Strabismus fixus convergens was first documented as a type of retraction syndrome It was originally thought to be a congenital structural anomaly (fibrosis of the medial rectus), but was later determined to differ from retraction syndromes.
Strabismus fixus can be associated with a number of conditions including congenital fibrosis, amyloidiosis and high myopes.
The underlying etiology of strabismus fixus still remains unclear.
Yokoyama et al provided the most recent explanation that the enlarged globe in high myopia herniates superotemporally and retroequatorially through the muscle cone.
Other etiologies include congenital, fibrosis of extraocular muscle secondary to myositis or myopathies and infiltration of extraocular muscle by amyloid.
- Congenital : Congenital strabismus fixus is believed to be caused by fibrosis which explains loss of elasticity of medial rectus
- Lateral rectus palsy: Lateral rectus palsy causes the secondary contracture and fibrosis of medial rectus
- Patients with high myopia have a weak intermuscular septa especially between superior rectus and medial rectus and lateral rectus and inferior rectus which causes superior rectus to slip medially towards medial rectus and lateral rectus to slip inferiorly, thus fixing the eye medially and down, esotropia and hypotropia, also termed as heavy eye syndrome
- Amyloidosis: Infiltration of extraocular muscle by amyloid causes the loss of elasticity of the extraocular muscle
- Unspecific, progressive fibrosis, myopathy and myositis is thought to be cause of strabismus fixus occurring in convergent form.
Histologic studies have demonstrated the disappearance of myofibrils in the external recti of highly myopic eyes, with added tissue changes of homogenization, acidophilia pyknosis and karyolysis. These findings suggest involvement of the external recti in some myopathic process in pathological myopia as hypothesized earlier. But it is still controversial as to which muscle is primarily affected in pathological myopia resulting in such ocular imbalances.
Patients are usually old aged, with high myopia, although few cases of strabismus fixus even in the absence of high myopia have been described.
Patients typically complain of their eyes being fixed and there the inability to move the eyes outward.
As a rule the eyes are anchored in a position of extreme adduction and is fixed in this position and there is limitation of extraocular movements laterally. Divergent form of strabismus fixus not accompanied by ptosis or generalized fibrosis of extraocular muscle is unusual.
Patients with bilateral strabismus fixus tend to prefer one eye for vision and the inability to move the eye necessitates an anomalous head posture.
Forced duction test: Done under topical anesthesia with forceps, forced ductions will confirm the immobility of eye and associated fibrosis and loss of elasticity.
Acquired bilateral sixth nerve palsy.
Congenital bilateral sixth nerve palsy resembles cross fixation and sometimes it may become impossible to decide whether the origin is myogenic or neurogenic.
Acquired bilateral sixth nerve palsy is commonly seen in middle-aged individuals with history of diabetes, head injury, or increased intracranial pressure symptoms. At a later stage, when secondary changes have set in, it may be difficult to differentiate it from strabismus fixus.
Computed tomography (CT) of the orbit can demonstrate relative localization of extra ocular muscle with respect to the globe and presence of any slippage of extra ocular muscle in patient of myopic strabismus fixus.
MRI Scan : In cases of congenital strabismus fixus, adherences among the extraocular muscles, posterior Tenon's capsule, and the globe within the muscle cone on MRI will be seen.
Treatment is mainly surgical.
For convergent strabismus fixus, complete disinsertion of medial rectus is done. In addition to this, the surgeon may consider lateral rectus resection and/or recession of conjunctiva. Although this will limit abduction beyond midline, it will help in cosmetic and functional improvement.
For myopic strabismus fixus (heavy eye syndrome), loop myopexy, the procedure proposed by Yamaguchi et al unites the lateral and superior rectus muscles with non-absorbable suture 15 mm behind the limbus. This procedure normalizes the vector forces of these muscles and eliminates the mechanical disturbance in eye movement. It can be combined with medial rectus muscle recession in cases of contracture as demonstrated by a positive forced duction test. The disadvantage of suture myopexy includes muscle strangulation, which may affect anterior ciliary circulation and may rarely cause cheese-wiring of the muscle.
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- Villaseca A : (1959) Strabismus fixus, Am J Ophthalmol 48:761-762
- Yokoyama T, Tabuchi H, Ataka S, Shiraki K, Miki T, Mochizuki K. The mechanism of development in progressive esotropia with high myopia. In: de Faber JT, editor. Transactions of the 26 th Meeting European Strabismological Association Barcelona, Spain, September 2000. Lisse, Netherland: Swets and Zeitlinger Publishers; 2000. p. 218-21.
- Yamaguchi M, Yokoyama T, Shiraki K. Surgical procedure for correcting globe dislocation in highly myopic strabismus. Am J Ophthalmol 2010;149:341-6.e2.