STRABISMUS
When we look at a near object 
two things happen: the lens 
rounds up because ciliary 
muscles contract, and the eyes 
turn in.

    The commonest cause of 
amblyopia in humans is 
strabismus, or squint, terms 
that signify nonparallel eyes--
cross-eye or wall-eye. (The term 
squint as technically used is 
synonymous with strabismus 
and has nothing to do with 
squinching up the eyes in 
bright light.) The cause of 
strabismus is unknown, and 
indeed it probably has more 
than one cause. In some cases, 
strabismus comes on shortly 
after birth, during the first few 
months when in humans the 
eyes would just be starting to 
fixate and follow objects. The 
lack of straightness could be 
the result of an abnormality in 
the eye muscles, or it could be 
caused by a derangement in the 
circuits in the brainstem that 
subserve eye movements.

    In some children, 
strabismus seems to be the 
result of long-sightedness. To 
focus properly at a distance, the 
lens in a long-sighted eye has 
to become as globular as the 
lens of a normal eye becomes 
when it focuses on a near 
object. To round up the lens for 
close work means contracting 
the ciliary muscle inside the 
eye, which is called 
accommodation. When a normal 
person accommodates to focus 
on something close, the eyes 
automatically also turn in, or 
converge. The figure to the left 
shows the two processes. The 
circuits in the brainstem for 
accommodation and 
convergence are probably 
related and may overlap; in any 
case, it is hard to do one 
without doing the other. When 
a long-sighted person 
accommodates, as he must to 
focus even on a distant object, 
one or both eyes may turn in, 
even though the convergence 
in this case is 
counterproductive. If a long-
sighted child is not fitted with 
glasses, turning in an eye may 
become habitual and eventually 
permanent. This explanation 
for strabismus must surely be 
valid for some cases, but not for 
all, since strabismus is not 
necessarily accompanied by 
long-sightedness and since in 
some people with strabismus, 
one or other eye turns out 
rather than in.

    Strabismus can be treated 
surgically by detaching and 
reattaching the extraocular 
muscles. The operation is 
usually successful in 
straightening the eyes, but 
until the last decade or so it was 
not generally done until a child 
had reached the age of four to 
ten, for the same reason that 
cataract removal was delayed--
the slight increase in risk.

    Strabismus that arises in an 
adult, say from an injury to a 
nerve or eye muscle, is of 
course accompanied by double 
vision. To see what that is like, 
you need only press (gently) on 
one eye from below and one 
side. Double vision can be most 
annoying and incapacitating, 
and if no better solution is 
available, a patch may have to 
be put over one eye, as in the 
Hathaway shirt man. The 
double vision otherwise 
persists as long as the 
strabismus is uncorrected. In a 
child with strabismus, 
however, the double vision 
rarely persists; instead, either 
alternation or suppression of 
vision in one eye occurs.

    When a child alternates, he 
fixes (directs his gaze) first 
with one eye, while the 
nonfixating eye turns in or out, 
and then fixes with the other 
while the first eye is diverted. 
(Alternating strabismus is very 
common, and once you know 
about the condition, you can 
easily recognize it.) The eyes 
take turns fixating, perhaps 
every second or so, and while 
one eye is looking, the other 
seems not to see. At any instant, 
with one eye straight and the 
other deviating, vision in the 
deviated eye is said to be 
suppressed. Suppression is 
familiar to anyone who has 
trained himself to look through 
a monocular microscope, sight 
a gun, or do any other strictly 
one-eye task, with the other 
eye open. The scene simply 
disappears for the suppressed 
eye. A child who alternates is 
always suppressing one or other 
eye, but if we test vision 
separately in each eye, we 
generally find both eyes to be 
normal.

    Some children with 
strabismus do not alternate but 
use one eye all the time, 
suppressing the other eye. 
When one eye is habitually 
suppressed, vision tends to 
deteriorate in the suppressed 
eye. Acuity falls, especially in 
or near the central, or foveal 
part of the visual field, and if 
the situation continues, the 
eye may become for practical 
purposes blind. This kind of 
blindness is what the 
ophthalmologists call 
amblyopia ex anopsia. It is by 
far the commonest kind of 
amblyopia, indeed of blindness 
in general.