Concept and location
Epidermoid and dermoid cysts (EC/DC) represent together the epithelial inclusion cysts.
These cysts account for a majority of cutaneous nodules found in children.
They may be present at birth or appear anytime in childhood.
EC/DC can appear on almost all skin surface,
but are more commonly located in the head and neck.
EC often occur on the forehead,
lateral corner of the eyebrow (which is the most frequent location),
anterior fontanelle or postauricular and periorbitary spaces. DC are typically found in the midline,
usually in the anterior fontanelle and occipital squama,
and may be associated with congenital malformations.
If they are located near the skull,
they may erode the bone surface and in the extremely rare situation may even penetrate into the brain.
Clinical manifestation
EC/DC are usually small,
mobile and slow-growing lumps located in the dermal or subcutaneous tissue.
Cysts may remain small for years,
or progressively increase in size due to descamation and accumulation of sebum,
varying from a few millimeters to several centimeters.
A giant cyst is one more than 5 cm in diameter.
Multilocular cysts also occur.
Inclusion cysts are often asymptomatic,
although may present acutely with infection or rupture,
becoming red and painful.
Spontaneous rupture of the wall results in a discharge of the content into the dermis,
ensuing in a granulomatous inflammatory response.
The sterile material either points and drains through the surface or is slowly reabsorbed.
Fibrosis may occur subsequently and if the wall is destroyed during the inflammatory process the cyst will not recur.
True secondary infection is quite rare,
more commonly due to repeated local trauma or in association with acne vulgaris.
Malignant degeneration (squamous cell tumour) is rarely reported.
Pathology and histology
EC/DC are true cysts with a wall of stratified squamous epithelium,
which produces keratohyaline granules.
These cysts commonly develop from the occlusion of pilosebaceous follicles,
although they can arise from the epithelium of any adnexal structure.
They may also grow from epidermal implants located in the dermis,
that remain there after embryologic development or are secondary to an injury that penetrates the epidermis.
The cysts may communicate with the surface through a narrow channel,
and the surface opening appears as a small,
round,
sometimes imperceptible,
keratin-filled orifice (central punctum or blackhead),
through which keratinous,
foul-smelling debris may be extruded.
They can only be ultimately differentiated by histology.
EC contain white,
cheesy keratin debris,
which is composed by epithelial elements and cholesterol crystals,
and may be bounded by a thin capsule.
DC typically contains thick,
foul-smelling yellow material in which accessory glandular structures,
such as sebaceous glands,
hair follicles and connective tissues can be found,
and are surrounded by a fibrous capsule.
Associated pathologies
Gardner’s syndrome should be suspected in children who present EC in unusual areas,
such as the legs,
during early childhood.
This autosomal dominant pathology is the extracolonic manifestation of a familial adenomatous polyposis (FAP),
a colorectal cancer syndrome characterized by hundreds of adenomas involving the large bowel,
with 50% risk of malignant degeneration (if untreated) by the fourth decade.
It is a condition characterized by multiple cutaneous cysts,
being EC the most common.
EC can occur in any location,
although they tend to occur most often on the face,
scalp,
and extremities.
Other cutaneous findings include multiple pilomatrixomas,
desmoid tumors,
osteomas,
fibromas,
lipomas,
abnormal dentition,
thyroid tumors,
and congenital hypertrophy of the retinal pigment epithelium (CHRPE).
Both FAP and Gardner syndrome have been mapped to the adenomatous polyposis coli (APC) gene,
a tumor-suppressor gene.
Members of affected families may now be screened for the genetic marker.
Milia represent miniature ECs that range from 1 to 3 mm in diameter,
tend to be multiple and are white in color.
Although they occur commonly sporadically in healthy newborns,
they may be acquired after acute and chronic cutaneous injury,
such as abrasions trauma,
surgery,
and recurrent blistering in epidermolysis bullosa or porphyria.
Although milia often resolve without treatment,
some remain indefinitely.
Curettage or gentle puncture and expression with a comedone extractor or 22-gauge needle is usually curative.
Differential diagnosis
EC/DC differential diagnosis includes pilar cyst of the scalp and face,
which is the second most common cyst occurring in the head and neck.
It is comprised of keratin and keratin breakdown products,
and the cyst is lined by cells resembling the outer root sheath of hair.
Approximately 2% of the pilar cysts have foci of proliferating cells that can develop into tumors called trichilemmal cysts.
Steatocystoma simplex may be confused with EC,
although steatocystomas can be distinguished by the yellow,
oily fluid that they contain.
Other tumors in the skin,
including pilomatrixoma,
lipomas and vellus hair cysts,
may be confused clinically with EC.
Midline DC can be confused with midline thyroglossal duct cysts.
However,
DC are more superficial and are usually mobile in the soft tissues,
but do not move cephalad when the tongue protrudes as they lack a connecting tract to the hyoid bone.
Fine needle aspirate may also be helpful in distinguishing an infected thyroglossal duct from a ruptured dermoid.
Treatment
Excision is the treatment of choice for all dermoid and epidermoid cysts,
especially those which are symptomatic,
enlarging,
or have ruptured.
It is important to completely remove the capsule and avoid intraoperative rupture to decrease the risk of recurrence.
Complete surgical removal is curative.
Inflamed lesions may be settled down first with intralesional injections of corticosteroids and oral antibiotics before surgery,
however,
this practice is unnecessary in most cases.