Discussion Author(s): Ioannis V.Papageorgiou - Vasilios P. Papakostas
Epidermoid cysts are benign congenital lesions of ectodermal origin. They have also been referred to in the literature as epidermoid tumors, epidermoid lesions, primary or congenital cholesteatoma and pearly tumor. They are believed to form between the 3rd and 5th week of embryonic development as a result of displaced epithelial remnants that remain after the neural tube closes. Although acquired epidermoid tumors may develop as a result of trauma, this is uncommon in the brain.
Epidermoids cysts are composed of an outer capsule of connective tissue that surrounds a layer of keratinized stratified squamous epithelium and inner cystic fluid that usually includes debris, keratin, water, and cholesterol. As the epithelial layer desquamates, the cells accumulate and form a cholesterol-rich layer that gives the tumor its characteristic pearly white appearance.
Most of the epidermoid tumors (90%) occur intradurally in the basal subarachnoid cisterns. Approximately 10% are extradural, located in the skull or spine. About half of intracranial epidermoid cysts (40-50%) are located in the cerebellopontine angle, where they represent 5% of overall lesions and the third most common mass behind vestibular schwannomas and meningiomas. Other common location is the sellar and parasellar region and the fourth ventricle. Less common locations include intraparenchymal locations (1.5%), the pineal gland, the thalamus, the septum pellucidum, the brainstem, and rarely in the lateral ventricles. Intracranial epidermoid cysts constitute 0.2%â€“1.8% of all primary intracranial neoplasms. There is no racial or sexual predilection. They are overwhelmingly benign, although they rarely can be malignant.
Because of the relative softness of epidermoid cysts and their tendency to include rather than displace adjacent structures, clinical symptoms occur only when the masses are large. The patients are usually asymptomatic until they are aged 20-60 years with peak at 40.
Intracranial epidermoid cyst is associated with the presence of occipital/ nasofrontal dermal sinus tract.
It is characterized by an insinuating growth pattern and lobulated excrescences. The outer surface of the cyst has shiny, glistening "mother of pearl" appearance. Cyst contents include keratinaceous debris and crystalline cholesterol. The cyst wall is composed by a layer of stratified cuboidal squamous epithelium covered by fibrous capsule.
Presenting symptoms may include headaches, visual problems, cranial nerve symptoms and seizures.
On CT scans, epidermoids usually have the same attenuation as that of CSF; this characteristic makes their differentiation from arachnoid cysts difficult. Enhancement is rare. Calcification occurs in only 15-20% of cases.
MR imaging is the best modality for evaluating these tumors. Characteristically, they are either isointense or slightly hyperintense relative to CSF on T1 and T2-weighted images. In many cases, especially for tumors in the cerebellopontine angle cistern or the sellar-parasellar region, the 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 completely 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. DWI is also crucial in the postoperative follow-up as it allows confirmation of the presence of a possible residual tumour. Finally, MR spectroscopy is also helpful as it shows only elevated lactate peaks in these tumours, which can be of interest in case of atypical epidermoid cysts.
The diagnosis of intracranial epidermoid cyst is confirmed by MR Imaging.
As with all tumors of the central nervous system, treatment decisions vary greatly depending on the patient and the specifics of each case. However, generally, epidermoid cysts that are causing symptoms are generally recommended to be treated. Treatment usually consists of a surgical resection of the tumor. The goal is total removal of the tumor but due to their cystic nature, they can be difficult to remove in their entirety, particularly if parts of the cyst wall are adherent to important structures such as nerves or the brainstem. While these are generally benign tumors, they can continue to enlarge slowly and recur after surgery if any of the cyst wall was not removed.
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