Strabismus Fixus

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Disease Entity

Strabismus fixus convergens was first documented as a type of retraction syndrome[1][2]and was thought to be a congenital structural anomaly (fibrosis of the medial rectus). It was Villascca who first described this condition as different from retraction syndrome and documented it in one of his cases which acquired it at a later age.[3]

Disease

Strabismus fixus can be associated with a number of conditions including congenital fibrosis, amyloidiosis and in rare occasions, high myopic patient may develop a specific squint called“myopic strabismus fixus”(MSF).Myopic strabismus fixus is a rare strabismus disorder. This disease may progress over several years, from a small degree of esotropia with free ocular movement to the end stage of large angle fixed esotropia.

Etiology

The underlying etiology of strabismus fixus still remains unclear.

Yokoyama et al[4] provided the most recent explanation that the enlarged globe in high myopia herniates superotemporally and retroequatorially through the muscle cone.

Other etiologies include congential, fibrosis of extraocular muscle secondary to myositis or myopathies and infiltration of extraocular muscle by amyloid.

Pathophysiology

  1. Congenital : Congenital strabismus fixus is believed to be caused by fibrosis which explains loss of elasticity of medial rectus
  2. Lateral rectus palsy: Lateral rectus palsy causes the secondary contracture and fibrosis of medial rectus
  3. 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
  4. Amyloidosis: Infiltration of extraocular muscle by amyloid causes the loss of elasticity of the extraocular muscle
  5. Unspecific, progressive fibrosis, myopathy and myositis is thought to be cause of strabismus fixus occuring in convergent form.


Mechanical

General Pathology

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 hypothesised earlier. But it is still controversial as to which muscle is primarily affected in pathological myopia resulting in such ocular imbalances.

Diagnosis

Symptoms

Patients are usually old aged, with high myopia, althogh 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.

Signs

Extraocular movements

Nine gaze picture showing Strabismus fixus in a high myope patient

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 accompained by ptosis or generalised fibrosis of extraocular muscle is unusual.

Visual acuity

Patients with bilateral strabismus fixus are severely handicapped, one eye is usually preferred for vision and immobility of the eyes necessitates an extreme degree of head turn in such patients.

Diagnostic procedures

Forced duction test: Done under topical anaesthesia with the help of Pierce Hoskin forceps, will further confirm the immobility of eye and associated fibrosis and loss of elasticity.

Differential diagnosis

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.

Investigations

CT Scan Coronal section of a patient of 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.

Management

Treatment is mainly surgical.

For the convergent form of strabismus fixus, complete disinsertion of medial rectus is done. In addition to this lateral rectus resection with recession of conjunctiva and tenons should be done, even though abduction beyond midline will not be possible after surgery , but will help in cosmetic and functional improvement.

For myopic strabismus fixus (heavy eye syndrome), loop myopexy, the procedure proposed by Yamaguchi et al[5] 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.

References

  1. Aebli R : (1933) Retraction syndrome, Arch Ophthalmol 10:602=610.
  2. Bielschowsky.A : (1939) Lectures on motor anomalies, Am J. Ophthalmol 22:357-367.
  3. Villaseca A : (1959) Strabismus fixus, Am J Ophthalmol 48:761-762
  4. 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.
  5. Yamaguchi M, Yokoyama T, Shiraki K. Surgical procedure for correcting globe dislocation in highly myopic strabismus. Am J Ophthalmol 2010;149:341-6.e2.