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Posterior orbital dermoids


Key points

  • Definition: Dermoids originate from totipotential germ cells. Present at birth, these differentiate abnormally, developing characteristics of mature dermal cells.

  • Synonym: Orbital dermoids, retrobulbar orbital dermoids, or deep dermoids.

  • Classic clue: Well-circumscribed encapsulated mass with low Hounsfield numbers, filled with high T1 signal material.

  • Displacement of globe and ocular muscles may impair ocular motility.

  • Compression of the optic nerve (ON) may result in visual impairment.

  • Dermoids may rupture inciting an inflammatory reaction.

  • Posterior retrobulbar orbital dermoids are rare and much less frequent than the anterior variety, a key difference which is well documented in the literature.

Imaging

Computed tomography features

  • Computed tomography (CT) is less preferred but may be the first imaging study performed.

  • Radiation to the orbit is undesirable.

  • No calcification of fluid-fluid levels are evident.

  • Occasionally calcifications may be present, which are better demonstrated by CT.

  • Occasionally teeth, hair tufts, and fat-fluid levels may be apparent.

  • Only approximately 35% show typical fat density on CT.

  • Approximately 40% appear cystic on CT.

  • Extraconal mass with adjacent bone thinning or notching should raise the possibility of dermoids, especially if rim calcification is identified.

Magnetic resonance imaging features

  • The preferred imaging procedure is magnetic resonance imaging (MRI) with Gd and fat saturation.

  • Better at defining disease extent.

  • Allows sequential follow-up of patient without increasing cumulative radiation dose.

  • Fat saturation imaging is helpful in evaluating subtle lesions.

  • MRI is the best method of revealing the thick low signal dermis-like wall covering the mass.

  • May see chemical shift artifact on T2 at the fat–fluid interface.

    • Shows up as a bright band on one side and a dark band on the other side of a fat–soft tissue interface.

    • In Figure 14-1 , B, the bright line is more conspicuous than the dark line.

      FIGURE 14-1 ■, A , Axial T1 MRI images showing a retrobulbar intraconal mass on the left with prominent hyperintensity relative to the vitreous humor. Exopthalmos is evident with a mass displacing the ON and the extraocular muscles. B , Axial T2 image shows heterogeneous hyperintensity, which is less bright than the vitreous humor. Obvious exopthalmos is evident OS. All images demonstrate the presence of a peripheral low signal capsule. A bright band is visible near the mid portion of the mass from a chemical shift artifact. C , Sagittal T1 image shows cephalic deviation of the ON over a well-circumscribed hyperintense mass.

  • T1-weighted images are typically hyperintense (due to cholesterol components).

  • T1 Gd typically exhibits a thin enhancing rim.

  • T2 signal varies from hypo to hyperintense.

Clinical issues

Presentation

  • Patients with posterior orbital dermoids may present with prominent proptosis and globe displacement.

  • Dermoids may become symptomatic in adulthood and exhibit considerable growth over a single year.

  • Some suggest these lesions may be dormant for many years with episodic growth.

  • Most orbital dermoids are clinically apparent anterior orbital dermoids, which are not usually a problem in diagnosis or treatment.

  • Neurologic findings:

    • ON compression may affect visual acuity, color vision, brightness perception, and pupillary reaction.

    • May produce diplopia by physical restriction of globe movement or compression of cranial nerves III, IV, or VI.

Natural history

  • Morbidity usually cosmetic.

  • Occasionally may result in visual loss, diplopia or orbital inflammation.

  • No sex or racial predilection.

  • Dermoids are most often noted in young children.

  • May appear or grow at any age.

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