Embryologically dermoid tumors are thought to result from inclusion of ectodermal elements at the time of the closure of the neural groove during the third to fifth embryonic life [1, 5, 10, 17, 18]. This is the same as epidermoid as opposed to teratomas where mesoderm also is involved.

In terms of time profile for development of dermoid tumors in relation to epidermoids, the current theory suggests that these lesions occur earlier in embryonic life than epidermoid tumors. Therefore, displaced cells lie near midsagittal plane and as sometimes noted are associated with defect in closure of overlying skin [5, 10]. This is in contrast to epidermoids which most often occupy a more lateral position, in particular in intracranial location. This is postulated to result either from inclusions of ectoderm at a later stage of embryogenesis or displacement during formation of secondary cerebral vesicles, i.e., the otic and optic vesicles [1]. On the other hand, lateral displacement of the primitive ectodermal and mesenchymal cells caused by the developing cerebral vasculature along Virchow-Robin spaces gives rise to laterally positioned dermoids [2, 6, 12].

The congenital dermal sinus that often is found in association with dermoids suggested to represent abnormal adhesion between the skin and neural ectoderm during development [9, 15] (Fig. 52.1a). This tract, depending on its degree of incomplete separation, may end up in subcutaneous tissue, bone, dura, subdural space, or extend at any length into the parenchyma and ultimately within the fourth ventricle. The dermal sinuses can be blind or connect to dermoid [15] (Fig. 52.1c). Rarely there may be more than one dermal sinus present [15].

In recent years, concurrent finding of a rare phenomenon of vertebral anomalies, namely, Klippel-Feil syndrome, with posterior fossa dermoids, has led some authors to propose theories on common embryological process, which suggests dermoid tumors being a late embryonic developmental anomaly [19–21]. It is proposed that this may be related to under-expression of pax gene that leads to segmentation failure of cervical somites that alters tissue tension at craniocervical junction leading to entrapment of dermal elements and development of posterior fossa dermoids [20, 21]. On the other hand, some have suggested that segmentation failure affects cervical flexure formation which in turn reduces fetal movements at rhombencephalo-cervical junction ultimately trapping ectodermal elements and leading to formation of dermoid tumors [19]. These theories remain unproven.

Other rare concurrent anomalies such as Dandy-Walker syndrome, callosal agenesis, and diastematobulbia in association with posterior fossa dermoid cyst have also been reported, but the significance of these anomalies remains to be explained [2, 22].

Most pediatric intracranial dermoids arise in the posterior fossa, in midline position, i.e., cerebellar vermis and adjacent meninges being more common locations [1, 2, 18]. These tumors are also found in the cavity of the fourth ventricle [2, 13, 18], although this is not the most common location within the posterior fossa. Of the 16 pediatric dermoids in one study, 8 (50 %) were in the posterior fossa with the cisterna magna as the most common location [2] (see Table 52.1). Intra-axial location, cerebellum or brainstem, is exceptionally rare [2, 23, 24]. Intracranial extradural dermoids are also reported as rare [6]. The significance of this location becomes apparent in the review by Martinez-Lage et al. [6] where four out of nine reported cases were closely related to venous sinuses with three intraoperative hemorrhages. The other favored intracranial location is in the parasellar region [10]. Interestingly in contrast to teratomas, dermoids rarely are reported in the pineal region [10].

Grossly well-circumscribed, opaque, oval, or round multi-lobulated masses are generally well demarcated from surrounding tissues. The wall is formed by a fibrous material that varies in thickness and degree of adherence to the surrounding structures [1, 10]. Often there is a layer of reactive gliosis in the adjacent brain tissue. Calcification may be seen in the wall of the cyst [10]. The material within the cyst can vary between a dense (sometimes described as “cheesy”) substance and a more brown, mucoid consistency fluid. The presence of hair is common within the tumor [5]. Pus-like fluid due to sebaceous secretions and desquamation, to a more yellowish-brown mucoid fluid, has also been described. Teeth are rarely seen. Some investigators have reported more than one cyst in the same location [10]. Dermoid tumors grow faster than epidermoids [9]. This is due to secretions as well as desquamation process. This may also explain their earlier presentation in life.

The presence of dermal sinus in the occipital region is an important clue to the underlying dermoid cyst and considered by some a diagnostic feature [10]. The importance of the dermal sinus lies in the fact that the open connection between the skin surface and the cyst is a direct route for bacterial infection and thus the source for recurrent meningitis or abscess formation. Most tumors are associated with a complete or incomplete dermal sinus; however, the IV ventricle tumors may not have an associated sinus [2, 18]. In McComb’s experience, the lesions involving the region of the IV ventricle have all been dermoids and all been associated with dermal sinus [9]. In another study, 89 % of dermal sinuses are associated with an inclusion tumor and were mostly dermoid tumors, with 18 % only found extradurally, and the rest were intradural [25]. The congenital dermal sinuses associated with dermoid tumor have both dermal and epidermal elements, and it can sometimes regress to a connective tissue band or a nodule. There may be enlargements along the tract forming other dermoid cysts [9]. The opening of the tract is usually very small and difficult to see without shaving the hair on occipital region [18].

Microscopically, the wall of the dermoid cyst (or the capsule of the tumor) is formed by stratified squamous epithelium as seen in the skin and includes hair follicles and sebaceous and sweat glands [1] (Figs. 52.2 and 52.3). Much of the lining of the cyst may be simple squamous epithelium supported by collagen, similar to epidermoid tumors, while in the thicker parts it is supported by dermis and its appendages [10, 17]. It is important to emphasize that dermoid cysts do not contain fat cells per se as this is of mesodermal origin, and the lipid metabolites seen are the result of breakdown products of hair and glandular secretions [26].

Symptoms from dermoid tumors generally result from mass effect, infection, inflammation, or occasionally hydrocephalus as presenting symptom [9, 15, 27–29]. Recurrent hyperpyrexia without meningism was the presenting symptom in 4 out of 8 pediatric posterior fossa dermoids, while only one presented with aseptic meningitis [2]. Another common finding in pediatric age is localized swelling secondary to inflammation and infection which may or may not be accompanied by discharge from the dermal sinus. These symptoms are often reported for one or more episodes before patient is brought to attention [2, 9].