Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
(a)
(b)
Figure 4.6 A partially accommodative esotropia in a
child not fully corrected by spectacles
cases are difficult to manage unless promptly
diagnosed and managed with occlusion therapy,
aimed at eliminating suppression and preventing
the onset of amblyopia. Congenital esotropia
often with low hypermetropia falls into this
group in many cases. Although glasses may not
be effective preoperatively, spectacle correction
of relatively small degrees of hypermetropia
postoperatively can improve alignment sufficiently
for monofixation syndrome to evolve.
Esotropia with high AC:A
ratio–convergence excess
High AC:A and convergence excess is said to
be present when the eyes overconverge for near
30
viewing of an object in a patient whose refractive
error is fully corrected and whose eyes are
straight for distance. Although the convergence
excess may be present in children of similar age
to those with fully accommodative esotropia, it
may occur before 6 months and be part of the
differential diagnosis of congenital esotropia. It
is essential to do a non-dissociating cover test
at distance to avoid the wrong diagnosis of
constant esotropia.
Such convergence excess esotropias are
characterised by accommodation-associated overconvergence,
with the esotropia at near often
more than 20 dioptres greater than for distance.
Patients may be hypermetropic, have a small
degree of hypermetropia, or be emmetropic.
More rarely, the patient is myopic and the
overconvergence only becomes manifest on
correction of the myopia and the need to focus on
near targets through the prescribed glasses is
attended by convergence excess. The convergence
excess may be worse if the patient is tense or
nervous. Patients with high AC:A ratios who have
central bifoveal fusion for distance may not
experience diplopia at near when the eyes
overconverge because of suppression. Overconvergence
associated with accommodation
tends to lessen with age and to disappear
completely by adolescence, though not invariably.
Management includes prescribing the full
hypermetropic correction to be worn. There is
no evidence that bifocal glasses improve long
term outcome. 11 A number of cases of high AC:A
ratio esotropias may require surgery because of
breakdown of normal binocular vision at
distance. 12 If further cycloplegic refraction
reveals no uncorrected hypermetropia, surgery
should be deferred to evaluate the full effect of
glasses.
Cyclic esotropia (alternate day syndrome)
This is an unusual condition where there are
alternating periods of an esotropia (often 30–40
dioptres) on one day followed by perfectly
straight eyes on the next day. The strabismus