• Visual axis ; line of vision ; extending from the point of fixation to the fovea• Strabismus ; misalignment of the visual axes• orthophoria ; perfect alignment of the visual axes. Most individuals have heterophoria• Heterophoria ; (latent squint) tendency of the eyes to deviate. Ocular alignment maintained with effort.• Heterotropia; (manifest squint) which is present at all times• Esophoria; latent squint inwards turning of the eyes• Esotropia; manifest squint inwards turning of the eyes• Exophoria; latent squint outwards turning of the eyes• Exotropia; manifest squint outwards turning of the eyes• Hyperphoria/hypertropia; latent/manifest squint upwards turning of eyes• Hypophoria/hypertropia; latent/manifest squint downwards turning of eyes• Anatomical axis ; is a line passing from the posterior pole through the centre of the cornia .• Angle kappa is the angle subtended by the visual and anatomical axes .
Extraocular muscles• Horizontal muscles; – Medial rectus-; Its sole action in the primary position is adduction. occulomotor nerve supply – Lateral rectus- Its sole action in the primary position is abduction.; abducens nerve supply• Vertical muscles; – Superior rectus- Primary action elevation (secondary actions are adduction and intorsion. Oculomotor r nerve supply – Inferior rectus ; The primary action is depression ; secondary actions are adduction and extortion. oculomotor nerve supply• Oblique muscles; – Superior oblque; Originates superomedial to the optic foramen. It passes forwards through the trochlea at the angle between the srperior and medial walls and is then reflected backwards and laterally to insert in the posterior upper temporal quadrant of the globe.The primary action is intorsion ;secondary actions are depression and abduction. – oculomotor nerve supply – Inferior oblique; Orginates from a small depression just behind the orbital rim lateral to the lacrimal sac. It passes backwards and laterally. To insert in the posterior lower temporal quadrant of the globe, close to the macula. The prmary action is extorsion;;secondary action are elevation and abduction . oculomotor nerve supply
• Listing plane is an imaginary coronal plane passing through the centre of rotation of the globe. The globe rotates on the X,Y and Z axes of Fick., which intersect in Listing plane .• The globe rotates left and right on the vertical Z axis.• The globe moves up and down on the horizontal X axis.• Torsional movements (wheel rotations) occur on the Y (sagittal ) axis which traverses the globe form front to back (similar to the anatomical axis of the eye )• Intorsion occurs when the superior limbus rotates nasally and extorsion on temporal rotation .
• OCULAR MOVEMENTS• Ductions – Ductions are monocular movements around the axes of Fick. They consist of adduction,abduction elevation, depression, intosion and extorrsion . They are tested by occluding the fellow eye and asking the patient to follow a target in each direction of gaze. Versions – Versions are binocular, simultaneous, conjugate movements ( in the same directon ) . – Dextroversion and laevoversion elevation and depression .These four movements bring the globe into the secondary positions of gaze by rotation around either a vertical or a horizontal X axis of Fick. – Dextroelevation and dextrodepression and laevoelevation and laevodepression . These four oblique movements bring the eyes into the tertairy positions of gaze by rotation around oblique axes in listing plane, equivalent to simultaneous movement about both the horizontal and vertical axes. – Torsional movements which maintain upright images occur on tilting of the head.
• VERGENCES• Vergences are binocular, simultaneous disjugate or disjunctive movements .Convergence is simultaneous adduction; divergence is outwards movement from a convergent position. Convergence may be voluntary or reflex. Reflex convergence has four components: – Tonic – Proximal – Fusional – Accommodative
• Positions of gaze• Six Cardinal positions of gaze are those in which one muscle in each eye has to move the eye into that position as follows: – Dextroversion – Laevoversion – Dextroelevation – Laevoelevation – Dextrodepression – Laevodepression Nine Diagnostic position of gaze are those in which deviations are measured. They consists of the six cardinal postions ,the primary position, elevation and depression .
Laws of ocular motility• Agonist ; antagonist – muscles of the same eye moving the eye in opposite direction; medial and lateral rectus• Synergists= muscles of the same eye moving it in the same direction; superior rectus and inferior oblique causing elevation• Yoke muscles= muscles of both eyes moving the eyes in same direction; medial rectus of both eyes• Sherrington law; increase in innervation to one muscle causes decreased innervation to its antagonist; medial and lateral rectus• Hering law; equal innervation flows to yoke muscles in eye movement ; medial rectus of both eyes
Consequences of squintSuppression , amblyopia , confusion and diplopia , postural changes according to strabismus1. Amblyopia – Definition – unilateral or bilateral decrease of best corrected visual acuity caused by stimulus deprivation or abnormal interaction for which there is no pathology of the eye or the visual pathway – Types • Strabismic amblyopia; abnormal interaction • Stimulus deprivation amblyopia ; form vision deprivation • Anisometropic amblyopia; difference of refractive errors in both eye ; one eye is amblyopic • Ametropic amblyopia; form vision deprivation of both eyes • Meridional amblyopia; because of astigmatism
Diagnosis• 1. visual acuity; difference of two lines in best corrected visual acuity in the absence of organic lesion• 2. neutral density filter; normal eyes have visual acuity reduced by two lines but there is no change in amblyopia
treatment• Sensitive period during which the amblyopia can be cured is below 10 years of age• 1. occlusion of the normal eye to encourage the use of the abnormal eye is the most effective treatment• 2. penalization; of the normal eye ; blurring of the vision with atropine
Objective and Principles of Therapy of Strabismus
Objective and Principles of Therapy ofStrabismus• Reversal of the deleterious sensory effects of strabismus (amblyopia, suppression and loss of stereoposis)• Best possible alignment of the eyes by medical or surgical treatment
Objective and Principles of Therapy ofStrabismus• Timing of treatment on children