Strabismus
Prof. Dr. R. K. Mishra (Presented in the month of
April 2003 - venue Krishna Hotel)
Strabismus is a fascinating and complex topic. It fails to draw attention
of the patient except for the cosmetic consideration. A small corneal
scar will bring him running to the doctor but amblyopic visual loss
does not bother him/her so much.
Treatment of strabismus is demanding for the patients
as well as the doctor. That the apparently normal looking eye could
be blind functionally is beyond comprehension of the patients.
Treatment is laborious time consuming and lengthy hence
dropouts are frequent.
Treatment seems to be unrewarding and frustrating both
for the patients and the doctors.
The nervous control of the ocular movements is complicated.
The muscles are supplied by nerves arising from nuclei in the mid-brain.
Their action is coordinated by intermediate centers
situated in this region by which reflex activities are governed.
Finally these intermediate centers are linked with the
vestibular apparatus where by they became associated with the equilibration
reflexes and with cerebral cortex.
These complexities run the ocular motor function. Any
disruption will lead to strabismus of one of the many varieties that
we are going to consider.
Prevalence
Strabismus mostly develops within the first 6-7 years
of life. Horizontal strabismus is far more common than vertical strabismus.
Approximately 7% of children of 6 to 7 years age develop strabismus.
Amongst infants about 1% may develop strabismus. And about 3% school
children have amblyopia related to strabismus.
Risk factors for strabismus
1. Cigarette smoking during pregnancy
2. Neurological disorders
3. Use of toxic drugs during pregnancy (opiates, marijuana,
barbiturates)
4. Lead poisoning
5. Twins
6. Heredity
7. Ethnicity
8. Refractive errors
9. Low birth weight (prematurity)
10. Anatomic factors (cranio-facial anomalies)
Predisposition
Under 5 yrs age, family history of strabismus, Hyperopia
> 4.00 D. Delay in refractive error correction could precipitate
strabismus.
Prevention
Prompt, periodical and complete refractive correction
has a strong preventive value. Refractive error and strabismus are
close associates.
Average refractive error
The average refractive error in children shows increasing
amounts of hyperopia in the first 5—7 years of life. Average
hyperopia in a 6-year child is approximately 2.00 D.
After 7 years of age, refractive errors move toward
the permanent distribution. 33% of children and young adults are myopic
worldwide
A relationship between esotropia and hyperopia was noted
in 1864 by Donders , Lepard, and later Duke- Elder and many others.
An exact threshold for esotropia to occur can not be predicted on
the basis of refraction alone.
AC : A ratio
High Convergence response to accommodation is more likely
to produce esotropia. Excessive Accommodative Convergence, high AC:A
ratio, and inadequate fusion amplitudes are risk factors for esotropia.
Hyperopia is more common in esotropic and even in most
of the exotropic children. Only a few exotropic children are myopic
Age & Refractive status
Rapid changes in refractive status of children under
the age of 1 year occur. In addition, the globe size and muscle relationship
changes. They stabilizes toward the end of the first year of life.
Hence preference of delayed surgery for an year.
Risks of Hyperopia
The risk of acquiring accommodative esotropia when hyperopia
exceeds 4.00D is substantial. High hyperopia are best corrected by
the earliest. Hyperopia in excess of 5.00 D is potentially amblyogenic
and may lead to bilateral amblyopia .
Amblyopia
It may be defined as vision after correction to be less
than 6/24. The incidence of amblyopia is about 2.%, less than
1% amblyopes develop strabismus.
Strabismic amblyopia
Decrease in vision in the non-fixing eye, as the result
of deviation. Strabismus prior to 6 years of age is a risk for developing
amblyopia. If free alternation of fixation continues, head tilt, face-turn,
or chin position allows the patient to fuse in a non-concomitant strabismus,
amblyopia may be avoided.
Every child with strabismus needs to be carefully, periodically
and repeatedly evaluated for refractive error correction and visual
acuity in the eyes. Equal vision in the two eyes is an insurance against
amblyopia
Under the age of 6 yrs, once fixation preference has
been established the non-fixing eye is at risk of amblyopia. In general,
fixation preference is more likely to occur in the smaller-angle esotropics
than in the larger-angle.
There is no evidence that a retinal or optic nerve abnormality
causes amblyopia. Instead, cortical defects of form and shape perception
are at the root of the defect.
Crowding Phenomenon
Amblyopic eyes have reduced ability to see small objects,
particularly with low contrast and more so if the objects are crowded
as in letters in a line of Snellens Chart
Detection - Amblyopia
In strabismus, measurement of visual acuity on all visits
is the only way of early detection of amblyopia. Strabismic amblyopia
is diagnosed when the vision in one eye is two lines or more worse
compared with the fellow eye.
In amblyopia visual acuity may be reduced to 20/80(6/24)
but is seldom worse. Visual acuity of 20/200(6/60) or less should
raise the suspicion of anisometropic amblyopia or other ocular disease.
Visual Acuity Measurement
Visual acuity measurements are still the gold standard
for the evaluation of the patient with amblyopia. Of all the visual
acuity measurements, Snellen measurements are preferred.due to the
"Crowding Phenomenon " value.
On set of amblyopia
From a clinical standpoint, strabismic amblyopia rarely
develops after age 6 to 8 years
Management of Amblyopia
Part-time Patching
Occlusion of the fixing eye is the standard therapy
for equalizing visual acuity in amblyopic patients. In total occlusion
the occluded eye may develop amblyopia if treated too long. Weekly
follow up is advised.
Full-time, Alternate-Day Occlusion
This could keep the "binocular slate clean" and help
to prevent amblyopia and anomalous retinal correspondence, both of
which require some degree of binocular interaction.
Patching therapy needs to be continued in the management
of strabismic amblyopia till visual acuity is equalized. Slippage
of vision after discontinuance of patching is known. Re-patching is
required for recovery.
Penalization
Penalization by cycloplegic drops in the fixing eye,
may be enough in patients with mild to moderate amblyopia.
Infantile Esotropia
Infantile Esotropia also called Congenital Esotropia
and Essential Infantile Esotropia (E I E) was referred to in the past
as congenital esotropia but is rarely, congenital and is more appropriately
designated Essential Infantile Esotropia.
It is characterized by a large, stable deviation, a
limited potential for single binocular vision, association with oblique
muscle dysfunction and dissociated vertical deviation. In addition,
may show latent or manifest nystagmus.
Presentation - I
Affected children typically have an esodeviation of
20 to 35 degrees, with an average refractive status for their age.
, rarely amblyopic. Good vision in either eye, as demonstrated by
an ability to alternate fixation.
Presentation - II
In addition, affected children show a habitual limitation
of abduction and a" cross fixation" i.e. preferring to fix objects
in their left visual field with their right eye, and vice versa, Features
of" inferior oblique" over action.
Management of EIE
These children are rarely amblyopic, have free alteration
of fixation and they do not have any significant refractive error.
If amblyopia or refractive error at all present, they be treated before
surgery.
EIE Surgical Management
Is invariably surgical. Many authorities believe that
unless, an anesthetic problem, early surgery by 2 years of age offers
the best prospect for the development of single binocular vision and
achievement of stable, long-term, ocular alignment
Over-action inferior oblique
Because EIE is so commonly associated with over-action
of the inferior oblique muscles, it may be necessary to deal with
this over-action either at the time of surgery of the horizontal muscle
or as a separate surgical procedure
Simultaneous correction of oblique dysfunction will
increase the frequency with which stable binocular single vision is
achieved post operatively
Divergent Vertical Deviation( DVD)
It is worth bearing in mind that the child with EIE
plus Divergent Vertical Deviation(DVD) is most unlikely to develop
useful binocular vision, and that the surgeon is dealing with an essentially
cosmetic defect.
Esotropia Secondary to Ocular Disease
Strabismus in children with ocular disease is equally
divided between esotropia and exotropia. So long as the ocular pathology
is sufficient to degrade the vision in one eye below a level of 6/24
to 6/36 (20/80 to 20/120), strabismus is likely to result.
Monocular deviation
With monocular pathology, the importance is on diagnosis
and management of the disease rather than the strabismus. In children
with retinoblastoma, 15% presented initially with strabismus (Parks,
1992).
In some older patients removing the underlying pathology
will also eliminate the strabismus but in children it require strabismus
surgery if they are to achieve binocular vision. .
Cautionary note
In children with deviation in one eye it is wise to
operate only in the effected eye. Scleral perforation with its vision-threatening
complications are reported in at least 3% of strabismus procedures
(Morris et a!., 1990),
Accomodative Esotropia
Accommodative esotropias arise due to excessive accommodational
demand, for a proportionate fixation distance (As in a hypermetrope)
AC:A
An Emetrope , viewing an object at 1. Meter will converge
one meter angle and accommodate 1 D. A hypermetrope of 4 D. will need
to accommodate 4. D and yet need to converge 1 meter angle. This strains
the synkinetic faculty of accommodation and convergence. binocularity
will be broken and esotropia result.
Types of Accommodative Esotropia
Fully accommodative esotropia –A
Full correction of Hypermetropia eliminates the deviation
Partially accommodative esotropia where full
correction of hypermetropia does reduce the angle but does not eliminate
it.
Fully accommodative esotropia can degrade to partially
accommodative Esotropia if attention is not paid to refractive correction
and to wards Amblyopia producing factors.
Not all children with hypermetropia will develop an
accommodative esotropia. As the level of hypermetropia increases,
so does the likelihood of strabismus and amblyopia. Why one child
with a +3.00 D refractive error develops esotropia and not another
remains obscure. Genetic influences may play a part, here.
Investigations
A careful cycloplegic refraction is the critical step
in diagnosis. Adequate cycloplegia is obtained with 2 drops of cyclopentolate
1%. But preferred regimen is atropine 1% ointment 3 times a day for
3 days prior to refraction.
Meticulous correction of both spherical and astigmatic
errors in both the eyes is essential in children. An "off axis" refraction
can be very misleading… All refractions should be accompanied
by a careful fundus examination
Management Of Fully Accommodative
Esotropia
A. Optical By full refractive error correction
of any error over 2D and in higher error near correction of 2to 4
D
B. Pharmacological. By use of drug like Pilocarpine
(2%-4% ),2 to 4 Times to reduce accommodational demand. The drug should
be given a trail of 2-4 months
C. Surgery
Discussion has centered on the role of surgery in fully
accommodative esotropia. Even with surgery, many children still require
spectacles for good vision and good alignment. As such in a fully
accommodative strabismus preference is for a non-surgical method of
treatment.
Partially Accommodative Esotropia
When the angle of deviation is reduced (by at least
10 Prism D.) but not corrected with adequate hypermetropic correction,
then these cases are considered as "Partial accommodative hypermetropia"
This condition is usually seen at the age of 3 yrs or so They frequently
need surgery.
Surgery
The amount of surgery needed will depend on the size
of the deviation and the surgical objective. Where amblyopia has been
eliminated and the prospects for binocular single vision are good,
the goal of surgery should be full correction of the deviation.
Prognosis
Accomodative Esotropia if treated sufficiently early,
carries a good prognosis. Care and management of Amblyopia and refractive
error are the main stay for success in treatment of Accommodative
Esotropia
Exodeviation