|
|
|
||||||||
ACCOMMODATIVE ESOTROPIA
Accommodative esotropia is defined as a "convergent deviation
of the eyes associated with activation of the accommodative reflex".
Esotropia that is related to accommodative effort may be divided
into three major categories: (1) refractive, (2) nonrefractive,
and (3) partial or decompensated.
Refractive Accommodative
Esotropia
Refractive accommodative esotropia usually occurs in a child
between 2 and 3 years of age with a history of acquired intermittent
or constant esotropia. Occasionally, children who are 1 year of
age or younger present with all the clinical features of accommodative
esotropia. Several investigators have demonstrated that the accommodative
mechanism is capable of functioning within the first few months
of life. Pollard reported two patients with onset of esotropia
at ages 4 1/2 and 5 months whose hyperopic correction (greater
than 3 diopters) resulted in resolution of the esodeviation. Baker
and Parks reported 21 patients with onset of accommodative esotropia
before the age of 1. Approximately 50 per cent of their patients
whose esodeviation was initially controlled with glasses decompensated
into nonaccommodative esotropia.
The refraction of patients with refractive accommodative esotropia
averages +4.75 diopters, with a range of +3.00 to +10.00 diopters.
The angle of esodeviation is the same when measured at distance
and near fixation, and is usually moderate in magnitude, ranging
between 20 to 40 prism diopters. Amblyopia is common, especially
when the esodeviation has become more nearly constant.
Pathogenesis The
mechanism of refractive accommodative esotropia involves three
factors: uncorrected hyperopia, accommodative convergence and
insufficient fusional divergence. Donders first described the
close relationship between accommodation and convergence. When
an individual exerts a given amount of accommodation, a specific
amount of convergence (accommodative convergence) is associated
with it. An uncorrected hyperope must exert excessive accommodation
to clear a blurred retinal image. This in turn will stimulate
excessive convergence. If the amplitude of fusional divergence
is sufficient to correct the excess convergence, no esotropia
will result. However, if the fusional divergence amplitudes are
inadequate or motor fusion is altered by some sensory obstacle,
an esotropia will result.
Treatment In
refractive accommodative esotropia, the full hyperopic correction,
determined by cycloplegic refraction, is initially prescribed.
If the child is orthophoric or has a small esophoria while wearing
glasses, the child can be followed at regular intervals, as often
as every 3 months or on a semiannual or annual basis once the
condition is stabilized. Extended-wear contact lenses have been
recommended in selected children with refractive accommodative
esotropia, with good results and acceptance.
Beginning around age 4 to 5 years, the strength of the hyperopic
correction can be reduced gradually to enhance fusional divergence
and maximize visual acuity. This can be performed by manifest
refraction instead of a cycloplegic refraction. Children with
moderate levels of hyperopia may be capable of developing enough
fusional divergence to be able to function without their glasses.
Children with extreme levels of hyperopia are unlikely to ever
"outgrow" their refractive error and will experience
asthenopia without their correction. Aggressive reduction of the
hyperopic prescription may not be warranted in these children.
It is important to warn parents of children with either refractive
or nonrefractive accommodative esotropia that the esodeviation
will appear to increase without glasses after the initial correction
is worn. Parents frequently state that, before wearing glasses,
their child had a small esodeviation, whereas after removal of
the glasses the esodeviation is now quite large. Parents often
blame the increased esodeviation on the glasses and note that
their child has become dependent on them. This situation can best
be explained on the basis of the child using the appropriate amount
of accommodative effort after the glasses have been worn. When
the child removes her glasses, she will continue to use an accommodative
effort to bring objects into proper focus and increase the esodeviation.
The strong desire these children have to wear their new glasses
may be secondary to the relief of asthenopia, benefits of single
binocular vision or both. Explaining these phenomena to parents
ahead of time is more effective than the same explanation after
the fact.
Nonrefractive Accommodative
Esotropia
Children with nonrefractive accommodative esotropia usually
present between 2 and 3 years of age with an esodeviation that
is greater at near than at distance fixation. The refractive error
in this condition may be hyperopia or myopia, although the average
refraction is +2.25 diopters.
Pathogenesis
In nonrefractive accommodative esotropia, there is a high accommodative
convergence to accommodation (AC:A) ratio: the effort to accommodate
elicits an abnormally high accommodative convergence response.
There are a number of ways of measuring the AC:A ratio: the heterophoria
method, the fixation disparity method, the gradient method, and
the clinical evaluation of distance and near deviation. Most clinicians
prefer to assess the ratio using the distance-near comparison.
This method allows the ratio to be evaluated more easily and quickly,
since it employs conventional examination techniques and requires
no calculations. The AC:A relationship is derived by simply comparing
the distance and near deviation. If the near measurement in an
esotropia patient is greater than 10 prism diopters, the AC:A
ratio is considered to be abnormally high.
Treatment The
management of nonrefractive accommodative esotropia may involve
a variety of modalities. Many pediatric ophthalmologists attempt
to correct the esodeviation at near with bifocals, provided that
the distance deviation is less than 10 prism diopters. Initially,
a +2.50 executive-type bifocal with the top of the lower segment
crossing the lower pupillary border is given. In follow-up, the
child should wear the least amount of hyperopic bifocal correction
to maintain straight eyes at near fixation.
The use of bifocals in treating the esotropia at near is not
without some controversy. Albert and Lederman reported on 69 patients
with excess esotropia at near. They found no difference in the
natural reduction of esotropia in those patients wearing bifocals
versus patients who had their bifocal discontinued. The reason
for the initiation of treatment was the parents' observation of
the crossing at near. Because the esotropia at near appeared not
to be cosmetically noticeable, the authors questioned the energetic
treatment of this disorder. Pratt-Johnson and Tillson reviewed
the long-term sensory status in 99 patients with excess esotropia
at near. Half were treated with bifocals while the others were
not. They found no difference in sensory status or deterioration
rate between the two groups. These authors suggested that a carefully
planned prospective and randomized multicenter study involving
large numbers of patients with high AC:A ratios was indicated
to provide a clearer understanding about the use of bifocal therapy
in these patients.
Miotics have been used successfully in patients with high
AC:A ratios. Parks observed that, although the AC:A ratio normalized
on miotics, it reverted to pretreatment levels with discontinuation.
Miotics have a number of ocular and systemic side effects previously
described, and are not recommended for long-term use.
Surgery for high AC:A ratio is usually performed when the
esodeviation at near fixation is no longer controlled with bifocals
or the distance deviation is higher than an acceptable level.
Surgery has been shown to decrease the AC:A ratio.
Partial or Decompensated
Accommodative Esotropia
Refractive or nonrefractive accommodative esotropias do not
always occur in their "pure" forms. These patients may
have a significant reduction in esodeviation when given glasses
with or without bifocals. However, a residual esodeviation persists
in spite of full hyperopic correction, which is the deteriorated
or nonaccommodative portion. This condition commonly occurs when
there is a delay of months between the onset of accommodative
esotropia and antiaccommodative treatment. It is also more prevelant
in patients who developed the esotropia before 24 months of age.
Sometimes the esotropia may initially be eliminated with glasses
but a nonaccommodative portion slowly becomes evident, in spite
of the patient's wearing the maximal amount of hyperopic correction
consistent with good vision.
The indications for surgery for partial or decompensated accommodative
esotropia remains controversial. Some ophthalmologists believe
that any esotropia greater than 10 prism diopters warrants surgery
to reduce the deviation to less than 10 prism diopters, in order
to enhance the development of the monofixation. These ophthalmologists
believe that if the monofixation syndrome develops, the patient
will function better because of the advantage of peripheral fusion.
Also, the prognosis for the permanency of the surgically created
alignment will be enhanced as a result of good motor fusional
vergences associated with the monofixation syndrome.
Other ophthalmologists consider that surgery should be performed
on the nonaccommodative portion only if it is cosmetically significant
as determined by the patient or family, or both. They feel that
there is no functional deficit that can be demonstrated consistently
in a real-world situation in patients who do not have peripheral
fusion, as would be present in the monofixation syndrome.
