StrabismusBY: SAYED HAMZAH

Introduction •Under normal binocular viewing conditions, the

image of the object of regard falls simultaneously on the fovea of each eye (bifoveal fixation), and the vertical retinal meridians are both upright.•Any deviation from perfect ocular alignment is

called "strabismus.“•Misalignment may be in any direction—inward,

outward, up, down, or torsional.

Epidemiology•Strabismus is present in about 4% of children. •Strabismus may also be acquired, caused by cranial nerve palsies, orbital masses, orbital fractures, thyroid eye disease, or other acquired disorders.

Physiology Motor Aspects

Individual Muscle Functions Field of Action Synergistic & Antagonistic Muscles (Sherrington's Law) Yoke Muscles (Hering's Law) Development of Binocular Movement

Physiology Sensory Aspects

Binocular Vision Sensory Fusion & Stereopsis Sensory Changes in Strabismus:

These changes include diplopia, suppression, anomalous retinal correspondence, and eccentric fixation.

Diplopia• each fovea receives a different image.• The foveal image is localized straight ahead,

while the peripheral image of the same object in the other eye is localized in some other direction. • Thus, the same object is seen in two places

(diplopia).

Suppression◦ The images seen by one eye become

predominant and those seen by the other eye are not perceived (suppression).

◦ Suppression is a common sensory adaptation in childhood strabismus.

◦ Suppression takes the form of a scotoma in the deviating eye only under binocular viewing conditions.

Amblyopia◦ This is reduced visual acuity in the absence of detectable

organic disease in one eye). ◦ The three clinical causes of amblyopia include amblyopia

due to visual deprivation (eg, congenital cataract or optic nerve hypoplasia), amblyopia due to strabismus, and amblyopia due to unequal refractive error (anisometropia). Often, more than one etiology is present.

Anomalous Retinal Correspondence

◦ This is a sensory adaptation that occurs in strabismus under binocular viewing conditions.

◦ Heterotropia leads to suppression in the nonfixating eye and a shift in the visual direction of the deviated eye.

◦ This shift in visual direction typically offsets the amount of motor deviation and prevents the perception of diplopia.

Eccentric Fixation◦ An eye with gross eccentric fixation will not point

toward the light source but will appear to be looking in some other direction (a positive or negative angle kappa).

◦ More subtle degrees of eccentric fixation can be detected by an ophthalmoscope that projects a small fixation target onto the retina.

Examination History

◦Family History◦Age at Onset◦Type of Onset◦Type of Deviation◦Fixation

Examination Visual Acuity

◦ Each eye is evaluated by itself, since binocular testing will not reveal poor vision in one eye.

◦ The target should be as small as the child's age, interest, and level of alertness allow.

◦ Fixation is described as being normal if it is centrally (foveally) and steadily fixated and maintained while the eye follows a moving object.

Inspection◦ Inspection alone may show whether the

strabismus is constant or intermittent, alternating or nonalternating, and whether it is variable.

◦ The quality of fixation of each eye separately and of both eyes together should be noted.

Determination of Angle of Strabismus (Angle of Deviation)

Prism and Cover Tests ◦ Cover test◦ Uncover test◦ Alternate cover test◦ Prism and alternate cover test.

Objective Tests ◦ Hirschberg Method◦ Prism Reflex Method (Correctly Known as the "Reverse"

Krimsky Test)

Cover testing. The patient is directed to look at a target at eye level 6 m (20 feet) away. Note: In the presence of strabismus, the deviation will remain when the cover is removed.

Ductions (Monocular Rotations)◦ With one eye covered, the other eye follows a

moving target in all directions of gaze. Any decrease of rotation indicates limitation in the field of action of that muscle due to weakness of contraction or failure of relaxation of its antagonist.

Versions (Conjugate Ocular Movements)◦ Hering's law states that yoke muscles receive equal

stimulation during any conjugate ocular movement.◦ Difference in rotation of one eye relative to the other is

noted as overaction or underaction.

Testing versions. Example of paretic left superior oblique.

Disjunctive Movements◦ Convergence

◦ Convergence is an active process with a strong voluntary as well as involuntary component. An important consideration in evaluating the extraocular muscles in strabismus is convergence.

Divergence◦ Electromyography has established that

divergence is an active process, not merely a relaxation of convergence. Clinically, this function is seldom tested except in considering the amplitudes of fusion.

Sensory Examination Stereopsis Testing Suppression Testing Fusion Potential

Medical Treatment Nonsurgical treatment of strabismus includes ◦ treatment of amblyopia◦ the use of optical devices (prisms and glasses)◦ pharmacologic agents◦ orthoptics

Surgical Treatment ◦Surgical Procedures

◦ Resection and Recession◦ Shifting of Point of Muscle Attachment◦ Faden Procedure

Esotropia (Convergent Strabismus, "Crossed Eyes")◦ Esotropia is by far the most common type of

strabismus.◦ It is divided into two types: paretic (due to

paresis or paralysis of one or both lateral rectus muscles) and nonparetic (comitant).

◦ Nonparetic esotropia is the most common type in infants and children; it may be accommodative, nonaccommodative, or partially accommodative.

Nonparetic Esotropia Nonaccommodative Esotropia

◦ Infantile Esotropia◦ Acquired Nonaccommodative Esotropia

Accommodative Esotropia

Accommodative esotropia occurs when there is a normal physiologic mechanism of accommodation with an associated overactive convergence response but insufficient relative fusional divergence to hold the eyes straight.

There are two pathophysiologic mechanisms at work, singly or together:

a. Accommodative esotropia due to hyperopiab. accommodative esotropia due to a high (AC/A ratio),

Partially Accommodative Esotropia

A mixed mechanism—part muscular imbalance and part accommodative/convergence imbalance—may exist.

Paretic (Incomitant) Esotropia (Abducens Palsy) • Incomitant strabismus results from paresis or restriction of

action of one or more extraocular muscles. • Incomitant esotropia is usually due to paresis of one or

both lateral rectus muscles as a result of unilateral or bilateral abducens nerve.

• Other causes of incomitant strabismus are fracture of the medial orbital wall with entrapment of the medial rectus muscle, thyroid eye disease with contracture of the medial rectus muscles, and Duane's retraction syndrome

Incomitant strabismus (paralytic). Paralysis of right lateral rectus muscle, with left eye fixing.

Pseudoesotropia• Pseudoesotropia is the illusion of crossed eyes in an infant or toddler when

no strabismus is present. • This appearance is usually caused by a flat, broad nasal bridge and

prominent epicanthal folds that cover a portion of the nasal sclera, giving the impression that the eyes are crossed.

Exotropia (Divergent Strabismus)◦ Exotropia is less common than esotropia,

particularly in infancy and childhood. ◦ Its incidence increases gradually with age. ◦ Not infrequently, a tendency to divergent

strabismus beginning as exophoria progresses to intermittent exotropia and finally to constant exotropia if no treatment is given.

Alternative Classification of Exotropia◦ Basic Exotropia◦ Divergence Excess◦ Pseudodivergence Excess◦ Convergence Insufficiency

Intermittent Exotropia◦ Intermittent exotropia accounts for well over half

of all cases of exotropia. ◦ The onset of the deviation may be in the first

year, and practically all have presented by age 5.◦ A characteristic sign is closing one eye in bright

light.◦ The manifest exotropia first becomes noticeable

with distance fixation.

Child with intermittent exotropia squinting in sunlight.

Constant Exotropia◦ Constant exotropia is less common than intermittent

exotropia. ◦ It may be present at birth or may occur when intermittent

exotropia progresses to constant exotropia.

Right exotropia.

A & V Patterns◦ A horizontal deviation may be vertically

incomitant, ie, the deviation is different in upgaze versus downgaze (A or V pattern).

◦ An A pattern is diagnostically significant when greater than 10 PD and a V pattern when greater than 15 PD.

Hypertropia Vertical deviations are customarily named according to the high eye, regardless of which eye has the better vision and is used for fixation.

Right hypertropia.

Head tilt test (Bielschowsky test).

Special Forms of Strabismus◦ Duane Retraction Syndrome◦ Dissociated Vertical Deviation◦ Brown's Syndrome (Superior Oblique Tendon

Sheath Syndrome)

Heterophoria◦ Heterophoria is deviation of the eyes that is held

in check by binocular vision.◦ The symptoms of heterophoria may be clear-cut

(intermittent diplopia) or vague ("eyestrain" or asthenopia).

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