|Discussion: In the CP angle, schwannoma, hemangioma, and meningioma should be considered in the differential diagnosis, but unlike our case, these usually enhance intensely on postcontrast MR imaging. Epidermoid tumor also may be located in the CP angle.Magnetic resonance is the imaging modality of choice for lesions of the CP angle and internal auditory canal. Lesions of the CP angles usually are divided into those native to the angle (vestibular schwannoma, meningioma, epidermoid, arachnoid cyst, metastases, lipoma, etc.) and those extending to the angle from adjacent structures (gliomas, ependymomas, choroid plexus papillomas, vascular malformations). Epidermoids, or less properly "epidermoid cysts" and dermoids are benign congenital tumours of developmental origin. They originate from ectopic ectodermic cells; the most frequent sites are the subarachnoid space at the level of the clivus, the CP angle, the perimesencephalic cistern. Rarely epidermoids and dermoids may be intraventricular. Vestibular schwannomas are by far the most important lesion of the CP angle. Recent literature suggests that epidermoid tumors have mostly been found at the cerebellopontine angle (37.3%) and in the parasellar region (30%), growing by spreading in the subarachnoid space of the basal cisterns. They rarely develop into malignant processes.
Symptoms associated with epidermoids usually appear after the second decade of life. These symptoms are caused by displacement of adjacent vascular and neural structures. The most frequent presenting symptom is trigeminal neuralgia. Other symptoms that can occur are similar to those caused by acoustic neuroma like vertigo, asymmetric hearing loss, and unilateral tinnitus. If the lesion gets very large, symptoms like headache, nausea, vomiting, diplopia, and ataxia, or symptoms of increased intracranial pressure and hydrocephalus can develop
Epidermoids have a pearly appearance and are well circumscribed and encapsulated. The epidermoids that are hypodense on CT may contain a soft, waxy material that consists of cholesterol crystals. The hyperintense epidermoids on the other hand are mostly cystic and contain fluid of various colors. Other sources suggest that the inner material is due to the progressive desquamation and breakdown of keratin.On imaging, most epidermoid tumors show a distinctive MRI appearance. They are usually hypointense on T1 MRI and hyperintense on T2 MRI. On T1, they can also show a characteristically marbled inner pattern. Diffussion-weighted imaging can provide additional information by showing a clear, hyperintense signal. On CT, an epidermoid can appear as a well-defined lobulated hypodense mass.
Epidermoid tumor is the third most common cerebellopontine angle internal auditory canal mass after vestibular schwannoma and meningiomathe main differential diagnostic consideration is an arachnoid cyst. The distinction is usually made with FLAIR and DW imaging. Arachnoid cysts follow the signal intensity patterns of CSF with all MR pulse sequences, whereas epidermoids are not hypointense on FLAIR images and display areas of hyperintense signal relative to CSF. On DW images, epidermoids typically show restricted diffusion, unlike arachnoid cysts. Finally, although most epidermoids do not enhance, up to 25% may show minimal rim enhancement .