Cruvellier in 1829 originally described these lesions as “pearly tumor” due to significant sheen and whitish appearance [1, 4]. Macroscopically, they consist of circumscribed, smooth, and irregular surface (described as “cauliflower shaped” by some authors) sometimes with foci of calcifications in the capsule, containing concentric lamellae of soft, white, and waxy flakes [1, 3, 4, 12]. The content is rich in cholesterol and results from progressive desquamation of epithelial lining of the cyst. Thick viscous and brown content secondary to breakdown products of keratin similar to that in dermoids has also been described [1, 4].

The main microscopic difference from dermoids is the lack of skin appendages (hair follicles, sebaceous and sweat glands). The cyst wall consists of simple stratified squamous epithelium, supported by an outer layer of collagenous tissue [4]. The wall of the epidermoids is thinner than that of dermoids and usually do not cause as much reactive fibrosis as the wall of dermoids do [1].

The growth rate of these lesions is said to be linear and similar to normal skin unlike the exponential growth of neoplastic tissues [4, 21], although malignant transformation has rarely been reported for epidermoid as well as for dermoid cyst [22, 23]. The germinal cell layer is spread out over the outer surface giving rise to vertically or radially orientated mitoses. Epidermoids grow slowly as dry keratin cells accumulate in the avascular center of the tumor [21]. Alvord suggests that due to increased pressure from within the tumor, the outer basal cells divide in tangential direction giving rise to increase basal cell layer and consequently the surface area. The theoretical surgical implication of the germinal cell layer being on the surface is that the capsule needs to be completely removed to minimize the risk of recurrence, although this must be weighed against risk of neurovascular and cranial nerve injury. Epidermoids tend to have a soft malleable consistency which allows them to extend between cranial nerves and vessels engulfing these structures [13, 17, 24, 25]. The expanding tumor conforms to the shape of the cavity. It occupies and does not displace the neurovascular structures until all the available space is filled [13]. The delay in onset of symptoms has been attributed to this characteristic of these lesions.

The embryogenesis of epidermoid lesions is generally agreed to be similar to that of dermoids, i.e., arising from inclusion of ectodermal elements at the time of neural tube formation between the third and fifth week of embryological life [1, 4, 26]. However, epidermoids, in particular intracranial ones, are thought to occur at a later stage of development, namely, during formation of secondary cerebral vesicles (optic and otic vesicles) at the fourth and fifth week, as they lay in a more lateral location and are differentially unipotent cells [1, 4].

Kaido et al. [27] reporting on intraparenchymal epidermoids put forward a hypothesis on development of these lesions explaining their locations. According to this theory, if sequestration of ectodermal elements occurs early during formation of primary central vesicles in the third week of gestation, the ectodermal cells are more likely to be “trapped” within the neural tube giving rise to intraparenchymal or intraventricular epidermoids, while sequestration later during the fifth week of gestation and the development of secondary cerebral vesicles (otic and optic) will lead to lesions located in CP angle, middle ear, and orbit.

The signs and symptoms caused by epidermoid tumors in general relate to their location, mass effect, or chemical meningitis. The tumor location is the key feature, and with slow growth recorded in adult patients, they are typically reported to cause symptoms for several years before detection [1, 11, 13, 16, 17]. With posterior fossa location in particular CP angle, clinical symptoms typically include cranial nerve abnormalities, such as facial palsy, hearing loss, vertigo, trigeminal neuralgia, as well as ataxia, hemiparesis, and hemianesthesia [1, 4, 13, 17, 28]. Particularly for cranial nerves VII, VIII, VI, and V, clinical signs have been reported [4, 5, 11]. Nystagmus and ataxia secondary to cerebellar compression have also been commonly observed [13, 29]. Chu et al. [28] have reported on cases of CPA epidermoids with audiovestibular deficits due to eighth nerve compression which returned to normal after surgery.

Raised intracranial pressure signs and symptoms including nausea, vomiting, headache, and transitory loss of consciousness have been reported with intraventricular or posteriorly located tumors within the infratentorial space [17, 24] causing CSF flow obstruction.

Rupture of epidermoids can result in granulomatous meningitis, which could lead to communicating hydrocephalus if repeat episodes do occur [1, 4], while the presence of dermal sinus can be a portal entry for bacterial meningitis [4].

Epidermoids usually are characterized by subtle and prolonged symptoms prior to diagnosis, and acute presentation (less than 1 week) is rare and associated with hydrocephalus and leakage of cyst contents into subarachnoid space causing chemical meningitis. Other acute presentations include bacterial meningitis, tension pneumocephalus, hemorrhage, and traumatic rupture [14, 30, 32]. Traumatic rupture of epidermoid cyst has also been reported with patient presenting few hours after cranial trauma with hemiparesis [30].

An interesting phenomenon reported in fourth ventricular lesions is the lack of correlation of ventricular enlargement and significant dimensions of the tumor [6]. Furthermore, patients have also been reported to have spontaneous improvement of their symptoms leading to the wrong diagnosis in some cases [6, 24]. The mechanism involved has been suggested to be either due to small leakages and subsequent tumor decompression or slow growth of the tumor and extension along the subarachnoid spaces without significant compression [6, 24]. The overall incidence of hydrocephalus has been reported to be less than 50 % with intraventricular lesions [24].

Bartal et al. [24] analyzed the clinical presentation of patients with posterior fossa epidermoids during CT era and divided them to two clinically distinguishable groups: those posteriorly located, which presented predominantly with the symptoms and signs of raised intracranial pressure, and those with anterior lesions which presented with cranial nerve deficit without features of raised intracranial pressure.

Association with other congenital abnormalities including nasal dermal sinus and congenital defects in the vertebral column has been reported [4, 30].

CT scan is the common starting point for diagnosis of intracranial lesions. However, the appearance and location of epidermoid tumors can be similar to several other lesions including arachnoid cyst, meningioma, dermoid tumor, and vestibular schwannoma. Epidermoids typically appear as low-density extra-axial lesion due to the fat content and therefore have similar attenuation to dermoids and CSF in arachnoid cysts. On occasions, the higher keratin content of the epidermoids can appear as hyperdensity on CT [1], in which case it may carry a differential diagnosis of meningioma or vestibular schwannoma. Li et al. [31] reported on 15 cases where hyperdense lesions were seen on CT. The incidence of such lesions was reported in 3 % of their cases over 20 years (15/596). It was hypothesized that recurrent leakage of the content with subsequent chemical inflammatory response may be responsible for this appearance [31]. Other causes of hyperdense appearance on CT imaging are thought to be intracystic hemorrhage [19]. Enhancement with contrast on CT scans is rare and occurs at the margins [1, 6]. Bone absorption surrounding the epidermoid cyst can occasionally be a clue to the diagnosis [1].

Magnetic resonance imaging (MRI) is the investigation of choice for diagnosis of epidermoid cysts allowing the differential diagnosis with dermoids, arachnoid cysts, lipomas, and cholesterol granulomas [33, 34]. MRI is especially useful in assessing the extent of the lesion and relationship to the surrounding anatomy using the images in various planes [35]. Generally, they have low intensity on T1W images and high intensity on T2W images (Fig. 53.1). This appearance may be very similar to that of CSF, and therefore the diffusion-weighted imaging (DWI) technique is the ideal sequence for discriminating between these two entities, where the epidermoid cysts appear hyperintense [1]. DWI is also useful in distinguishing between epidermoids and encephalomalacia [1]. Signal intensity varies depending on the relative lipid, cholesterol, and keratin composition [35]. They generally do not show contrast enhancement [35]. In postoperative cases, cholesterol crystals are clues for capsule remnant [7]. Proton density (PD)-weighted images are also helpful in differential diagnosis, where structural heterogeneity of the lesion can be a distinguishing feature from brain tissue and CSF [17].

The first line of treatment of epidermoid tumors is surgical resection. Ideally, complete excision should be the aim of the surgery, as they carry risk of recurrence and do not respond to adjuvant therapy such as chemotherapy or radiation [4, 5, 11, 13]. However, it should be emphasized that when a risk of neurological deficit due to surgery exists, a conservative approach is more appropriate, as these lesions are histologically benign and very slow growing [5, 6]. Furthermore, even a subtotal resection alleviates patient’s symptoms, allowing normal life for many years.

Standard surgical approaches have been used for removal of these lesions. The principle approach undertaken is to firstly access the cistern where the lesion arises. Depending on the size of the tumor and location, this is then followed by evacuation or removal of the contents carefully avoiding spillage, and then attempt to excise the cyst wall by tracing the wall along various compartments. Epidermoid tumors are less likely to have dense reactive involvement of the arachnoid membranes compared to dermoids and have a thinner wall [13].

Yasargil et al. [13] in their series of 43 cases approached all parasellar lesions from a pterional route, while the posterior fossa tumors were accessed via midline suboccipital route for fourth ventricle or centrally located tumors and through a retromastoid approach for masses in CP angle. Following the conduit created by the tumor and creating a plane between the capsule and the overlying arachnoid, Yasargil and colleagues [13] reported complete removal of the lesions in one-stage operations. Another important observation in this series was the fact that these lesions did not cross the midline to the contralateral subarachnoid spaces, although they may bulge across the midline [13]. This was thought to be due to possible fenestration existing between some cisterns, while others have no such communication.