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Clinical Examination

• Onset: the earlier the onset (i.e., younger age), the more likely surgical correction is needed
• Decompensation of a pre-existing heterophoria?
  • Discomfort, blurred vision, and possibly intermittent diplopia
• Recent, acquired, usually paretic condition?
  • Sudden onset of double vision
• Compulsory head posture?
• General health status
• History: Birth history, family history, eye diseases
• Visual acuity testing

Eye Movements

• Ductions = monocular eye movements around the Fick’s axis
• Versions = binocular, simultaneous, conjugate movements in the same direction
  • Dextroversion, levoversion, elevation, depression, cycloduction
• Vergences = binocular simultaneous, disjunctive movements (in opposite directions)
  • Convergence (tonic, proximal, fusional, accommodative), divergence

Laws of Eye Movements

• Agonist-Antagonist Pairs: muscles of the same eye moving the eye in opposite directions
  • e.g., right lateral rectus and right medial rectus
• Synergists: muscles of the same eye moving the eye in the same direction
  • e.g., right superior rectus and right inferior oblique (common elevation)
• Yoke Muscles = contralateral synergists: muscle pairs, one muscle in each eye, producing conjugate eye movements
  • e.g., right inferior rectus and left superior oblique
• Sherrington’s Law of Reciprocal Innervation (Inhibition): increased innervation of an extraocular muscle (e.g., right medial rectus) is associated with reciprocal decrease in innervation of its antagonist (e.g., right lateral rectus)
  • when medial rectus contracts, lateral rectus automatically relaxes and vice versa
  • also applies to versions and vergences
• Hering’s Law of Equal Innervation: during conjugate eye movements, yoke muscles receive equally strong and simultaneous innervation

Sensorics of Binocular Vision

• Normal binocular single vision
  • Simultaneous bifoveal fixation by both eyes, contributing to a single perception of the target object
• Projection is the subjective interpretation of an object’s position in space based on stimulated retinal elements
• Retinomotor Values: to enable fixation of an object falling on an extrafoveal retinal element in the peripheral field, a saccade with precise amplitude is required -> each extrafoveal retinal element thus has a retinomotor value, proportional to its distance from the fovea, guiding the amplitude of the saccade required to look at the object
• Horopter: imaginary plane in external space, relative to the observer’s eyes for a specific target object, where all points stimulate corresponding retinal elements and are thus seen simply and in the same plane
• Panum’s Fusional Area (“Volume”): zone before and behind the horopter, where objects stimulate not precisely corresponding retinal elements (retinal disparity)
  • Objects within the limits of the fusional area are seen simply with stereopsis
  • Objects before or behind the Panum area appear double (physiological diplopia)

Sensory Adaptation to Strabismus

• Suppression: active inhibition of the image of one eye by the visual cortex when both eyes are open. Stimuli for suppression include diplopia, confusion (simultaneous perception of two superimposed but unequal images due to stimulation of corresponding points), and a blurred image from astigmatism/anisometropia
  • Central or peripheral
  • Monocular or alternating
  • Optional or obligatory
• Abnormal Retinal Correspondence (ARC)
  • When non-corresponding retinal areas develop a common subjective visual direction
  • i.e., fusion is possible in manifest strabismus with a small angle of deviation -> the fovea of the fixing eye corresponds with a non-foveal element of the deviating eye

Motor Adaptation to Strabismus

• Compulsory head posture: abnormal head position in which binocular single vision is maintained
  • Head turn, head tilt, chin elevation or depression

Sources

• The Wills Eye Manual: Office and Emergency Room Diagnosis and Treatment of Eye Disease; Nika Bagheri MD, Brynn Wajda MD, et al; Lippincott Williams&Wilkins; 7th Edition (2016)
• Kanski’s Clinical Ophthalmology: A Systematic Approach; Jack J. Kanski MD, Brad Bowling MD; Saunders Ltd.; 8th Edition (2015)