Shadi Awwad, MD

I. Action of extra ocular muscles

    A. Basic Concepts
        a. Primary action of a muscle
            -When the eye is in the primary position of gaze
        b. Subsidiary action of a muscle
            -Secondary & tertiary action
    B. Horizontal Recti
        - Pure horizontal action in the primary position
    C. Vertical Recti
        -Run parallel to the orbital axis. They make therefore an angle of 23 degrees with the
          optical axis.
        -Superior Rectus:
            -Elevates in the primary position
            -Adducts & intorts as a subsidiary actions
            -When the globe is abducted 23 degrees, the orbital axis coincides with the optical
              axis. Therefore, the Superior Rectus purely and maximally elevates.
            -If the globe is adducted 67 degrees, the angle becomes 90 degrees and the SR
             purely intorts ( no elevation or adduction)
        -Inferior Rectus:
            -By the same logic, the IR depresses, adducts, and extorts
    D. Obliques
        -Superior Oblique
            -Inserts posterior to the equator, and forms an angle of 51 degree with the optical
             axis
            -It intorts in the primary position
            -Its subsidiary actions are depression and abduction
            -When the globe is adducted 51 degrees, SO purely and maximally depresses
            -When abducted 39 degrees, SO intorts only
        -Inferior Oblique
            -Same logic
            -Extorts, elevates, abducts
    E. A way to remember

    F. The six cardinal positions of gaze

II. Ocular movements

    -Ductions
    -Versions
    -Vergences

    A. Ductions
        -Definition
            -monocular eye movements consisting of adduction, abduction, elevation, and depression
        -Agonist: primary muscle moving the eye in a given direction
        -Synergist: acting in conjunction with the Agonist
        -Sherington’s Law of reciprocal innervation: when a muscled is activated, its antagonist in 
          the  same eye is suppressed
         
    B. Versions
        -Definition
            -Binocular movements with the two eyes moving synchronously & symmetrically in the
              same direction
        -Yoke Muscles: a muscle of one eye is paired with another muscle of the fellow eye to
          produce a cardinal gaze
            -Example: RLR & LMR in dextroversion
         -Hering’s Law: During any conjugate eye movement, equal & simultaneous innervation flows
          to the yoke muscles
            -The amount of innervation of both eyes is determined by the fixating eye
            -Hence if an eye with right LR palsy(primary deviation) is made to fixate, it needs high
             innervation flow to the LR. The medial rectus of the left eye will receive equal innervation
             producing  Left eye esodviation and hence Secondary deviation    
    C. Vergences

        -Definition
            -Binocular movements with the two eyes moving synchronously & symmetrically in
              opposite directions
        -Convergence can be voluntary or reflex, such as :
            a. Tonic
            b. Proximal: induced by the psychological awareness of a near object
            c. Fusional: to fuse a near object due to bitemporal retinal image disparity
            d. Accomodation: induced by the act of accomodation. 
                -Each diopter of accomodation produces a constant increment of convergence ( AC/A)
        -Divergence:
            -Fusional divergence
                -Like fusional convergence, initiated by binasal retinal disparity

III. Motility Concepts

    -Comitent deviation: 
        -Definition: 
            -Same amplitude of deviation in all gazes
        -Present in cases of squint due to occulomotor inbalance
    -Non-comitent deviation: 
        -Definition:
            -Amplitude of the deviation changes with gaze. Found in paretic or 
          restricted muscles.
        -Example: a right VIth nerve palsy will initially produce a esodeviation which is comitent: i.e 
          the deviation increases with right gaze and decreases on left gaze. With time, however, the 
          right medial rectus might become restricted due to the continuous  tonic unopposed 
          contraction. By Hering's law, the yoke muscle of the right medial rectus (left lateral rectus) 
          will be stimulated as well. Now by Sherington's law, the antagonist of the left lateral 
          rectus( left medial rectus) will be inhibited. This will produce esotropia mostly when gazing 
          to the left. The end result is esodeviation that seems to be pretty much constant in all gazes, 
          and  whether the initially diseased muscle is the right lateral rectus or the left medial rectus 
          is difficult to know by routine exam. Hence, a initially non comitent deviation might end 
          up becoming comitent. 
          
    -AC/A:
        -Definition:
            -Accomodative Convergence / Accomodation  ratio
        -Normal values: 4-5 diopers
            -Every 1 diopter of accomodation gives 4-5 diopters of convergence
        -High ratio : convergent squint during convergence or accomodation
        -Low ratio : divergent squint when looking at near object
    -Fusional Vergence:
        -Produces corrective eye movements to overcome retinal image disparity
    -Fusional Amplitude
        -Maximum amount of eye movement produced by fusional vergence
    -Fusional convergence:
        -Normal: 15 diopters for distance, 35 diopters for near
    -Fusional divergence:
        -Less in amplitude
    -Fusional convergence helps control a latent divergent squint (exophoria) whereas Fusional
      divergence helps controlling esophoria
    -Fusional vergence amplitude :
        -May decrease by fatigue or illness : Phoria becoming Tropia
        -It might be increased by orthoptic exercises, which are done best for near( for convergence
          insufficiency)
    -Binocular Single Vision
        -Definition:
            -Cortical Phenomenon gained by unifying the separate retinal images
        -Macular Binocular Single Vision
            -Begins to function in the first 2-3months of life. If deprived, it never develops®  poor
              vision & nystagmus
            -Serves fixation
            -Once developed, it needs reinforcement until age 9 years or so otherwise amblyopia
             settles in
            -Cones & Parvocellular ganglions : 14 " arc of stereopsis
        -Extra-Macular Binocular Single Vision
            -Mature at birth
            -Is needed for fusional vergence while the macular is needed for fine tuning
            -Rods , Magnocellular ganglions : 200" arc of stereopsis (best is 60)
            -Misalignment tolerance for fusion to be maintained: 8 prism diopters
    -Fusion is different from Stereopsis
        -Periphery is better for fusion ( faster)
        -Center is better for stereopsis (shape & color)
    -Double Vision
        -Defintion:
            -Misalignment of the visual axes
        -It can be latent (phoria) or manifest (tropia)
        -If it’s manifestÞ confusion and diplopia
    -Confusion
        -Definition
            -Superimposition of 2 different objects into the same position
        -Two different objects stimulating corresponding retinal points
    -Diplopia
        -Definiton
            -One object stimulating 2 non corresponding retinal points
        -Compensatory mechanisms
            -Suppression
            -Amblyopia
            -Abnormal Retinal Correspondence
    -Suppression:
        -Definition:
            -Subconscious active neglect of one eye’s input that occurs only when both eyes are open
        -When the fixating eye is covered®: suppression in the fellow eye ceases immediately and the
          squinting eye takes up fixation
    -Strabismic Amblyopia
        -Definition:
            - ¯ BCVA > = 2 Snellen lines when the involved eye is made to fixate
    -Abnormal Retinal Correspondence
        -Defintion:
            -Crude type of binocular vision obtained despite the presence of squint
    -Classification of Amblyopia
        -Strabismic
        -Anisometropic
        -Stimulus deprivation
        -Meridional (astigmatism)
    -Amblyopia/squint relationship
         -Stimulus deprivation gives amblyopia, which disrupts fusional vergence, giving rise to  
          sensory deviation (squint)
    -Visual Maturity
        -Critical Period 
            -From 16-18 weeks of age
            -Fixation reflex starts to develop
            -To produce foveal fixation
            -To maintain foveal fixation
        -Sensitive Period: 
            -Birth till 9-12 years of age
            -Visual system is still plastic& sensitive to any change in the visual acuity
    -Example of management of a case of esotropia
               -3 year-old child developing esotropia with fixation preference in OD:  LET  
               -Suppression is then followed by Amblyopia OS
               -Patching OD will stimulate OS and might reverse the amblyopia
                -Patient is assessed at intervals of age in years x 1 week/ 1 year (e.g 3 years gives a F/U Q 3 
                 weeks)
                -When amblyopia resolves, surgery is performed

IV. Misalignments in the visual axes

    A. Heterophoria
        -Latent tendency of the eyes to deviate. It is normally controlled by fusional mechanism which 
         provide binocular vision or avoid diplopia
    B. Heterotropia
        -Misalignment of the eyes which is manifest
        -Can be unilateral or alternating
            -In Unilateral, only one eye is preferred for fixation, while the other deviates constantly® 
             prone to defective central vision during visual maturity
            -In Alternating, either eye deviates, and hence developing similar vision
    C. Orthophoria
        -Exact ocular balance
        -Oculomotor apparatus is in perfect equilibrium: the visual axes always intersect at the
         object of visual regard 

V. Tests for Motility & Strabismus

        -Used to approximate the extent of deviation
        -A light is shone into the eyes, and its reflex on the cornea is assessed.
        -Normally, the reflex should be slightly nasal to the center of the cornea
        -If the reflex is on the pupillary border, the deviation is around 15°
        -If the reflex is on the limbal margin, the deviation is around 45°

        
         Fig. 1: The light reflex is more or less in the 
         center of the pupils in both eyes. 
 
        
         Fig. 2: The light reflex is nearly on the temporal 
         pupillary border in the left eye, suggesting < 15
         degree of esotropia.

    B. Krimski’s Test

        -A prism is used to make the deviated reflex come to the center of the pupil, symmetrical to 
         the fellow eye. The deviation equals the necessary prism used in diopters
        -Note that A prism shifts the light towards its base, and the image towards its apex

    C. Prism Cover Test

        -Measures the exact heterotropic deviation
        -The patient is made to fixate at a target, then one eye is covered. I the uncovered eye moves 
          to take fixation, then heterotropia is diagnosed.
        -If the eye moves nasally: exotropia
        -If the eye moves temporally: esotropia
        -If the eye doesn’t deviate, repeat same for the fellow eye

    D. Alternate Cover Test

        -Measures Heterophoria+Heterotropia
        -One eye is covered for 2 seconds while the other is made to fixate, then the cover is shifted 
          to the other eye.

    E. 4D Base Out Prism

        -Assesses macular binocular vision and suppression
        -A 4D prism is put base out in front one eye while the patient is looking at a target
        -If the patient is fixating with the eye in question, it should deviate inward to follow the new 
          image
        -Patients who fail the test in one eye are called monofixators. These can fuse an object (by 
          extra macular binocular vision), but don’t have much of stereopsis (macular binocular vision)