Dermoid cysts are common cystic lesions that represent a developmental choristoma. They are congenital lesions that enlarge very slowly as they accumulate sebum and keratin. Dermoid cysts can be found in any part of the body. Approximately 7% occur in the head and neck,¹ with the most commonly reported locations being periorbital, nasal, submental, neck, and suprasternal.²,³,⁴,⁵ In a study of the incidence and clinical characteristics of periocular dermoid cysts diagnosed among a population-based cohort of children older than 20 years of age, 54 cases were diagnosed yielding a birth incidence of 1 in 638 live births.⁶ The mean age at diagnosis was 15 months (range 1-60 months), and 50% of patients were diagnosed by the age of 1 year. There was no gender predilection. The clinical presentation was a periocular mass, without inflammation or erythema.

In a large series of orbital and periorbital dermoid cysts, Shields et al⁵ proposed a classification that included soft-tissue orbital lesions that were mostly of conjunctival origin, sutural dermoids that extended into bony sutures and were immobile, and juxtasutural dermoids near but not into bony sutures. Most of the superficial periorbital lesions were of the latter type. When dermoid cysts are associated with or near periorbital bony sutures, most show growth outward into the eyelid where they present as a palpable mass and are typically noted early in young children. More rarely they may extend inward into the orbit in which case they tend to present later in life, in teens or young adults.

Orbital and periorbital dermoids together represent 35% to 47% of orbital pediatric lesions and 89% of all orbital cystic lesions.⁷ In most patients, they present with a prominent extra orbital component, and in 90% to 99% of cases, presentation is with a slow-growing, painless, subcutaneous mass in the eyelid.⁴,⁵,⁸ Lesions most often occur superotemporally, associated with the frontozygomatic suture at the superolateral orbital rim in 55% to 75% of cases,⁴,⁶,⁸,⁹ and less frequently in a superomedial periorbital location within the frontoethmoid or frontonasal sutures. Cysts located deep in the orbit are uncommon, seen in 0.5% to 10% of cases,⁵,⁸ and may present with proptosis, optic nerve compression, or restriction of eye movements.⁵,¹⁰ In very rare cases, a dermoid cyst can be located within an orbital extraocular muscle and when large can cause visual compromise.¹¹,¹²,¹³ There are also reports of dermoid cysts involving the lacrimal gland and the caruncle.¹⁴,¹⁵ A case of tarsal dermoid has also been described.¹⁶

Rupture of the cyst wall can occur as a result of trauma or can be spontaneous. Cyst rupture can cause intense inflammatory sequelae manifesting as local pain, edema, and erythema.¹⁷ Since most periorbital dermoids are superotemporal, a ruptured cyst may present with a clinical picture of preseptal cellulitis or dacryoadenitis.¹⁸ Very rarely, a dermoid cyst can present with chronic osteomyelitis or with inflammatory symptoms due to bony involvement adjacent to cyst leakage.¹⁹ Clinical or histologic evidence of spontaneous cyst rupture was identified in a series of 86 cases by Siah et al¹⁷ In that series, the median patient age was 5.5 years (range, 1-63 years). Clinical evidence of rupture was identified in 29 cases (33.7%), and of these, 2 (6.9%) ruptured spontaneously before presentation, and 27 (93.1%) ruptured during surgery. The remaining 57 cases (66.3%) had histologic evidence of subclinical spontaneous rupture. This was consistent with an earlier study by Lane et al²⁰ who reported that 75% of 40 orbital dermoid cysts had histologic evidence of rupture and inflammation, and Shields et al⁵ who reported chronic granulomatous inflammation in the walls of 34% of 197 dermoid cysts.

Imaging of superficial dermoid cysts presenting as a cystic superotemporal eyelid mass, especially when they are mobile, is usually not necessary to make the diagnosis. In
a radiological review of 70 patients with a diagnosis of lateral dermoid cysts, Sathananthan et al²¹ showed that in cases where the lesion is both superficial and mobile in all directions, a scan is not required. However, when the cyst is fixed to the underlying bone, and because of the rare occurrence of the cyst extending through a bony suture with a dumbbell configuration and simultaneous masses in the orbit or intracranial compartment, neuroimaging is recommended before any surgical intervention. On computed tomography, a dermoid cyst appears as a round to oval, well-defined cystic lesion.⁸,²² The cyst is almost always extraconal and has a cystic center generally with low density in 72% of cases, is usually homogeneous, but occasionally can be heterogeneous depending on the proportions of lipid and keratin content. Denser foci within the cyst represent flecks of keratin and sebum.⁸ The cyst cavity is surrounded by a thin rim of tissue density that may have areas of calcification in about 6% of cases.²³ Adjacent bone commonly shows remolding from long-standing pressure, seen in 53% of cases.⁸ Contrast administration produces mild enhancement of the cyst rim, but not of the lumen. On magnetic resonance imaging (MRI) T1-weighted sequences, the cyst cavity can be isointense to hypointense to fat and hypointense to muscle because of its water content. In lesions with high fat content, the T1-WI will be hyperintense, closer to that of orbital fat. On T2-weighted sequences, the signal is isointense or hypointense with respect to vitreous and hyperintense to fat. Images may be homogeneous to heterogeneous depending upon the cyst contents. A fat-fluid level is seen in some cases, with the upper lipid layer giving a brighter signal on T1 and a lower signal on T2 relative to the lower water-keratin layer. Calcifications appear as signal voids. With gadolinium, the cyst rim shows moderate enhancement, but the lumen does not enhance. In a series of 280 orbital and periorbital dermoid cysts, Pushker et al²³ reported a fat-fluid level in 24% of cases, and 75% had bone excavation adjacent to the cyst. Of the orbital lesions, 6% were dumbbell-shaped with connection to the temporalis fossa through a defect in the lateral orbital wall.