Orbital dermoid is a congenital benign cystic teratoma (choristoma) that originates from aberrant ectodermal tissue. In these tumours, there is embryonic displacement of epidermis to a subcutaneous location along embryonic lines of closure. As two suture lines of skull close during embryonic development, epidermal or dermal elements may pinch off and form cysts. Orbital dermoids are not attached to the skin.
Orbital dermoid may appear as:
Dermoid cyst: Dermoid cysts have a fibrous wall and are lined by keratinised stratified squamous epithelium and contain dermal appendages such as sweat glands, hair follicles and sebaceous glands. Dermoid cyst may be:-
- Superficial dermoid cyst: Superficial dermoid cysts are located anterior to the orbital septum (anatomic boundary between lid and orbital tissue). These are more common than deep dermoid cysts and become apparent during first decade of life.
- Deep dermoid cysts: Deep dermoid cysts are located posterior to the orbital septum. Deep dermoid cysts appear typically in adolescence or adult life.
Epidermoid cyst: Epidermoid cysts, unlike dermoid cysts, do not contain adnexal structures (accessory visual structures) such as hair follicles, sweat glands or sebaceous glands.
Kanski,Jack J. Clinical Ophthalmology, A Systematic Approach .Third Edition.UK. Butterworth Heinemann, 1994. P 47-48.
Orbital dermoid may present as:
- Painless superotemporal mass in orbit.
- Painless superonasal mass in orbit.
- Mechanical ptosis (drooping of upper eyelid).
- Proptosis (protrusion of eyeball).
- Diplopia (double vision).
Dermoids are produced due to entrapment of embryonic epithelial nests when foetal suture lines close during embryogenesis.
Fronto-zygomatic suture is the most commonly involved suture.
Dermoids may also involve fronto-ethmoid or fronto-maxillary sutures.
Diagnosis depends upon the presenting history and clinical examination.
Superficial dermoid cyst:
Superficial dermoid cyst presents typically in infancy as an asymptomatic, firm, round localised lesion in upper temporal aspect of the orbit with attachment to fronto-zygomatic suture or in upper nasal aspect with attachment to fronto-lacrimal suture. It may be mobile or affixed to the bone. It may induce granulomatous inflammation in adjacent tissues, if it leaks or ruptures. Large cysts may cause mechanical ptosis.
Deep dermoid cyst:
Deep dermoid cyst present typically in adolescence or adult life with ocular displacement and non-axial proptosis or a mass lesion with indistinct posterior margins. Deep orbital dermoids may be palpable or non-palpable. Some deep dermoids may extend into temporalis fossa or intracranially. These dermoids may be associated with bony defects. Deep dermoids may cause progressive proptosis or diplopia.
Dermoid may show following signs:-
- Palpable mass.
- Non-axial proptosis.
- Ptosis of eye lid.
- Restricted eyeball movements.
- Diplopia may be due to restriction of eyeball movement or due to compression of cranial nerves controlling movement of eyeball viz. IIIrd, IVth or VIth.
- Orbito-cutaneous fistula.
- Optic nerve compression leading to reduced visual acuity, colour vision and brightness perception. There may be relative afferent pupillary defect.
X-rays may show radiolucent bony defects due to erosion by the cyst. These defects may be large with distinct margins and may show sclerosis.
On CT scan, cyst lumen is generally homogeneous but it can also be heterogeneous depending upon the amount of lipid and keratin within it. Lumen does not show enhancement with use of contrast.
Magnetic Resonance imaging (MRI):
MRI shows cystic appearance, internal fat attenuation (on T1 hyperintensity), internal calcification, and fluid levels. Cyst wall shows enhancement but not the lumen with gadolinium based contrast. Dermoids are high in signal intensity on MRI diffusion-weighted imaging.
On ultrasound, dermoid cysts show a smooth contour and variable echogenicity.
Colour Doppler imaging:
Colour Doppler imaging of dermoid cyst shows absence of intralesional blood flow.
Dermoid cyst wall has variable thickness and may be very thin. Cyst may be connected to periorbita by fibrovascular tissue. Epidermoid cysts are lined by epithelial cells. Dermoid cysts contain adnexal tissue such as sebaceous gland and hair follicles. Cyst may contain oily liquid or solid mass, often high in cholesterol. The cysts may be inflamed.
The dermoid cyst should be differentiated from conditions like:-
- Congenital encephalocele.
- Lateral dermoid cyst should be distinguished from lacrimal gland tumours.
- Exophthalmos (protrusion of eyeball).
- Compressive optic neuropathy.
- Orbital tumours.
- Sebaceous gland carcinoma.
Management should be carried out under medical supervision.
No medical treatment is normally required for an orbital dermoid.
Ruptured dermoid cyst may cause inflammation. Inflammation may be controlled with oral steroids.
Mainstay of treatment is surgical for dermoid cyst.
Superficial dermoid is excised for cosmetic reasons.
Deep dermoid may require anterior, lateral or combined orbitotomy to avoid their leakage into adjacent tissues.
Dermoid cyst may lead to complications like:-
- Displacement of globe.
- Inflammation in adjacent tissues, if the cyst leaks or ruptures.
- Neurologic complications if the cyst compresses optic nerve or cranial nerve IIIrd, IVth or VIth.