Strabismus Lecture Notes PDF

Lecture 1 Strabismus Dr. Ahmed Rassol Introduction The visual axis passes from the fovea, through the nodal point of the eye, to the point of fixation. In normal binocular single vision (BSV) the visual axes of the two eyes intersect at the point of fixation,...

Lecture 1 Strabismus Dr. Ahmed Rassol Introduction The visual axis passes from the fovea, through the nodal point of the eye, to the point of fixation. In normal binocular single vision (BSV) the visual axes of the two eyes intersect at the point of fixation, the images being aligned by the fusion reflex and combined by binocular responsive cells in the visual cortex to give BSV. Orthophoria implies perfect ocular alignment in the absence of any stimulus for fusion; this is uncommon. od Heterophoria (‘phoria’) implies a tendency of the eyes to deviate when fusion is blocked (latent squint). ○ Slight phoria is present in most normal individuals and is overcome by the fusion reflex. The phoria can be either a small inward imbalance (esophoria) or an outward imbalance (exophoria). ○ When fusion is insufficient to control the imbalance, the phoria is described as decompensating and is often associated with symptoms of binocular discomfort (asthenopia) or double vision (diplopia). 59NJ Heterotropia (‘tropia’) implies a manifest deviation in which the visual axes do not intersect at the point of fixation. ○ The images from the two eyes are misaligned so that either double vision is present or, more commonly in children, the image from the deviating eye is suppressed at cortical level. ○ A childhood squint may occur because of failure of the normal development of binocular fusion mechanisms or as a result of oculomotor imbalance secondary to a difference in refraction between the two eyes (anisometropia). 1  Lecture 1 Strabismus Dr. Ahmed Rassol ○ Failure of fusion, for example secondary to poor vision in one eye, may cause heterotropia in adulthood, or a squint may develop because of weakness or mechanical restriction of the extraocular muscles, or damage to their nerve supply. ○ Horizontal deviation of the eyes (latent or manifest) is the most common form of strabismus. ○ Upward displacement of one eye relative to the other is termed a hypertropia and a latent upward imbalance a hyperphoria. ○ Downward displacement is termed a hypotropia and a latent imbalance a hypophoria. The anatomical axis is a line passing from the posterior pole through the centre of the cornea. Because the fovea is usually slightly temporal to the anatomical centre of the posterior pole of the eye, the visual axis does not usually correspond to the anatomical axis of the eye. Angle kappa is the angle, usually about 5°, subtended by the visual and anatomical axes. 2  Lecture 1 Strabismus Dr. Ahmed Rassol ○ The angle is positive (normal) when the fovea is temporal to the centre of the posterior pole resulting in a nasal displacement of the corneal reflex and negative when the converse applies. ○ A large angle kappa may give the appearance of a squint when none is present (pseudosquint) and is seen most commonly as a pseudoexotropia following displacement of the macula in retinopathy of prematurity, where the angle may significantly exceed +5°. 3