described as demonstrating expansion of the mastoid air cells. Multiple theories regarding a possible association between these two entities, including various types of ex-vacuo response of the skull to absence of adjacent brain parenchyma, have been proposed but not validated. Intracranial arachnoid cysts may sometimes be associated with complex skull abnormalities of uncertain etiology. Progressive unilateral hyperplasia of mastoid air cells in isolation, adjacent to an intracranial arachnoid cyst, has not previously been reported, and may represent a distinct variant of the PD spectrum. Clear understanding of each patient’s overlying bony anatomy should be fully defined prior to any attempted surgical approach to an intracranial arachnoid cyst.

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