Describe the extraocular muscles and the physiology of binocular single vision.

The Extraocular Muscles

• The rule that makes sense OF all OF IT: the recti act purely in abduction and the obliques act purely in adduction. This follows from the 23-degree angle between the muscle plane and the visual axis, and it dictates the positions in which each muscle is tested
• Blood supply — the muscular branches of the ophthalmic artery, giving the anterior ciliary arteries. Each rectus carries these except the lateral rectus, which carries only one — which is why operating on three or more recti at once risks anterior segment ischaemia

Binocular Single Vision

• Binocular single vision is the ability to use both eyes together so that the two images are perceived as one, with the added benefit of stereopsis — true depth perception
• The three grades (worth), which are the framework for both assessment and treatment:
• Grade 1 — simultaneous perception: both images are seen at once, though not necessarily fused

Sensory Adaptations to Misalignment

• When the eyes are misaligned, the same object falls on the fovea of one eye and on a non-corresponding peripheral point of the other. Two symptoms follow, and the brain must deal with both:
• Diplopia — one object seen in two places.
• Confusion — two different objects seen superimposed, because both foveae are stimulated by different images and each claims the straight-ahead direction. This is the more disturbing of the two and is less often described
• The adult brain, being no longer plastic, cannot suppress, so an adult with a new squint suffers intractable diplopia
• The child’S brain adapts, and the adaptations are what cause the lasting damage:
• Suppression — active cortical inhibition of the image from the deviating eye. It is rapid, effective and the reason children with squint do not complain of double vision. It may be central (causing amblyopia) or peripheral
• Amblyopia — sustained suppression leads to a permanent reduction of vision in the deviating eye
• Abnormal retinal correspondence (arc) — the cortex re-maps the peripheral retina of the deviating eye to correspond with the fovea of the fixing eye, restoring a crude form of binocularity in the presence of a manifest squint. It is an obstacle to surgical alignment, since straightening the eyes may then produce diplopia
• Eccentric fixation — the amblyopic eye fixes with a non-foveal point even when the other eye is covered; it indicates dense amblyopia and a worse prognosis
• The practical consequence IS the central paradox OF squint: an adult with a squint has diplopia but normal vision in each eye; a child has NO diplopia but loses vision in one eye. The child’s comfort is bought at the cost of the eye.
• The medial rectus is the only muscle with one action, and is the strongest of the recti.