What is Squint?

Technically know as Strabismus, Squint is a condition where the two eyes are misaligned. That is both eyes do not appear to be looking in the same direction. One eye may be looking at an object and the other eye may be turned in or out or up or down. A squint may be constant (always apparent) or intermittent (seen only at times – usually when tired). Squints are more commonly seen in children. The condition affects 2% of children under 3 years and 3% of children and young adults. It may be seen always in one eye or it may alternate and appear to shift from one eye to the other. For example the Right eye which is straight to start with, turns in as the Left eye looks straight or visa versa.

Types of Squint

Squints are classified depending on direction of the squinting eye
  • Convergent squint or esotropia when one eye is turning in
  • Divergent squint or exotropia when the eye is turning out
  • Vertical squint (Hypertropia / Hypotropia) when one eye is pointing upwards/downward


The precise cause of squint is unclear in a large proportion of childhood squints. It is understood that the condition may be due to mis-functioning of the brain in moving the eye muscles synchronously. There is no defect in the eye muscles themselves. The other causes leading to a squint can be refractive errors like hypermetropia (long-sightedness), myopia (short-sightedness), astigmatism or unequal refractive error in both eyes.

1. Trauma:

Brain trauma and certain brain tumours may also cause the eye to turn in. If a child develops a sudden in-turning of the eye associated with nystagmus (involuntary eye movements), an urgent assessment is required as it suggests a build up of intracranial pressure or a possible tumour.

2. Abnormality of the eye muscles or in their nerve supply as in:

  • Myasthenia gravis.
  • Sixth Cranial Nerve Palsy (in children, this occurs frequently post a viral infection and may cause sudden onset of esotropia).
  • Trauma to the eye.
  • Duane’s Syndrome.
  • Strabismus Fixus.
  • In adults Usually an untreated childhood squint.
  • Following previous Squint surgery.
  • Decompensating squint, previously under control but now getting worse.
  • Paralytic squint – sudden onset in patients with underlying hypertension or diabetes.
  • Thyroid Eye Disease, due to inflammation/swelling caused by scarring of the eye muscle. Once the inflammation has subsided, surgery is advised if symptoms of double vision are present.
  • Trauma to eye muscles e.g., as a complication of sinus surgery.
  • Early Diagnosis: If the proper visual impulses are not transmitted to the brain in the first few months after birth, which is the critical period of visual development, amblyopia will develop and there will be loss of 3D (binocular) vision.

Non Surgical Treatment:

Prisms: These are specialised glasses used to treat double vision of sudden onset. They can be used as a temporary measure and are later incorporated into the usual spectacles. They can also improve the appearance of a squint in some patients where surgery is not an option. Botox: In adults and occasionally in children, Botulinium toxin may be the first line of treatment. The toxin is injected into the muscle at the surface of the eye. It temporarily paralyses the muscle in the direction of the squint and balances the forces producing the misalignment. It causes an overcorrection initially and the eye may move in the direction opposite to the squint in the first few weeks. The eye will gradually straighten and then return to its original position. Repeat injections are usually required to maintain the corrected position of the eye.

Surgical Treatment:

To achieve surgical correction, the eye muscles in one or both eyes (depending on the degree and direction of the squint), are tightened or loosened. The muscles are shortened and reattached to the eye to tighten or moved further back which has the effect of loosening. If the eye is turning in (convergent squint), the muscle which pulls the eye in (the medial rectus) is moved back and the outer muscle (the lateral rectus) is tightened to straighten the eye. The amount of surgery is based on normograms that are tables which have been developed based on results of muscle surgery in thousands of patients
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