Ch 14 .Strabismus /disease of occular motility

• Extraocular muscles


  A set of six extraocular muscles (4 recti and 2 obliques) control the movements of each eye
• Rectus muscles


  • Superior (SR), inferior (IR), medial (MR) and lateral (LR)
• Oblique muscles


  • Superior (SO) and inferior (IO)
• Origin and insertion of rectus muscles

  1. Originate from a common tendinous ring (the annulus of Zinn), which is attached at the apex of the orbit, encircling the optic foramina and medial part of the superior orbital fissure

  2. Medial rectus arises from the medial part of the ring, superior rectus from the superior part and also the adjoining dura covering the optic nerve, inferior rectus from the inferior part and lateral rectus from the lateral part by two heads which join in a 'V' form

  3. All the four recti run forward around the eyeball and are inserted into the sclera, by flat tendons (about 10 mm broad) at different distances from the limbus

  
• Origin and insertion of superior oblique muscle

  Arises from the bone above and medial to the optic foramina, runs forward and turns around a pulley—'the trochlea' (present in the anterior part of the superomedial angle of the orbit) and is inserted in the upper and outer part of the sclera behind the equator

  
• Origin and insertion of inferior oblique muscle


  Arises by a rounded tendon from the orbital plate of maxilla just lateral to the orifice of the nasolacrimal duct, passes laterally and backward to be inserted into the lower and outer part of the sclera behind the equator
• Nerve supply of extraocular muscles

  • Third cranial nerve (oculomotor) supplies the superior, medial and inferior recti and inferior oblique muscles
  • Fourth cranial nerve (trochlear) supplies the superior oblique
  • Sixth nerve (abducent) supplies the lateral rectus muscle
• Actions of extraocular muscles

  • Rotate the eyeball around vertical, horizontal and anteroposterior axes
  • Medial and lateral rectus muscles are almost parallel to the optical axis of the eyeball, so they have got only the main action
  • Superior and inferior rectus muscles make an angle of 23° and reflected tendons of the superior and inferior oblique muscles of 51° with the optical axis in the primary position, so they have subsidiary actions in addition to the main action

  
• Types of ocular movements

  • Unilateral movements (ductions): adduction, abduction, supraduction, infraduction, incycloduction, excycloduction
  • Binocular movements: versions (dextroversion, levoversion, supraversion, infraversion, dextroelevation, dextrodepression, levoelevation, levodepression, dextrocycloversion, levocycloversion), vergences (convergence, divergence)

  
• Synergists


  Muscles having similar primary action in the same eye
• Antagonists


  Muscles having opposite actions in the same eye
• Yoke muscles (contralateral synergists)

  Pair of muscles (one from each eye) which contract simultaneously during version movements
• Contralateral antagonists


  Pair of muscles (one from each eye) having opposite action
• Hering's law of equal innervation


  Equal and simultaneous innervation flows from the brain to a pair of muscles which contract simultaneously (yoke muscles) in different binocular movements
• Sherrington's law of reciprocal innervation

  Increased flow of innervation to the contracting muscle is accompanied by decreased flow of innervation to the relaxing antagonist muscle
• Diagnostic positions of gaze

  • Primary position, secondary positions, tertiary positions, cardinal positions
• Supranuclear control of eye movements

  • Highly accurate, still not fully elucidated, keeps the two eyes yoked together so that the image of the object of interest is simultaneously held on both fovea despite movement of the perceived object or the observer's head and/or body
  • Includes saccadic system, smooth pursuit system, vergence system, vestibular system, optokinetic system, position maintenance system

  
• Vergence movement system


  1. Vergence movements allow focussing of an object which moves away from or towards the observer or when visual fixation shifts from one object to another at a different distance

  2. Vergence movements are very slow disconjugate movements
• Vestibular eye movement system

  1. Vestibular movements are usually effective in compensating for the effects of head movements in disturbing visual fixation

  2. These movements operate through the vestibular system and constitute the vestibulo ocular reflex (VOR)

  
• Optokinetic system


  1. The system helps to hold the images of the seen world steady on the retinae during sustained head rotation

  2. It becomes operative when the vestibulo-ocular reflex (VOR) gets fatigued after 30 seconds

  3. It consists of a smooth pur suit movement following the moving scene, succeeded by a rapid saccade in the opposite direction (Optokinetic nystagmus)

  
• Position maintenance system


  1. This system helps to maintain a specific gaze position by means of rapid micromovements called 'flicks' and slow micromovements called 'drifts'

  2. This system co-ordinates with other systems

  3. Neural pathway for this system is believed to be the same as for saccades and smooth pursuits
• Binocular single vision


  When a normal individual fixes his visual attention on an object of regard, the image is formed on the fovea of both the eyes separately; but the individual perceives a single image

  
• Development of binocular single vision


  • It is a conditioned reflex which is not present since birth but is acquired during first 6 months and is completed during first few years
  • At birth there is no central fixation and the eyes move randomly
  • By the first month of life fixation reflex starts developing and becomes established by 6 months
  • By 6 months, the macular stereopsis and accommodation reflex is fully developed
  • By 6 years of age full visual acuity (6/6) is attained and binocular single vision is well developed

  
• Prerequisites for development of binocular single vision

  • Straight eyes starting from the neonatal period, with precise coordination for all directions of gaze (motor mechanism)
  • Reasonably clear vision in both eyes so that similar images are presented to each retina (sensory mechanism)
  • Ability of visual cortex to promote binocular single vision (mental process)

  
• Grades of binocular single vision

  • Simultaneous perception
  • Fusion
  • Stereopsis

  
• Simultaneous perception


  The power to see two dissimilar objects simultaneously
• Fusion


  The power to superimpose two incomplete but similar images to form one complete image
• Stereopsis


  The ability to perceive the third dimension (depth perception)
• Anomalies of binocular vision


  • Suppression
  • Amblyopia
  • Abnormal retinal correspondence (ARC)
  • Confusion
  • Diplopia

  
• Suppression


  A temporary active cortical inhibition of the image of an object formed on the retina of the squinting eye

  
• Amblyopia


  A partial reversible loss of vision in one or both eyes, for which no cause can be found by physical examination of the eye

  
• Types of amblyopia


  • Strabismic amblyopia
  • Stimulus deprivation amblyopia
  • Anisometropic amblyopia
  • Isoametropic amblyopia
  • Meridional amblyopia
• Clinical characteristics of an amblyopic eye

  • Visual acuity is reduced
  • Effect of neutral density filter
  • Crowding phenomenon is present
  • Fixation pattern may be central or eccentric
  • Colour vision is usually normal, may be affected in deep amblyopia with vision below 6/36

  
• Treatment of amblyopia


  • Occlusion therapy
  • Penalization
  • Pleoptic exercises
  • Pharmacologic manipulation using levodopa/carbidopa
  • Perceptual learning
  • Computerized vision therapy
• Abnormal retinal correspondence (ARC)

  An active cortical adjustment in the directional values of the two retinae where the fovea of the normal eye and an extrafoveal point on the retina of the squinting eye acquire a common visual direction

  
• Types of diplopia


  • Binocular diplopia
  • Uniocular diplopia
• Binocular diplopia


  Occurs due to formation of image on dissimilar points of the two retinae
• Uniocular diplopia


  An object appears double from the affected eye even when the normal eye is closed

  
• Causes of uniocular diplopia

  • Subluxated clear lens
  • Subluxated intraocular lens
  • Double pupil
  • Incipient cataract
  • Keratoconus

  
• Strabismus


  A misalignment of the visual axes of the two eyes