PART III. CLASSIFICATION OF UVEITIS
STRABISMUS Definition of Strabismus
The visual axis is an imaginary line that connects an object in space with the fovea. In a person with normal ocular, sensory and motor systems, the visual line in each eye intersects at the object in space and there is binocular fixation. If visual lines are not directed at the same fixation point, fixation is by one eye only.
Normal development of stereoscopic vision requires binocular, simultaneous use of each fovea during the critical time that occurs early in life.
Amblyopia is a condition in which there is a unilateral or bilateral decrease in visual acuity that is not fully attributable to organic ocular abnormalities. It is usually caused by opacities in the media, high refractive errors, anisometropia (difference in refractive errors of the 2 eyes) or ocular misalignment or strabismus during visual immaturity. The correction of amblyopia depends on the maturity of the visual system at the onset and the duration of the abnormal visual experience. Treatment consists of occlusion (patching) of the better eye to force the use of the amblyopic eye and the correction of the underlying cause.
STRABISMUS Definition of Strabismus
Strabismus means ocular misalignment of whatever cause. When the eyes are dissociated or not aligned, strabismus is present.
Orthophoria refers to the ideal condition of ocular balance, so eyes are aligned in all directions of gazes at all distances even after occluding one eye.
Types of Strabismus:
A. According to direction of deviation:
1. Horizontal - esodeviation, exodeviation 2. Vertical - hyperdeviation, hypodeviation 3. Torsional - excyclodeviation, incyclodeviation B. According to age of onset:
1. Congenital, infantile - documented prior to age 6 months 2. Acquired
C. According to fusion status (whether the deviation can be controlled by fusuin mechanism.
1. Phoria - latent deviation, controlled by fusion mechanisn so that under binocular condition, the eyes remain aligned.
2. Intermittent phoria or tropia - fusion control present part of the time 3. Tropia - manifest deviation in which fusion control is not present.
D. According to variation of deviation with gaze position or fixating eye
1. Comitant - deviation does not vary with direction of gaze or fixationg eye.
2. Incomitant - deviation varies with direction of gaze or fixationg eye. Most incomitant strabismus is paralytic.
E. According to fixation
1. Alternating - there is spontaneous alternation of fixation from one eye to the other.
2. Monocular - there is preference for fixation with one eye.
Examination of the Patient
A. History taking : information should be obtained about the following 1. Chief complaint
2. Age of onset - document onset with photographs 3. Direction of deviation
4. Constant or intermittent 5. Magnitude of deviation
6. Associated eye complaints – diplopia, blurring of vision
7. Antecedent or concurring illness - seizures , diabetes, thyroid disease 8. Trauma
9. Previous consultation, treatment - patching, glasses, surgery 10. Maternal and birth history - prematurity
11. Developmental history 12. Family history
B. Ocular examination
Various tests are available for visual acuity determination . Children often pose a difficult assessment problem. For verbal and cooperative children, charts using tumbling E or pictures can be used. The child’s fixation pattern will give a clue as to the comparative vision of the two eyes. For example, achild who can alternate fixation in his 2 eyes will probably have equal visual acuity. Another child who prefers one eye to fixate probably has a better vision in that eye compared to the fellow eye.
2. Ocular motility examinations
The following clinical protocol may be used to assess ocular movements :
a. Sit facing the patient. Hold your finger or a small fixation target 10-14 inches in front of the patient, with the patient in primary position (straight ahead)
b. Ask the patient to follow target as you move it into the six cardinal positions. Elevate upper eyelid with your finger of your free hand to observe downgaze.
c. Note whether the amplitude of eye movements is normal or abnormal in both eyes. Rate amplitude for all fields of gaze by considering normal amplitudes as 100%, and rate lesser amplitudes
accordingly. To record relative over or underactions, designate normal as 0, that is , no over or underactions are present. Use 4 to designate maximum over or underaction. Underactions are rated -1 to -4 while overactions are rated +-1 to +4.
d. Note any nystagmus and if presen,t record its direction and amplitude in specific field of gaze.
3. Tests for ocular alignment
A. Corneal light reflex test ( Hirschberg method )
a. Ask the patient to seat facing you with head straight and eyes directed in primary gaze.
b. Hold a penlight in front of the patient’s eyes at a distance of about 2 ft, directing the light between the patient’s two eyes. Instruct the patient to look directly at the light.
c. Compare the position of the light reflex and record the estimated degrees of deviation. See Fig. 4.
B. Prism Test ( Krimsky Test)
This test is usually perfomed in patients unable to fixate with both eyes because of poor vision in one eye or in uncooperative patients.
a. Ask the patient to fixate on a light.
b. Place increasing amount of prism on the straight eye until the corneal reflex on the deviating eye is centered..
c. Prisms placed on the deviating eye is preferred in patients with incomitant or paralytic deviations. ( See Fig 5)
Fig. 4 . Corneal Light Reflex
15 ° Esotropia
30 Esotropia
45 ° Esotropia
B. Cover tests
The validity of a cover test depends upon the patient’s ability to maintain constant fixation on an accommodative target. Each eye must be able to move adequately when fixating . The cover-uncover test is done to establish the presence of either a manifest deviation (heterotropia) or a latent deviation
(heterophoria). The alternate tests are then performed to measure the deviation.
B.1. Cover-uncover test
a. Ask the patient to look at a distance fixation and position yourself directly opposite the patient, at an arm’s reach.
b. Cover the fixating eye with an occluder or your hand and observe the other eye for any movement. Note its direction.
c. Uncover the eye and allow about 3 seconds for both eyes to be uncovered.
d. Cover the other eye and observe its fellow for any movement.
e. After about one second, uncover the eye and observe it for any movement.
f. Repeat the test for near, using a near fixation point.
g. Repeat the distance and near tests using patient’s eyeglasses, if applicable.
B.2. Alternate cover test (prism and cover test)
a. With the patient seated upright and looking at a distance fixation point, rapidly shift the occluder from one eye to the other several times, not allowing any period of binocularity. The examiner should be seated slightly to the side of midline, facing the patient and at an arm’s length to the patient.
b. Place a trial prism over one eye, while continuing to shift the cover from one eye to the other.
Orient the prism apex towards the direction of the deviation. Choose the strength of the initial prism to approximate the deviation estimated by the Hirschberg’s test.
c. Continue to place prisms of progressively higher power until no movement is noted in either eye (neutralization).
d. Repeat test for near.
Figure . Prism Test
Fig. 5. Krimsky Prism Test 4. Ophthalmoscopy
Abnormalities in the fundus should be noted such as abnormal optic disc, macular lesions and retinopathy of prematurity.
5. Refraction
It is important to know the refractive state of the patient in assessing his ocular deviation. Cycloplegia is an adjunct to refraction in young strabismic patients. Cycloplegic agents that can be used include atropine, cylopentolate and tropicamide.
Common Types of Strabismus A. Comitant Strabismus
1. Congenital Esotropia or Infantile Esotropia
Congenital esotropia is usually noted shortly after birth or up to 6 months of age. The esodeviation is big and constant . Cross fixation (infant uses right eye to look at left visual field and left eye to see right visual field) may be present. There may be overaction of the inferior obliques , causing elevation of the adducting eye. Refraction is usually appropriate for the patient’s age. Aside from the esodeviation, the patient is usually otherwise normal. The child is best treated with surgery before the age of 18 months.
Fig. 6. Congenital Esotropia
Right gaze Primary Gaze Left gaze Figure 7. Overacting Inferior Obliques. Elevation of the adducting eye.
2. Refractive Accommodative Esotropia
Refractive accommodative esotropia usually starts at age 2 years. The child has a significant grade of hyperopia ( +3.00 to +10.00 diopters) . In order to see clearly, he accommodates. Accommodation is however accompanied by convergence of the eyes. Convex or plus lenses are prescribed to correct the hyperopia.
Fig 8 Accommodative Esotropia A. Esotropia of the right eye B. Eyes aligned with eyeglsses
A B
3. Sensory Esotropia
An esodeviation occurs in a patient with monocular or binocular condition that prevents good vision ( e.g. corneal opacity, cataract, retinal scars, inflammation , tumors, optic mreuropathy, anisometropia).
Treatment consist of the following : attempt to correct the cause of the poor vison, full cycloplegic refraction, muscle surgery to correct the deviation.
4. Intermittent Exotropia
Exotropia is an outward deviation of the eye which usually starts out as intermittent. It becomes manifests when patient is fatigued, sleepy or inattentive. He closes one eye when exposed to bright sunlight . The frequency and the duration of deviation may increase as the patient grows older. The exotropia can later become constant. Usually, the patient can use either eye for fixation . Vision is usually good for both eyes.
Treatment is surgical.
A B
Fig 9.Alternating Exotropia A. Left eye fixating B. Right eye fixating 5. Sensory Exotropia
An eye that does not see well for any reason may turm outward. Principles in treatment of sensory exotropia is the same as that of sensory esotropia.
B. Incomitant Strabismus.
1.. Paralytic Strabismus
There is limitation of action of involved muscle. The deviation is bigger when the involved eye is fixating and in the direction of action of involved muscle. Lateral rectus is the most frequently involved muscle as a result of abduscens nerve palsy. The patient should have a neurologic and systemic evaluation.
Strabismic Syndromes
Motility disorders may demonstrate typical feature of a particular syndrome. Examples are Duane syndrome, Brown syndrome, Mobius syndrome and congenital fibrosis syndrome.
Duane syndrome is a congenital motility disorder, usually unilateral, characterized by limited abduction, or limited adduction or both. The globe may retract and the eyelid fissure may narrow on adduction.There may also be upshooting or downshooting of the eye. There may be a face turn to allow the patient to use both eyes together. Muscle surgery is indicated to correct significant face turn or a significant deviation on primary gaze.
Left gaze Primary Gaze Left gaze
Fig.10 Duane syndrome, bilateral. Patient is orthophoric on primary gaze. There is limitation of abduction in both eyes.
There is narrowing of palpebral fissures on adduction.
Systemic illness associated with strabismus
A. Thyroid disease : Grave’s ophthalmopathy is an autoimmune disaease affecting the extraocular muscles, orbital fat, lacrimal glands and orbital connective tissue. Lid retraction, exophthalmos and ophthalmoplegia are some of the clinical findings. Limitation of elevation because of inferior rectus restriction is the most common motility findings. Patients complain of diplopia most severe in upgaze.
B. Diabetes mellitus : Diabetes is a complex metabolic disease involving small vessels and causing widespread damage to tissues, including the eyes Patients may have acute onset diplopia due to infarction of a cranial nerve and subsequent paresis of an extraocular muscle. The abdusens nerve and the lateral rectus is most often affected. If the occulomotor nerve is involved, the pupil is usually spared. Recovery of ocular motor function usually happens within 6 months.
C. Myasthenia gravis is characterized by abnormal fatigability of striated muscles which improves after rest.
Presenting complaints are ptosis and diplopia from involvement of one or more extraoular muscle.
D. Neurologic conditions : Cerebrovascular disorders and CNS space occupying lesions may have strabismus as one of the clinical presentations..
Principles of Management of a Strabismic Patient Aims of strabismus treatment:
1. Good vision 2. Binocularity 3. Good alignment
1. Accurate refractive correction. Treat amblyopia, if present, by patching the better eye. An alternative to patching in certain types of patients may be instilling atropine eye drops to the better eye.
2. Manipulation of accommodation. Esodeviations are treated with antiaccommodative therapy ( plus lenses for hyperopia ) and exodeviations by stimulating accommodation. (overcorrect myopia and undercorrect hyperopia)
3. Prisms. May be useful in patients with acute onset of strabismus and diplopia and those with small deviations.
4. Surgery. Muscles are chosen depending on the type and amount of deviation in the various directions of gaze.. A muscle is strengthened by resection. A measured amount is cut from the muscle which is then sutured back to its original insertion site. Recession in a weakening procedure whereby a muscle is detached from the eye, freed from its fascial attachments and then sutured to the eye at a measured distance from the original insertion.
Fig 11. Muscle recession
Fig 12 Muscle resection
SUMMARY
Under normal binocular viewing conditions, the eyes are aligned and the image of the object of regard falls simultaneously on the fovea of the two eyes. One of the eyes maybe misaligned (strabismus), so that only one eye at a time views the object of regard. Constant strabismus at an early age can result to amblyopia. In addition, any condition which can result to poor vision can lead to strabismus. It is important that a physician is able to detect strabismus at early age so that treatment can result in good vision, binocularity and good alignment.
REFERENCES
1. Del Monte, M.A. ed Pediatric Ophthalmology and Strabismus. 1996, San Francisco : American Academy of Ophthalmology.
2. Riordan-Eva, P , Whitcher, J Vaughan and Sahbury’s General Ophthalmology. Lange Medical Books : New York , 2004.
3. Valbuena, M.N. Strabismus and Amblyupia :. An instructional module for Level V Medical Students.
1999
4. Wilson, F.F. ed. Practical Ophthalmology . 1996, Indianapolis : American Academy of Ophthalmology.
SELF –TEST
2. The superior division of the oculomotor nerve supplies the superior rectus and A. superior oblique
B. inferior oblique C. orbicularis oculi D. levator palpebrae
3. This test will distinguish phoria from tropia A. cover uncover test
B. alternate cover test C. prism cover test D. modified Krimsky test 4. Example of incomittant squint is
A. congenital esotropia B. intermittent exptropia C. accommodative esotropia D. Graves ophthalmopathy
5. Prism measurement of lateral rectus palsy is done with the prism’s base oriented A. in
B. out C. up D. down
6. When the angle of deviation is greater in one direction of gaze the strabismus is A. monocular
B. comittant C. incomitant
D. sensory deprivation
7. After removing the cover in one eye, the eye moved inward. The patient is A. orthophoric
B. hyperopic C. esotropic D. exotropic
8. Accommodative esotropia is best treated A. medically
B. optically C. surgically D. by observation