Squint eyes

A squint (strabismus) is an abnormal deviation of the eye.

Epidemiology
Approximately 2% of the population suffer with varying degrees of a squint.

Pathology
A squint may be concomitant or incomitant, depending on the deviation of the eye with the position of the gaze. In a concomitant squint the deviation is constant regardless of the position of gaze. This is a typical squint of childhood and patients do not have double vision because the image of one eye is suppressed. In an incomitant squint, the deviation varies according to the direction of gaze, usually due to a paretic muscle or muscle underaction.

Clinical features
It is important to distinguish monocular diplopia from binocular, and whether the diplopia is horizontal or vertical. Sudden onset of diplopia may be associated with ischaemia, and intermittent diplopia may be associated with tiredness (phoria). Diplopia can also be associated with fatigue (myasthenia) and may be progressive (compressive lesion) or stationary (thyroid eye disease). Pain associated with a squint may be due to idiopathic orbital inflammation or aneurysm of the posterior communicating artery (causing IIIrd nerve palsy). Associated neurological symptoms such as abnormalities of gait, balance, urinary problems and headache may be due to multiple sclerosis (internuclear ophthalmoplegia), or raised intracranial pressure (VIth nerve palsy).

Examination may reveal an abnormal head posture (left head turn for a left lateral rectus weakness), abnormal eye alignment (globe displacement with orbital tumour and thyroid eye disease), proptosis (thyroid eye disease or tumour), enophthalmos (orbital fracture), ptosis (IIIrd nerve palsy, myasthenia) or eyelid retraction (thyroid eye disease). Globe injection and chemosis are associated with thyroid eye disease and idiopathic orbital inflammation.

Ocular motility examination will establish the weak or restricted muscle. The cover-uncover test is performed with the patient fixating on a near or distance target. The patient's eye is covered and uncovered; if the uncovered eye moves to take up fixation there is a manifest squint. If there is not a manifest squint, the alternate cover test is used to reveal a phoria (tendency for eyes to deviate into a squinting position normally prevented when both eyes are open). Each eye is occluded alternately several times. In a latent squint the occluded eye drifts under the occluder and has to move to regain fixation.

Investigations
Hess chart
A Hess chart test is performed by dissociating the two eyes by red and green filters. A target is presented in one colour (e.g. red) and the patient is then asked to position a pointer of another colour (e.g. green) in the same position as the initial target. Once the target is presented in all position of gaze, the colour of the pointer and target is reversed and the other eye examined.
Refraction
If the phoria is intermittent it is important to refract the patient as an outward squint may be found in uncorrected myopia and an inward squint with uncorrected hypermetropia.

Management
Glasses
Patients with refraction errors are managed by appropriate glasses, prisms and exercises to aid near focusing.
Address underlying cause
It is important to treat or address any underlying cause of a squint, e.g. myasthenia, as this may correct the symptoms without any further treatment.
Squint surgery
Surgery is contemplated if the deviation is stable for 6 months. The principles of squint surgery are to weaken muscles (recession, myomectomy), to strengthen them (resection) or to alter their mode of action (transposition).

Prognosis
The recovery of a squint depends on the underlying cause which includes nerve palsies,Compressive lesions, arteritis,raised intracranial pressure, congenital demyelination,neoplasm and trauma as well as thryoid eye disease. Squints associated with refraction errors are easily corrected. Occasionally, addressing the underlying cause (myasthenia, multiple sclerosis) may improve the squint. Some causes of a squint are refractory to treatment (tumours).



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