Strabismus (Squint) PDF

Strabismus (squint) Introduction: The visual axis (optical axis):it is a line passes from the fovea, through the nodal point of the eye, to the point of fixation. The anatomical axis: is a line passing from the posterior pole through...

Strabismus (squint) Introduction: The visual axis (optical axis):it is a line passes from the fovea, through the nodal point of the eye, to the point of fixation. The anatomical axis: is a line passing from the posterior pole through the centre of the cornea. Angle kappa is the angle, usually about 5°, subtended by the visual and anatomical axes (fig. 1) Fig. 1. Angle kappa Negative angle kappa occur in myopia due to large globe. Large positive angle kappa occur in hypermetropia. Orthophoria: there is no deviation of the eye even when the fusion is broken. 1 Ocular motility( fig. 2) Fig. 2. Extra-ocular muscles origin and insertion Ocular movement: 1. Ductions: are monocular movements around the axes of Fick. They consist of adduction, abduction, elevation, depression, intorsion and extorsion. 2. Vergence: are binocular, simultaneous, disjugate movements (disjugate- in opposite directions) e.g Convergence is simultaneous adduction (inward turning); divergence is outwards movement. 2 3. Versions: are binocular, simultaneous, conjugate movements (conjugate – in the same direction, so that the angle between the eyes remains constant). Dextroversion and laevoversion (gaze right and gaze left), elevation (upgaze) and depression (downgaze). Dextroelevation and dextrodepression (gaze up and right; gaze down and right) and laevoelevation and laevodepression (gaze up and left; gaze down and left) (fig. 3). Fig. 3. Cardinal position of the gaze. Laws of ocular motility: Agonist–antagonist pairs are muscles of the same eye that move the eye in opposite directions. Synergists are muscles of the same eye that move the eye in the same direction. Yoke muscles (contralateral synergists) are pairs of muscles, one in each eye, that produce conjugate ocular movements. The Sherrington law of reciprocal innervation states that increased innervation to an extraocular muscle (e.g. right medial rectus) is accompanied by a reciprocal decrease in innervation to its antagonist (e.g. right lateral rectus) The Hering law of equal innervation states that during any conjugate eye movement, equal and simultaneous innervation flows to the yoke muscles. 3  Binocualr single vision(BSV) It involves the simultaneous use of both eyes with bifoveal fixation, , so that each eye contributes to a common single perception of the object of regard, it required. 1. Approximately equal image clarity and size for both eyes. 2. Accurate neuromuscular development and coordination. 3. Normal retinal (retinocortical) correspondence (NRC) resulting in ‘cyclopean’ viewing. 4. Normal routing of visual pathways with overlapping visual fields. 5. Binocularly driven neurones in the visual cortex. GRADES OF BSV (fig. 4): Grade 1 (Simultaneous Perception): It refers to the simultaneous perception of two dissimilar objects simultaneously from the two eyes, by the cerebral cortex. Grade 2 (Fusion): It is producing a composite picture of two similar objects, each of which is incomplete in a different manner. Grade 3 (Stereopsis): It refers to the ability to obtain an impression of depth by the superimposition of two pictures of the same object taken from a slightly different angle. Fig. 4. Grades of BSV. 4 Advantages of BSV: 1. Stereopsis. 2. Large field. 3. Visual field defect in one eye masked by the other eye. Abnormalities of BSV: Confusion: This occurs when corresponding retinal points (foveae) are stimulated by dissimilar stimuli and fusion cannot occur.. Diplopia: The stimulation of non-corresponding retinal points by the same image produces diplopia, or double vision. Adaptations to abnormalities of BSV: Suppression: This is a cortical mechanism to ignore one of the images causing diplopia or confusion. Later on amblyopia developed. Abnormal retinal correspondence: If the fovea of one eye is paired with a non-foveal area of retina in a deviating fellow eye. Abnormal head posture: This occurs as a motor adaptation to diplopia in adults who cannot suppress, or in people with strabismus who have good binocular potential. Horizontal deviations produce face-turns horizontally, vertical deviations result in elevation or depression of the chin, and torsional deviations cause a head tilt to one or other shoulder. Visual acuity (VA) Visual acuity is defined as the power to differentiate object from each other and to appreciate their details. It is highly complex function. Optically, the visual acuity is expressed as the minimum visual angle substended at the nodal point when accommodation is entirely relaxed (fig. 5). It called 20/20, 6/6 or 1.0 vision. 5  Fig. 5. The visual angle. Visual development: At birth: blinking response to bright light At 2 months: fixation well developed At 6 months: following. At 2 years: Snellen acuity at adult level At 7 years: stereoacuity at adult level Testing of VA in preverbal children(5 years old). VA can be tested using Snellen’s chart or log mar test. Amblyopia Amblyopia (amblyos, blunt, and opia, vision) is the unilateral, or rarely bilateral, decrease in best corrected visual acuity (VA) caused by form vision deprivation and/or abnormal binocular interaction. Types of Amblyopia:- 1. Strabismic amblyopia results from abnormal binocular interaction where there is continued monocular suppression of the deviating eye. 2. Stimulus deprivation amblyopia results from vision deprivation. is typically caused by opacities in the media (e.g. cataract) or ptosis that covers the pupil. 3. Anisometropic amblyopia is caused by a difference in refractive error between the eyes and may result from a difference of as little as 1 dioptre. 4. Bilateral ametropic amblyopia results from high symmetrical refractive errors, usually hypermetropia. 5. Meridional amblyopia results from image blur in one meridian. It can be unilateral or bilateral and is caused by uncorrected astigmatism (usually >1 D). 7 Management of amblyopia: Prevention: Screening programmes to identify amblyopia in preschool children. Correct underlying causes: Any refractive error should be corrected with full-time spectacle wear and any amblyogenic stimuli, such as cataract or ptosis, ameliorated as soon as possible. Treatment: Amblyopia must be treated before the end of the sensitive period of visual development (about age 8–9). 1. Occlusion: Patching of the better-seeing eye to encourage use of the amblyopic eye is the mainstay of treatment, Careful monitoring is essential as excessive patching may induce amblyopia in the better eye. 2. Atropine penalization: Atropine 1% drops can be given to blur the vision in the better eye to around 6/18. Strabismus (squint) It is a misaligenment of the visual axis which is normally In BSV the visual axes of the two eyes intersect at the point of fixation. Apparent squint (pseudo- strabismus)) Classification of Latent squint squint (heteropohoria) True squint Concomitant squint Manifest squint (heterotropia) Incomitant squint (paralytic or restrictive) 8 I. Apparent squint: 1. Apparent esotropia: causes of apparent esotropia include :myopia(negative angle kappa)-enophthalmos- epicanthus(fig. 8)- narrow interpupillary distance. 2. Apparent exotropia: causes of apparent esotropia include :hypermetropia(large angle kappa)-exophthalmos- lateral tarsorrhaphy- wide interpupillary distance(fig. 8). 3. Apparent hypertropia; occure in lid coloboma. 4. Apparent hypotropia: occur wih ptosis(fig. 9). Fig. 8. (right) epicanthus. (left) wide interpupillary distance. Fig. 9. Left eye ptosis 9  II. True squint A. Latent squint (heterophoria): a latent deviation controlled by fusional mechanisms; may manifest under certainconditions (fatigue, illness, stress, or tests that dissociate eyes [alternat e cover test]. It is either esophoria, exophoria and hyperphoria. Aetiology: refractive error (hypermtropia– esophoria) (myopia– exophoria) muscle weakness. Clinically: eye strain, diplopia on prolonged work. Treatment: 1. Correct the refractive error 2. Surgical treatment. B. Manifested squint (heterotropia) It either concomitant (non paralytic) or incomitant (paralytic or restrictive). In a concomitant squint the variability of the angle of deviation is within 5 Δ in different horizontal gaze positions. In an incomitant deviation the angle differs in various positions of gaze as a result of abnormal innervation or restriction. I. Concomitant esotropia (manifest convergent squint) Classification of esotropia: 1. Primary esotropia: may be intermittent or constant, accommodative or non-accommodative. 2. Secondary or sensory esotropia: it is caused by a unilateral reduction in visual acuity that interferes with or abolishes fusion; causes can include cataract, optic atrophy or hypoplasia, Fundus examination under mydriasis is therefore essenttial in all children with strabismus. 3. Consecutive esotropia: following previous exotropia, with or without prior surgery. 10  Infantile esotropia: it is Primary constant non-accommodative esotropias it is an idiopathic esotropia developing within the first 6 months of life, with no significant refractive error.  The angle is usually fairly large (>30 Δ) and stable (fig. 10).  There is cross-fixating in side gaze, so that the child uses the left eye in right gaze and the right eye on left gaze.  Latent nystagmus (LN) is seen only when one eye is covered.  Inferior oblique overaction may be present initially or develop later. Fig. 10. Large angle infantile esotropia. Differential diagnosis includes bilateral congenital sixth nerve palsy, secondary (sensory) esotropia due to organic eye disease. Initial treatment: the eyes should be surgically aligned by the age of 12 months, and at the very latest by the age of 2 years. Either by recession of both medial recti or unilateral medial rectus recession with lateral rectus resection. Accommodative esotropia It is either: 1. Non refractive accommodative esotropia. 2. Refractive accommodative esotropia (fully refractive or partially refractive) Non-refractive accommodative esotropia: normally near vision involves both accommodation and convergence, and have a fairly constant relationship to each other Accommodative convergence to accommodation 11 (AC/A). Abnormalities of the AC/A ratio ( excess accomodaton) are an important cause of this types of esotropia. Treatment; bifocals may be prescribed to relieve accommodation (fig. 12) thus allowing the child to maintain bifoveal fixation and ocular alignment at near. Fig. 12. Bifocal glasses as treatment of non refractive accommodative esotropia Refractive accommodative esotropia: Fully refractive accommodative esotropia is characterized by hypermetropia with esotropia when the refractive error is uncorrected. The deviation is eliminated and BSV is present at all distances following optical correction of hypermetropia (fig. 13). 12  Fig. 13. The esotropia totally corrected with glass Partially refractive accommodative esotropia: the deviation is reduced partially with optical correction of hypermetropia (fig. 14), but not eliminated by full correction of hypermetropia. Amblyopia is frequent. It needs surgical correction for residual angle. Fig. 14. The esotropia partially corrected with glass 13  II. Concomitant exotropia (manifest divergent squint). Classification of exotropia Intermittent exotropia. Constant exotropia: it is either 1. Congenital exotropia. 2. Sensory exotropia: it is caused by a unilateral or bilateral reduction in visual acuity that interferes with or abolishes fusion; causes can include cataract, optic atrophy or hypoplasia, Fundus examination under mydriasis is therefore essenttial in all children with strabismus. 3. Consecutive exotropia: it developed spontaneously in amblyopic eye or following previous surgical overcorrection of esotropia Intermittent exotropia. It presented with exophoria which breaks dwon to exotropia under condition of visual inattention or fatigue (fig. 15), start around age of 2 years. Classification: 1. Distance exotropia(divergence excess): the angle of deviation is greater for distance than near, it either true or simulated. 2. Near exotropia (convergence insufficiency): the deviation greater for near. 3. Basic exotropia: the deviation are same for distance and near. Treatment: spectacle correction, orthoptic exercise and surgery for poor controlled or deteriorated cases 14  Fig. 15. Intermittent exotropia. Congenital infantile exotropia It is an idiopathic exotropia developing within the first 6 months of life, with no significant refractive error. The angle is usually fairly large and stable. Neurological anomalies may present. Differential Diagnosis: apparent exotropia- Intermittent exotropia sensory exotropia Treatment; surgical correction III. Incomittant strabismus (paralytic squint) Oculomotor nerve (CNIII) palsy: Note. the pupillomotor : parasympathetic fibres are located superficially in the superomedial part of the third nerve, They derive their blood supply from the pial blood vessels, whereas the main interior trunk of the nerve is supplied by the vasa nervorum (fig. 16). ‘Surgical’ lesions such as aneurysms and trauma characteristically involve the pupil by compressing the pial blood vessels and the superficially located pupillary fibres. 15  ‘Medical’ lesions such as occur in hypertension and diabetes usually spare the pupil. Fig. 16. Blood supply of oculomotor nerve Clinical picture of oculomotor nerve palsy (fig. 17):  Profound ptosis due to weakness of the levator muscle, so that diplopia may not be volunteered. Abduction and depression in the primary position Normal abduction as the lateral rectus is intact Limitation other ocular movement. Pupil dilated not reacted if the cause is surgical Fig. 17. Right oculomotor nerve palsy. 16 Investigation: required if surgical cause was suspected CT angiography MRI brain and orbits with venography lumbar puncture. Treatment:- Observation is usually appropriate in presumed microvascular cases; the majority will resolve over weeks or months. Surgical treatment of the ocular motility element and ptosis should be contemplated only after spontaneous improvement has ceased, usually not earlier than 6–12 months from onset. Or for surgical cause Trochlear nerve (CNIV) palsy: Clinical picture (fig. 18): Hypertropia in the primary position, increasing on opposite gaze Limitation of depression, most marked in adduction. A compensatory head posture: To compensate for weakness of intorsion there is contralateral head tilt to the opposite side. To alleviate the weakened depression of the eye the chin is slightly depressed; as this is most marked in adduction, the face may also be turned slightly to the opposite side. Normal abduction and elevation of the eye Fig. 18. Left trochlear nerve palsy. 17 Parks three-step test (fig. 19): Step one. In the primary position, the hypertropic eye is identified. Step two. The eyes are examined in right and left gaze to determine where the hypertropia is greater, the deviation is worse on opposite gaze – WOOG. Step three. The head is tilted to each side in turn in order to assess the muscles responsible for cyclotorsion, with observation to determine the position in which the hypertropia is worse. the deviation is better on opposite tilt – BOOT. Fig. 19. Parks three-step test of left trochlear nerve palsy. Abducent nerve (CNVI) palsy: Clinical picture (fig. 20) Double vision is characteristically worse for a distant target and less or absent for near fixation. Esotropia in the primary position due to relatively unopposed action of the medial rectus. 18  Limitation of abduction on the side of the lesion, A compensatory face turn is towards the side of the paralysed muscle in unilateral palsy. Investigation and treatment: as 3rd nerve palsy Fig. 20. Right abducent nerve palsy. Evaluation a case of strabismus A. History: A careful history is crucial for appropriate diagnosis and management of strabismus.  Duration/age of onset: this can give an indication of the nature of the strabismus.  Variability: intermittent deviations suggest some degree of binocularity.  Variation with near and distance fixation suggests an accommodative element or convergence disorder.  Laterality: a constant squint with one eye indicates high risk of amblyopia in a child; alternating fixation suggests equal acuities.  Diplopia: constant or intermittent? monocular or binocular?  Previous treatment: including spectacles, patching, and surgery.  Medical history. Family history.  In adults, occupation, hobbies, and driving history may all have a bearing on decisions about strabismus management. In children, birth and developmental history are important. 19 B. General examination A number of general examinations should be made to supplement the more specific strabismus examinations: Observation: for abnormal head posture, facial and orbital abnormalities, etc. Visual acuity. Refraction: to identify any accommodative or ametropic strabismic drive. Fundoscopy: this is mandatory to identify any underlying pathology. C. Extraocular movements should be examined to exclude a incomitant squint. D. Measurement of deviation: 1. Hirschberg test: The distance of the corneal light reflection from the centre of the pupil is noted; each millimetre of deviation is approximately equal to 7° (1° ≈ 2 prism dioptres). For example, if the reflex is situated at the temporal border of the pupil (assuming a pupillary diameter of 4 mm), the angle is about 15°; if it is at the limbus, the angle is about 45 (fig. 21) Fig. 21. Hirschberg test: (up) with angle 15°. (Down) with angle 45° 2. Krimsky and prism reflection tests: The Krimsky test involves placement of prisms in front of the fixating eye until the corneal light reflections are symmetrical, The prism reflection test involves the placement of prisms 20  in front of the deviating eye until the corneal light reflections are symmetrical (fig. 22). Fig. 22. Krimsky test 3. Cover tests: These are accurate and objective tests of ocular deviation. They give information, including: presence of any heterophoria or heterotropia, size of deviation, effect of accommodation Cover–uncover test This has two elements  Cover test This test looks for manifest deviations. One eye is covered with an opaque occluder and the other eye is observed. If the uncovered eye moves to take up fixation, a manifest deviation is present in that eye (fig. 23). Temporal movement indicates an esotropia. Nasal movement indicates an exotropia. Downward movement indicates a hypertropia. Upward deviation indicates a hypotropia. If no movement occurs, there is either no manifest deviation or there is eccentric fixation. Fig. 23. Cover test. 21  Uncover test This test looks for latent deviation. One eye is covered and then the same eye observed as the cover is removed for any corrective movement. Temporal movement indicates an esophoria. Nasal movement indicates an exophoria (fig. 24). Downward movement indicates a hyperphoria. Upward deviation indicates a hypophoria. Alternate cover test This dissociates the eye to reveal the full deviation (phoria plus tropia). The cover is placed over each eye in turn at about 2 second intervals and movement of the eye being uncovered noted as the cover moves to the other eye. Patients with heterophoria may break down to a manifest deviation after dissociation during this test. Fig. 24. Uncover test. 4. The synoptophore This is a haploscopic device in which the two eyes are dissociated so that each eye views a picture on a different slide on a plane mirror. its uses include: measurement of interpupillary distance, measurement of angle of deviation, and stereoacuity, measurement of angle kappa. 5. Maddox rod: it is a subjective dissimilar image test used to measure the angle of a heterophoria, The Maddox Rod effectively dissociates the eyes 22 since dissimilar images are presented to the eyes, removing the stimulus to fuse Treatment of strabismus Spectacle correcyoin of refractive error Treatment of amblyopia Orhtoptic exercise Surgical treatment: 1. Weakening procedures: Recession slackens a muscle by moving it away from its insertion. Disinsertion(or myectomy) involves detaching a muscle from its insertion without reattachment. Posterior fixation suture: The principle of this procedure is to suture the muscle belly to the sclera posteriorly so as to decrease the pull of the muscle in its field of action without affecting the eye in the primary position. 2. Strengthening procedures: Resection shortens a muscle to enhance its effective pull. Advancement of the muscle nearer to the limbus 23 24