Incomitant Strabismus/Module 7: Restrictive Strabismus

Characteristics of Acquired Restrictive Strabismus
Mechanical restriction of ocular movement is caused by factors which interfere with muscle contraction or relaxation, or somehow inhibit free rotation. An acquired mechanical restriction can be caused by such things as trauma, where there is damage to or a lesion of the bony structures of the orbit and its contents, inflammatory disease of the orbit, or tumours and other space occupying lesions, which cause limitation of movement. Apart from this, diplopia and discomfort, or pain on movement, are common occurrences.

Orbital injuries: (i) orbital floor (antral) blow-out fractures, and (ii) medial wall (ethmoidal) blow-out fractures. A blow-out fracture may result from impact to the eye and soft tissues of the orbit from a blunt, non-penetrating convex object which is of greater diameter than the orbital margin, some examples being a fist, tennis ball, or dashboard or steering wheel of a car. The force is relayed to the weak walls of the orbit, the floor and the medial wall, which may fracture.

Soft tissue injuries: Oedema and haemorrhage after trauma may cause a mechanical restriction of ocular movement and diplopia which resolves as swelling subsides. Also, scar tissue may cause a limitation of ocular movement, due to inability to relax and stretch the muscle.

Tumours and space occupying lesions: A mechanical restriction of ocular movement may be caused by tumours or cysts. These may be discrete and affect one muscle only. Exophthalmos may be present and diplopia is a usual presenting symptom.

Retinal detachment surgery: Sceral buckling as part of retinal detachment surgery can also produce a mechanical restriction of ocular movement.

Management of Acquired Restrictive Strabismus
The management of blow-out fractures in particular involves regular orthoptic assessment (measurements, Hess, field of BSV and diplopia charting) to monitor progress and spontaneous recovery. If surgery is necessary a forced duction test pre-op, will show the resistance to enforced movement. Surgical treatment aims to remove entrapped muscles and tissue from the fracture, and to permanently close the bony defect. Surgery is often preformed in the first two weeks following the injury, as soon as swelling has subsided. If left too long, fibrosis of the involved tissues may occur, resulting in more permanent damage to the muscles and nerves. This early intervention approach, however, has attracted much debate in more recent years.

Characteristics of Congenital Restrictive Strabismus
Duane's retraction syndrome: This condition is typified by limitation of abduction and/or adduction, depending on the type, with retraction of the globe and narrowing of the palpebral fissure on adduction.

Brown's syndrome: This condition is typified by a limitation of elevation in adduction (mimicking an inferior oblique palsy).

Strabismus fixus: This is a congenital anomaly in which the eye(s) is/are anchored in a position of extreme adduction or abduction, as the MR or LR respectively becomes a thick fibrous band.

Adherence syndromes: This is a developmental anomaly occurring between the extraocular muscles and their fascial sheaths. Abnormal connections most commonly occur between the LR and IO, and between the SR and SO muscles.

Generalised and other fibrosis syndromes:' These genetic syndromes cause gross restrictions of ocular movement and quite often involve the eyelids, causing ptosis. Apart from a detailed examination of the ocular motility status, the emphasis of the orthoptic assessment is also to examine compensation and binocular functions.

Management of Congenital Restrictive Strabismus
Usually, if the condition is well controlled in the primary position or with an inconspicuous and comfortable Abnormal Head Posture (AHP), it is best left untreated and regularly monitored. Indications for surgery may be uncosmetic strabismus in the primary position, an uncomfortable and/or unsightly AHP, and troublesome diplopia.