Orbital cysts generally arise from epithelium sequestered within the orbit during embryological development, by implantation after trauma or due to expansion of epithelial-lined sinus lesions into the orbit.
These lesions arise from surface epithelium implanted at sites of embryological folding and, if situated anteriorly within the orbit, are commonly noted soon after birth. Due to the accumulation of epithelial debris and sebaceous oil in the lumen of the cyst, the cysts slowly enlarge and leakage of the contents into the surrounding tissues may cause marked inflammation - with deeper dermoid cysts tending to present in this fashion. A dermoid cyst contains dermal structures (hairs, sebaceous glands), whereas more rarely there is only an epithelial (epidermoid cyst) or a conjunctival lining (conjunctival dermoid).
Implantation cysts may arise and develop in a similar fashion to congenital dermoids, but do not respect the characteristic anatomic sites of the latter and will generally present in patients with a past history of periocular trauma.
The commonest dermoid cysts are firm and smooth, mobile preseptal masses overlying the supero-temporal quadrant of the orbit and, less commonly, the supero-nasal quadrant. Many cysts have a variable periosteal attachment near the underlying fronto-zygomatic or fronto-ethmoidal sutures, but occasionally the dermoid will pass into or through defects in the neighbouring bone. In some cases the dermoid is incompletely separated from the skin surface and presents as a chronically inflamed and discharging sinus (Figure 12.1).
A clinically characteristic lesion presenting in childhood does not require radiological investigation if anteriorly situated and mobile. Likewise, fixed anterior masses do not necessitate imaging, provided the orbital surgeon is adequately experienced to follow the lesion to its limits - if necessary within the orbital depths. Deep orbital dermoids, often presenting as orbital inflammation or proptosis, require thin-slice CT with bone
windows to show associated clefts or canals in the bone (Figure 12.2); MRI is a poor investigation for these orbital abnormalities. CT will often show a smooth, "scalloped" erosion of the neighbouring bone as a result of pressure from the mass, although this is a nonspecific sign suggesting a longstanding benign orbital lesion.
All cysts develop inflammatory changes and should be removed, generally at preschool age before childhood trauma encourages rupture.
They are excised through an incision hidden in the upper lid skin-crease or brow line and it is important to divide tissues right down to the cyst (there being a tendency to dissect a plane too far from the surface of the cyst) and then follow the plane by blunt dissection; in some cases it is necessary to remove some of the underlying periosteum or follow the lesion into or through the orbital walls.
Deep orbital dermoids often involve the greater wing of the sphenoid and may require lateral orbitotomy or complicated anterior orbitotomy for their removal.
Rupture of the cyst during surgery may lead to a marked post operative inflammation and any spilt contents should be removed. Incomplete excision of the epithelial lining will lead to recurrent inflammation with formation of a discharging cutaneous fistula to the operative site.
Although dermolipomas are not cystic, they are conveniently classified with dermoid cysts as they arise from cutaneous epithelium sequestered within the conjunctival recesses -typically laterally and occasionally associated with a minor clefting of the outer canthus or with Goldenhar's syndrome. The abnormal epithelium, which often bears hairs and sebaceous glands that cause chronic conjunctivitis, is associated with localised prenatal formation of cutaneous-type fat, which is evident as an underlying bright yellow mass. The differential diagnosis is subconjunctival fat prolapse, which tends to present in obese adults and is not associated with an abnormal conjunctival surface.
Dermolipomas require removal if they are causing chronic conjunctivitis or if easily visible at the palpebral aperture. Excision should be performed under the operating microscope as this aids preservation of all except the abnormal epithelium and reduces the risk of damage to the lacrimal gland ductules. To avoid adherence between the lateral rectus and the orbital rim, only fat anterior to the rim should be removed and this abnormal cutaneous fat has a subtle plane of dissection free from the normal orbital fat.
Incautious excision of dermolipomas is a source of medico-legal cases, as there is a significant risk of damage to the lacrimal gland ductules, restriction of eye movements and ptosis if the lesions are not managed properly.
Mucocoeles, most commonly in the anterior ethmoid and frontal sinuses, are slowly-enlarging, mucus-filled, cystic lesions that arise from paranasal sinus mucosa and gradually encroach into the orbit; occasionally the contents of a mucocoele become infected, which may lead to orbital cellulitis. Most mucocoeles will present with a gradual displacement of the globe and proptosis (Figure 12.3), although those of the maxillary sinus may lead to collapse of the orbital floor and secondary enophthalmos.
The CT appearance of a mucocoele is a cystic cavity smoothly expanding the bone of a paranasal sinus and necessary with patchy thinning or loss of bone. T1- and T2-weighted MR images can show a wide variation in signal intensities due to continuing changes in the contents of the mucocoele.
Severe acute sinusitis or orbital cellulitis requires admission for intravenous antibiotic
therapy and drainage of the orbital abscess if threatening vision. Once the infection has been shown to be under control, the mucocoele and other sinus disease should receive definitive treatment under the care of an otorhinolaryngologist. In general, treatment involves removal of the mucocoele lining and re-establishment of a new drainage pathway for the affected sinus.
Orbital cellulitis secondary to infected mucocoeles may lead to an orbital abscess or the formation of a transcutaneous fistula, typically in the medial aspect of the upper eyelid. Late presentation of mucocoeles within the sphenoid or posterior ethmoid sinuses can lead to compressive optic neuropathy and irreversible visual loss.
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