The correct answer is (b): acoustic neuroma

The MRI reveals a fairly homogeneous slightly hyperintense solid lesion in the right CPA that exhibits a rounded contour medially and pointed contour laterally, where the lesion, in fact, extends into the internal auditory meatus (porous acousticus). There's minimal local mass effect and no associ-ated aggressive features such as adjacent edema, bone destruction or regional infiltration. By presenta-tion and the MR appearance, this lesion most likely represents an acoustic neuroma.

Acoustic neuromas are benign primary central nervous system neoplasms, which aren’t un-common, comprising 8-10% of all intracranial tumours in adults. The term most frequently associated with this lesion, ‘acoustic neuroma,’ is really a misnomer. The 8th cranial nerve consists of two sepa-rate nerve divisions: acoustic (cochlear) and vestibular. In fact, this neoplasm most often arises from the Schwann (sheath) cells of the vestibular portion of the 8th cranial nerve and as such the more cor-rect term would be ‘vestibular schwannoma.’

The most frequent associated symptom is asymmetric SNHL (as with the patient presented). Less common presenting symptoms may include: tinnitus (ringing or hissing in the ears), dizziness and vertigo (with nausea and vomiting). Rarely, if the tumour grows large enough, symptoms of local mass effect on adjacent structures may result in ipsilateral facial weakness (including Bell’s palsy) from pressure on the adjacent 7th cranial nerve, as well as trigeminal neuralgia (5th cranial nerve), and oc-casionally generalized headache and altered consciousness.

Most acoustic neuromas occur sporadically, but these lesions are also known to be closely as-sociated with both neurofibromatosis Type I (von Recklinghausen disease) and Type II. In Type II cases, the typical presentation is actually bilateral acoustic neuromas.

The differential diagnosis of a lesion at the cerebellopontine angle is broad but usually includes meningioma (the most common solid lesion of the CPA after acoustic neuroma), and arachnoid cyst (which wouldn't fit in this case as this lesion is a fluid density mass, not solid). Identification of intra-tumoral microhemorrhage within the lesion on T2-weighted gradient echo MR images heavily favours the diagnosis of acoustic neuroma over meningioma. The CPA is an atypical location for a thrombosed aneurysm. Choroid plexus papillomas develop within the intracranial ventricles where the choroid plexus resides, ruling out that diagnosis.

References:

1. Thamburaj K, Radhakrishnan VV, Thomas B. Am J Neuroradiol 2008;29(3):552-7.
2. Obholzer RJ, Rea PA, Harcourt JP. J Laryngol Otol 2004;118(5): 329-32.
3. Bonneville F, Savatovsky J, Chiras J. Eur Radiol 2007;17(11):2908-20.