Strabismus

Published on 08/03/2015 by admin

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Last modified 08/03/2015

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18 Strabismus

Strabismus is present when the foveas of both eyes are not simultaneously aligned on the object of regard; for distance fixation this means that the visual axes are not parallel. Strabismus is classified by the direction of the deviation: if the visual axes converge there is esodeviation; if they diverge there is exodeviation. If the visual axes differ in vertical direction there is hyperdeviation or hypodeviation, depending on whether the eye described is higher or lower than its fellow. Strabismus may be manifest—a ‘tropia’—if the deviation is present with both eyes open or latent—a ‘phoria’—when the deviation is demonstrable only with the eyes dissociated and binocular visual reflexes disrupted. Strabismus is concomitant when the angle of deviation remains constant (or nearly so) irrespective of the position of gaze or the eye that fixates or incomitant when the angle of deviation varies with the direction of gaze and fixing eye. Incomitant deviations are generally associated with ocular muscle paresis or mechanical restriction of rotation of the globe.

ANATOMY

Ocular posture is controlled by the six extraocular muscles, the four recti and two obliques. Pulleys of connective tissue attached to orbital bones and extraocular muscles form a fascial sling around the globe which helps to maintain ocular position. The rectus muscles arise at the orbital apex and insert anterior to the equator of the globe. A useful way to consider the actions of the extraocular muscles is to think of them as antagonistic pairs. Two fundamental laws govern their innervation: Hering’s law of equal innervation states that an impulse to one muscle to contract is accompanied by an identical impulse to the contralateral synergist to contract and Sherrington’s law of reciprocal innervation states that an impulse to one muscle to contract is accompanied by an identical stimulus to the ipsilateral antagonist to relax.

Table 18.1 shows the primary, secondary and tertiary actions of the extraocular muscles. Torsional movements are described as intorsional or extorsional in relation to the midline.

VISUAL ACUITY AND AMBLYOPIA

Measurement of acuity (see Ch. 1) is critical in the assessment and treatment of amblyopia and strabismus. It should be measured both binocularly and uniocularly, with and without correcting glasses and with and without associated head posture, particularly if nystagmus is present. Testing acuity in children demands time and patience; the method depends on the child’s development (Table 18.2).

BINOCULAR SINGLE VISION (BSV)

BSV starts developing by about 4 months of age in normal infants; it combines the visual stimuli from each eye in a single stereoscopic image. The advantages of BSV are accurate depth perception (stereopsis), improved acuity compared with uniocular vision, an enlarged field of vision, compensation for the blindspot scotoma, and maintenance of ocular position. Binocular vision can be divided into three processes: simultaneous macular perception, fusion, and stereopsis. The presence and quality of binocular vision needs to be assessed in any patient with strabismus or diplopia. Most childhood strabismus is associated with cortical suppression by which the brain ignores the image of the deviating eye to avoid diplopia. Strabismus that is acquired after the development of BSV usually results in diplopia.

AMBLYOPIA

Amblyopia is defined as deficient visual acuity in the absence of structural abnormality of the eye or visual pathways. The physiological basis is the lack of a clear focused retinal image eventually leading to structural changes in the lateral geniculate body and visual cortex. Although amblyopic eyes have reduced acuity, they still retain normal colour vision, movement detection and visual field; an afferent pupillary defect is found only when the amblyopia is very dense. Amblyopia is classified into anisometropic amblyopia due to unequal refractive errors; strabismic amblyopia due to nonalignment of the visual axes and least common is stimulus deprivation amblyopia due to ptosis, cataract, etc., which can be the most difficult to treat. Visual loss due to amblyopia can be distinguished from that due to other causes by putting a neutral density filter in front of the eye; when the cause is amblyopia acuity is maintained but other causes result in reduced acuity.

DIAGNOSIS AND EVALUATION OF STRABISMUS

The majority of children with strabismus are otherwise entirely healthy but a number of conditions are associated with a higher than expected incidence. These include intrauterine problems, prematurity and small-for-date babies, birth trauma, a family history of strabismus and congenital abnormalities or cerebral palsy. Strabismus is more common in children with debilitating disease and may present after a severe illness or injury, presumably from decompensation of a pre-existing phoria.

Strabismus is sometimes associated with other signs, the most common of which is an abnormal head posture. When examining for abnormal head posture it is important that the patient fixates on a distant target. A cover test should be performed with and without the head posture. Not all abnormal head postures are adopted for ocular reasons (e.g. torticollis, vestibular disease, habitual), but those that are usually help the patient to achieve a limited area of binocular vision (Table 18.3). Occasionally a head posture will be used to separate images or to fixate with a better seeing eye.

THE COVER TEST

The cover test is the fundamental method for assessing strabismus. Interruption of fixation of one eye causes the fellow eye to move to fixate the same object if a manifest strabismus (tropia) is present. No movement in the presence of an obvious strabismus indicates very poor acuity or mechanical restriction. The alternate cover test detects latent strabismus (phoria).