Strabismus

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      • Definition
        • Normally visual axis of the two eyes are parallel to each other in the ‘primary position of gaze’ and this alignment is maintained in all positions of gaze.
        • A misalignment of the visual axes of the two eyes is called squint or strabismus.
      • Classification of strabismus
        • Apparent squint or pseudostrabismus.
        • Latent squint (Heterophoria) (MCQ)
        • Manifest squint (Heterotropia) (MCQ)
          • Concomitant squint
          • Incomitant squint.
      • Pseudostrabismus
        • In pseudostrabismus (apparent squint), the visual axes are in fact parallel, but the eyes seem to have a squin
        • Pseudoesotropia or apparent convergent squint is associated with
          • a prominent epicanthal fold (which covers the normally visible nasal aspect of the globe and gives a false impression of esotropia)
          • negative angle kappa.
        • Pseudoexotropia or apparent divergent squint may be associated with
          • hypertelorism, a condition of wide separation of the two eyes
          • positive angle kappa.
      • Heterophoria
        • Heterophoria also known as ‘latent strabismus’,
        • the tendency of the eyes to deviate is kept latent by fusion.
        • Therefore, when the influence of fusion is removed the visual axis of one eye deviates away.
        • Practically a small amount of heterophoria is of universal occurrence and is known as ‘physiological heterophoria’.
      • Types of heterophoria
        • Esophoria.
          • It is a tendency to converge.
          • It may be:
            • Convergence excess type
              • esophoria greater for near than distance
            • Divergence weakness type
              • esophoria greater for distance than near
            • Non-specific type
              • esophoria which does not vary significantly in degree for any distance
        • Exophoria.
          • It is a tendency to diverge
          • It may be:
            • Convergence weakness type
              • exophoria greater for near than distance).
            • Divergence excess type
              • exophoria greater on distant fixation than the near
            • Non-specific type
              • exophoria which does not vary significantly in degree for any distance
        • Hyperphoria.
          • It is a tendency to deviate upwards,
        • Hypophoria
          • it is a tendency to deviate downwards
        • Cyclophoria.
          • It is a tendency to rotate around the anteroposterior axis.
          • When the 12 O’clock meridian of cornea rotates nasally, it is called incyclophoria
          • when the 12 O’clock meridian of cornea rotates temporally it is called excyclophoria.
      • Etiology
        • Anatomical factors responsible for development of heterophoria include:
          • Orbital asymmetry.
          • Abnormal interpupillary distance (IPD)
            • A wide IPD is associated with exophoria and small with esophoria.
          • Faulty insertion of extraocular muscle.
          • A mild degree of extraocular muscle weakness.
          • Anomalous central distribution of the tonic innervation of the two eyes.
          • Anatomical variation in the position of the macula in relation to the optical axis of the eye.
        • Physiological factors
          • Age
            • Esophoria is more common in younger age group
            • Exophoria which is more often seen in elderly.
          • Role of accommodation.
            • Increased accommodation is associated with esophoria (as seen in hypermetropes and individuals doing excessive near work)
            • Decreased accommodation is associated with exophoria (as seen in simple myopes).
          • Role of convergence.
            • Excessive use of convergence may cause esophoria (as occurs in bilateral congenital myopes)
            • Decreased use of convergence is often associated with exophoria (as seen in presbyopes).
          • Dissociation factor such as prolonged constant use of one eye may result in exophoria as occurs in
            • individuals using uniocular microscope
            • watch makers using uniocular magnifying glass
          • Depending upon the symptoms ,heterophoria can be divided into compensated and decompensated.
        • Compensated heterophoria.
          • It is associated with no subjective symptoms.
          • Compensation of heterophoria depends upon the
            • reserve neuro-muscular power to overcome the muscular imbalance
            • individual’s desire for maintenance of binocular vision.
        • Decompensated heterophoria.
          • Symptoms of muscular fatigue
            • Headache and eyeache after prolonged use of eyes, which is relieved when the eyes are closed.
            • Difficulty in changing the focus from near to distant objects of fixation or vice-versa.
            • Photophobia due to muscular fatigue is not relieved by using dark glasses, but relieved by closing one eye.
          • Symptoms of failure to maintain binocular single vision
            • Blurring or crowding of words while reading;
            • Intermittent diplopiadue to temporary manifest deviation under conditions of fatigue
            • Intermittent squint (without diplopia) which is usually noticed by the patient’s close relations or friends.
          • Symptoms of defective postural sensations
            • Cause problems in judging distances and positions especially of the moving objects.
            • This difficulty may be experienced by cricketers, tennis players and pilots during landing.
      • Examination of a case of heterophoria
        • Testing for vision and refractive error.
          • It is most important, because a refractive error may be responsible for the symptoms of the patient or for the deviation itself.
        • Cover-uncover test. (MCQ)
          • It tells about the presence and type of heterophoria.
          • To perform it, one eye is covered with an occluder and the other is made to fix an object.
          • In the presence of heterophoria, the eye under cover will deviate.
          • After a few seconds the cover is quickly removed and the movement of the eye (which was under cover) is observed.
          • Direction of movement of the eyeball tells the type of heterophoria (e.g., the eye will move outward in the presence of esophoria)
          • speed of movement tells whether recovery is slow or rapid.
        • Prism cover test.
          • Maddox rod test.
            • A Maddox rod consists of many glass rods of red colour set together in a metallic disc
            • The Maddox rod converts the point light image into a line
            • Patient is asked to fix on a point light in the centre of Maddox tangent scale  at a distance of 6 metres.
            • A Maddox rod is placed in front of one eye with axis of the rod parallel to the axis of deviation
            • The Maddox rod converts the point light image into a line. Thus, the patient will see a point light with one eye and a red line with the other.
            • Due to dissimilar images of the two eyes, fusion is broken and heterophoria becomes manifest.
        • Maddox wing test.
          • Maddox wing is an instrument by which the amount of phoria for near (at a distance of 33 cm) can be measured.
          • It is also based on the basic principle of dissociation of fusion by dissimilar objects.
      • Treatment
        • It is indicated in decompensated heterophoria (i.e., symptomatic cases).
        • Correction of refractive error when detected is most important.
        • Orthoptic treatment.
          • It is indicated in
            • patients with heterophoria without refractive error
            • those where heterophoria and/or symptoms are not corrected by glasses.
        • Aim of orthoptic treatment is to improve convergence insufficiency and the fusional reserve.
        • Orthoptic exercises can be done with synoptophore.
        • Prescription of prism in glasses
        • Surgical treatment.
          • Aim of the surgical management is to strengthen the weak muscle or weaken the strong muscle.
      • Concomitant strabismus
        • It is a type of manifest squint
        • amount of deviation in the squinting eye remains constant (unaltered) in all the directions of gaze
        • there is no associated limitation of ocular movements.
      • Etiology
        • Binocular vision and coordination of ocular movements
          • not present since birth
          • starts by the age of 3-6 months
          • completed up to 5-6 years.
          • Therefore, any obstacle to the development of these processes may result in concomitant squint.
          • These obstacles can be arranged into three groups, namely: sensory, motor and central.
        • Sensory obstacles.
          • These are the factors which hinder the formation of a clear image in one eye.
          • These include:
            • Refractive errors
            • Prolonged use of incorrect spectacles
            • Anisometropia
            • Corneal opacities,
            • Lenticular opacities
            • Diseases of macula (e.g., central chorioretinitis),  
            • Optic atrophy
            • Obstruction in the pupillary area due to congenital ptosis.
        • Motor obstacles.
          • These factors hinder the maintenance of the two eyes in the correct positional relationship in primary gaze and/or during different ocular movements.
          • A few such factors are:
            • Congenital abnormalities of the shape and size of the orbit,
            • Abnormalities of extraocular muscles such as faulty insertion, faulty innervation and mild paresis,
            • Abnormalities of accommodation, convergence and AC/A ratio.
        • Central obstacles.
      • Clinical features of concomitant strabismus (in general)
        • Ocular deviation can be
          • Unilateral (monocular squint)
          • alternating (alternate squint).
          • Inward deviation (esotropia)
          • outward deviation (exotropia)
          • vertical deviation (hypertropia).
        • Primary deviation is equal to secondary deviation
        • Ocular deviation is equal in all the directions of gaze.
        • Ocular movements are not limited in any direction
        • Refractive error may or may not be associated
        • Suppression and amblyopia may develop as sensory adaptation to strabismus..
        • A-V patterns may be observed in horizontal strabismus.
      • Types of concomitant squint
        • Convergent squint (esotropia)
        • Divergent squint (exotropia)
        • Vertical squint (hypertropia)
      • Convergent squint
        • denotes inward deviation of one eye
        • It can be
          • unilateral (the same eye always deviates inwards and the second normal eye takes fixation) or
          • alternating (either of the eyes deviates inwards and the other eye takes up fixation, alternately).
      • Clinico-etiological types
        • Accommodative esotropia.
          • It occurs due to overaction of convergence associated with accommodation reflex.
          • It is of three types: refractive, non-refractive and mixed.
          • Refractive accommodative esotropia:
            • It usually develops at the age of 2 to 3 years
            • associated with high hypermetropia (+4 to +7 D).
            • Mostly it is for near and distance
            • It is  fully correctable by use of spectacles.
          • Non-refractive accommodative esotropia:
            • It is caused by abnormally AC/A (accommodative convergence/accommodation) ratio.
            • This may occur even in patients with no refractive error.
            • Esotropia is greater for near than that for distance
            • It is fully corrected by adding +3 DS for near vision.
          • Mixed accommodative esotropia
        • Non-accommodative esotropias.
          • Essential infantile esotropia.
            • It usually presents at 1-2 months of age.
            • It is characterised by  fairly large angle of squint (> 30o)
          • Essential acquired or late onset esotropia.
        • Secondary esotropia.
          • Sensory deprivation esotropia.
          • It results from monocular lesions (in childhood) which either prevent the development of normal binocular vision or interfere with its maintenance.
          • Examples of such lesions are: cataract, severe congenital ptosis, aphakia, anisometropia, optic atrophy, retinoblastoma, central chorioretinitis
        • Consecutive esotropia.
          • It results from surgical overcorrection of exotropia.
      • Divergent squint
        • Concomitant divergent squint (exotropia) is characterised by outward deviation of one eye while the other eye fixates.
      • Evaluation of a case of concomitant strabismus
        • Testing of vision and refractive error.
          • It is most important, because a refractive error may be responsible for the symptoms of the patient or for the deviation itself.
        • Cover tests
          • Direct cover test
            • It confirms the presence of manifest squint.
            • To perform it, the patient is asked to fixate on a point light.
            • Then, the normal looking eye is covered while observing the movement of the uncovered eye.
            • In the presence of squint the uncovered eye will move in opposite direction to take fixation, while in apparent squint there will be no movement.
            • This test should be performed for near texation (i.e., at 33 cm) distance tixation(i.e., at 6 metres).
          • Alternate cover test.
            • It reveals whether the squint is unilateral or alternate
            • It differentiates concomitant squint from paralytic squint (where secondary deviation is greater than primary).
        • Estimation of angle of deviation
          • Hirschberg corneal reflex test. (MCQ)
            • It is a rough but handy method to estimate the angle of manifest squint.
            • In it the patient is asked to fixate at point light held at a distance of 33 cm
            • deviation of the corneal light reflex from the centre of pupil is noted in the squinting eye.
            • Roughly, the angle of squint is 15o and 45o when the corneal light reflex falls on the border of pupil and limbus, respectively
          • The prism and cover test
          • Krimsky corneal reflex test.
          • Measurement of deviation with synoptophore.
        • Tests for grade of binocular vision and sensory functions.
          • Worth’ s four-dot test.
          • Test for fixation.
          • After-image test.
          • Sensory
          • Synoptophore (major amblyoscope) tests for sensory functions
        • Treatment of concomitant strabismus
          • Spectacles with full correction of refractive error
            • correct the squint in accommodative squint. (MCQ)
          • Occlusion therapy.
            • It is indicated in the presence of amblyopia.
            • Occlusion helps to improve the vision in children below the age of 10 years.
          • Preoperative orthoptic exercises.
          • Squint surgery.
            • Basic principles of squint surgery.
            • These are to
              • weaken the strong muscle by recession (shifting the insertion posteriorly)
              • to strengthen the weak muscle by resection (shortening the muscle).
            • Type and amount of muscle surgery
              • 1 mm resection of medial rectus (MR) will correct about 1°-1.5°
              • 1 mm recession will correct about 2°-2.5°.
              • 1 mm resection and recession of lateral rectus (LR) muscle will correct 1°-2°.
            • The maximum limit allowed for
              • MR resection is 8 mm
              • MR recession is 5.5 mm.
              • LR muscle resection is 10 mm
              • LR recession 8 mm
          • Postoperative orthoptic exercises

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Kenneth Wright – Strabismus And Amblyopia

Eyes before strabismus surgery