Year : 2003 | Volume
: 13 | Issue : 2 | Page : 139--144
Cystic lesions of the orbit : A CT spectrum
S Vashisht, S Ghai, P Hatimota, S Ghai, SM Betharia
Department of Radio-diagnosis, All India Institute of Medical Sciences, New Delhi, India
Department of Radio-diagnosis, All India Institute of Medical Sciences, New Delhi
Cystic lesions of the orbit are frequently encountered in the radiological assessment of orbital pathologies. Knowledge of the various cysts and cyst-like lesions with recognition of specific imaging abnormalities enables the radiologist to make the correct diagnosis. This pictorial essay illustrates the spectrum of CT appearances, which may be encountered, with both developmental and acquired cystic lesions of the orbit.
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Vashisht S, Ghai S, Hatimota P, Ghai S, Betharia S M. Cystic lesions of the orbit : A CT spectrum.Indian J Radiol Imaging 2003;13:139-144
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Vashisht S, Ghai S, Hatimota P, Ghai S, Betharia S M. Cystic lesions of the orbit : A CT spectrum. Indian J Radiol Imaging [serial online] 2003 [cited 2019 Nov 19 ];13:139-144
Available from: http://www.ijri.org/text.asp?2003/13/2/139/28646
A large variety of cystic lesions may involve the orbit. The list may be subdivided into developmental anomalies and acquired lesions . In clinical practice these include dermoid cysts, colobomatous cysts, certain vascular lesions with major cystic components such as lymphangiomas, lacrimal duct cysts, haematic cysts, epithelial and appendage cysts, parasitic cysts, cephaloceles and mucoceles. Rare lesions such as optic nerve cysts and enterogenous cysts of the orbit have also been described in the literature ,,.
Clinically, superficial cysts may present as a subcutaneous or subconjunctival nodule while larger or deep cysts usually present with orbital signs: vision changes, strabismus, orbital congestion, globe displacement or proptosis.
Dermoid cysts represent the most common congenital lesion of the orbit and account for one third of all childhood orbital tumors . The superior temporal quadrant at the frontozygomatic suture is the most common location, followed by the nasal aspect of upper orbit at the frontoethmoidal suture. These arise as developmental sequestration of the ectoderm within the suture lines or diploe of the orbital bones. This explains the close relationship of these lesions with the sutures of the orbit. Orbital dermoids may be subdivided into superficial [Figure 1],[Figure 2],[Figure 3] or deep lesions [Figure 4][Figure 5]. The superficial dermoids present in infancy as painless subcutaneous nodules near the orbital rim and are not associated with proptosis; deep lesions are usually larger, seen in adults, most commonly arise behind the orbital rim and may show proptosis or displacement of the globe. Focal thinning or scalloping (fossa formation) of the adjacent bone may be seen in large deep dermoids. Long standing lesions can produce remodelling of the adjacent bones [Figure 5].
Dermoid cysts may have cystic or solid components. On imaging these may show fat, fluid or soft tissue signal and occasionally may show calcification. On CT scan the lesion is seen as a low density, extraconal, nonenhancing mass with smooth margins. Fat fluid level may be present. Ruptured dermoids may show adjacent inflammatory changes.
A coloboma is a congenital or acquired notch, gap or fissure in which a portion of normal tissue is absent. Any part of the eye may be affected, however, typically the cleft appears in the inferonasal quadrant of the globe . The optic nerve head is also commonly involved resulting in central excavation of the disc [Figure 6] a&b. More than 60% of cases are bilateral . A coloboma may be an isolated finding or a part of a complex syndrome ,. On imaging, the affected eye may be normal sized or microphthalmic [Figure 7]. A cleft in the globe may be visualized however, the tunnel-like connection between the eye and the cyst is often very thin and difficult to image. CT can demonstrate the anatomic relationship of the cyst to the globe [Figure 6] a and b &[Figure 7] prior to surgical intervention as well as any associated developmental anomalies of the brain.
Lymphangiomas are composed of delicate endothelium lined, lymph-filled sinuses which invade the surrounding connective tissue stroma. These are relatively infrequent orbital lesions, being less common than hemangiomas; occur in children and young adults and are more common in the extraconal space. These benign lesions produce marked proptosis and diplopia secondary to the bulk of the mass. Optic nerve function is spared until late in the course. Unlike the capillary hemangiomas, lymphangiomas slowly enlarge with age. The slow growth of these masses produces orbital expansion and remodelling rather than true bony destruction. Spontaneous hemorrhage within the lesion is common and results in sudden proptosis. On CT scan, lymphangiomas appear as poorly circumscribed, multilocular, often heterogenous masses of increased density in the extraconal or intraconal space [Figure 8]. Bony expansion may be present, calcification is rare. Minimal to marked contrast enhancement may be present but is less than that seen in a hemangioma.
Lacrimal Gland Cysts
Cysts of the lacrimal glands can occur due to blockage of the excretory ducts and may be located in the orbital or palpebral lobes of the main gland; in the accessory lacrimal glands of Krause and Wolfring; or in ectopic (choristomatous) lacrimal glands ,. These cysts appear as low-density, nonenhancing lesions on CT [Figure 9].
Hematic cysts are deeply placed, incompletely resorbed hematomas which may remain unchanged for long periods of time. These may develop as a complication of head trauma or from prolonged retention of an orbital foreign body. Hematic cysts usually develop because the hemorrhage is too big to be quickly resorbed. The orbital roof is the most common site for hematic cysts . These patients usually present with diplopia and painless unilateral globe displacement. History of orbital trauma or prior surgery may be present. The CT appearance of chronic hematic cyst includes a well defined, extraconal, non enhancing mass in the subperiosteal [Figure 10], medullary or diploic spaces. The lesion often shows high CT attenuation value due to hemosiderin deposition and protein-rich fluid. It is commonly associated with erosion and expansion of the adjacent bones and at times specks of calcification may be present.
These acquired cysts develop from the skin of eyelids; skin appendages (glands and cilia); or conjunctiva  and include sebaceous cysts, apocrine and eccrine hidrocystoma, milia, epidermal and conjunctival inclusion cysts. On CT, these epithelial lesions are seen as small, well-circumscribed cysts in the superficial orbit. Rarely, rim calcification may be seen in these cysts [Figure 11].
Hydatid cysts are seen in patients from endemic regions. Hydatid is the larval form of the parasitic tapeworm Echinococcus granulosus. Humans are infected by eating undercooked meat of an infected animal. Patients usually present with slowly progressive, painless orbital signs . On CT scan, these cysts may appear as well defined, uni-locular or multi-locular large cysts, with or without globe displacement [Figure 12],[Figure 13].
C. Cellulosae Cysts
Cysticercosis is the most common intra-orbital parasitic infestation. It may be intraocular (involving the vitreous and subretinal space) or extraocular (affecting extraocular muscles, eyelid, lacrimal gland or optic nerve). Extraocular muscles are the commonest structure to be affected in the orbit .
Cysticercus cellulosae is the larval form of the pork tapeworm, taenia solium. Human cysticercosis is acquired by ingestion of tapeworm eggs shed in faeces. The eggs mature and the larvae penetrate the intestinal mucosa to enter the portal circulation, from where they are carried to various target organs. The common systemic sites of involvement are subcutaneous tissues, muscles, brain and eyes ,. The larval form may enter the eye through the choroidal circulation and migrate into the subretinal space or enter the vitreous body. Although the disease may be asymptomatic the cyst may occasionally give rise to iridocyclitis, chorioretinal lesion and retinal detachment. Death of the larvae within the ocular tissues can induce a severe inflammatory reaction which may lead to blindness. Cysts embedded in the extraocular muscles may cause ocular movement disorders or proptosis.
On computed tomography (CT) the intraorbital cysts may be seen as a small round, welldefined, nonenhancing area of low attenuation. The scolex, which is the invaginated head of the larva, may or may not be visualized. When visualized it is seen as an eccentrically placed hyperdense (due to presence of calcareous corpuscles) nodule of pin head size on the inner aspect of the cyst wall. A cystic lesion with a scolex confirms the diagnosis of cysticercosis ,. On contrast enhanced CT scans, the pericystic inflammation is manifested as thick, irregular enhancing cyst walls, thickening of involved muscle and streaky soft tissue densites in the orbital fat [Figure 14],[Figure 15],[Figure 16].
Adjacent Structure Cysts
Orbital cephalocele is a rare nonhereditary congenital abnormality in which the meninges, CSF and brain herniate through a bone defect or through a natural anatomic opening. It accounts for less than 1% of orbital masses in children . If only meninges and CSF herniate, the lesion is termed a meningocele, when neural tissue is also included in the herniation, the lesion is termed a meningoencephalocele.
Orbital cephaloceles are divided into anterior and posterior types. The posterior variety usually manifests in childhood as gradually progressive unilateral proptosis. The cephalocele sac can herniate through a defect in the bony orbit or through a natural opening such as the optic foramen or the orbital fissures.
Orbital cephaloceles typically manifest with exophthalmos, medial canthal mass lesions, globe displacement, lacrimal sac mass or ocular hypertelorism. Microphthalmos, colobomas and anophthalmos are other associated ocular anomalies. Anterior orbital cephaloceles may sometimes present as congenital, midline, pulsating cystic mass lesions that increase in size with crying and may be reduced with gentle pressure. Though MR imaging is the study of choice to illustrate cephaloceles, CT scanning in the sagittal and coronal plane identifies the bony defect as well as the herniated mass [Figure 17],[Figure 18].
Mucoceles are expanding cystic lesions covered by a mucous membrane. These are secondary to obstruction of the main ostium of the sinus. There is a definite predilection for the frontal and ethmoid sinuses, presumably because of the dependent position of their ostia. Approximately two thirds of all mucoceles involve the frontal sinuses. The persistent expansion of the mucocele causes erosion of the surrounding bone with extension of the mass into the adjacent orbit. The cyst continues to expand within the orbital cavity producing orbital symptoms. Proptosis or displacement of the eye, puffiness of the upper eyelid, mild ophthalmoplegia, some degree of visual disturbance, and a palpable mass are the clinical features encountered with an orbital mucocele. The frontoethmoidal mucocele usually enters the more anterior portion of the orbit in the upper nasal quadrant.
On CT scan the mucoceles usually appear as homogeneous, low density, nonenhancing soft tissue masses causing expansion of the involved sinus and erosion of the adjacent orbital walls with protrusion of the mass in to the orbit [Figure 19].
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