Concomitant Squint: types and aetiology

CONCOMITANT STRABISMUS
It is a type of manifest squint in which the amount of
deviation in the squinting eye remains constant
(unaltered) in all the directions of gaze; and there is
no associated limitation of ocular movements.
Etiology
It is not clearly defined. The causative factors differ
in individual cases. As we know, the binocular vision
and coordination of ocular movements are not present
since birth but are acquired in the early childhood.
The process starts by the age of 3-6 months and is
completed up to 5-6 years. Therefore, any obstacle to
the development of these processes may result in
concomitant squint. These obstacles can be arranged
into three groups, namely: sensory, motor and central.
1. Sensory obstacles. These are the factors which
hinder the formation of a clear image in one eye. These
include:
Refractive errors,
Prolonged use of incorrect spectacles,
Anisometropia,
Corneal opacities,
Lenticular opacities,
Diseases of macula (e.g., central chorioretinitis),
Optic atrophy, and
Obstruction in the pupillary area due to congenital
ptosis.
2. Motor obstacles. These factors hinder the
maintenance of the two eyes in the correct positional
relationship in primary gaze and/or during different
ocular movements. A few such factors are:
Congenital abnormalities of the shape and size of
the orbit,
Abnormalities of extraocular muscles such as
faulty insertion, faulty innervation and mild
paresis,
Abnormalities of accommodation, convergence
and AC/A ratio.
3. Central obstacles. These may be in the form of:
Deficient development of fusion faculty, or
Abnormalities of cortical control of ocular
movements as occurs in mental trauma, and
hyperexcitability of the central nervous system
during teething.
Clinical features of concomitant strabismus (in
general)
The cardinal features of different clinico-etiological
types of concomitant strabismus are described
separately. However, the clinical features of
concomitant strabismus (in general) are as below :
1. Ocular deviation. Characteristics of ocular
deviation are:
Unilateral (monocular squint) or alternating
(alternate squint).
Inward deviation (esotropia) or outward deviation
(exotropia) or vertical deviation (hypertropia).
Primary deviation (of squinting eye) is equal to
secondary deviation (deviation of normal eye
under cover when patient fixes with squinting
eye).
Ocular deviation is equal in all the directions of
gaze.
2. Ocular movements are not limited in any direction.
3. Refractive error may or may not be associated.
4. Suppression and amblyopia may develop as
sensory adaptation to strabismus. Suppression may
be monocular (in monocular squint) and alternating
(in alternating strabismus). Amblyopia develops in
monocular strabismus only and is responsible for
poor visual acuity.
5. A-V patterns may be observed in horizontal
strabismus. When A-V patterns are associated, the
horizontal concomitant strabismus becomes vertically
incomitant (see page 334).
Types of concomitant squint
Three common types of concomitant squint are :
1. Convergent squint (esotropia),
2. Divergent squint (exotropia), and
3. Vertical squint (hypertropia).
CONVERGENT SQUINT
Concomitant convergent squint or esotropia denotes
inward deviation of one eye (Fig. 13.14). It can be
unilateral (the same eye always deviates inwards
and the second normal eye takes fixation) or
alternating (either of the eyes deviates inwards and
the other eye takes up fixation, alternately).
Depending upon the clinico-etiological features
convergent concomitant squint can be further
classified into following types:
1. Accommodative esotropia. It occurs due to
overaction of convergence associated with
accommodation reflex. It is of three types: refractive,
non-refractive and mixed.
i. Refractive accommodative esotropia: It usually
develops at the age of 2 to 3 years and is
associated with high hypermetropia (+4 to +7 D).
Mostly it is for near and distance (marginally
more for near) and fully correctable by use of
spectacles.
ii. Non-refractive accommodative esotropia: It is
caused by abnormally AC/A (accommodative
convergence/accommodation) ratio. This may
occur even in patients with no refractive error.
Esotropia is greater for near than that for distance
(minimal or no deviation for distance). It is fully
corrected by adding +3 DS for near vision.
iii. Mixed accommodative esotropia: It is caused by
combination of hypermetropia and high AC/A
ratio. Esotropia for distance is corrected by
correction of hypermetropia; and the residual
esotropia for near is corrected by an addition of
+3 DS lens.
2. Non-accommodative esotropias. This group
includes all those primary esodeviations in which
amount of deviation is not affected by the state of
accommodation. It includes:
i. Essential infantile esotropia. It usually presents
at 1-2 months of age. However, it may be detected
shortly after birth or any time within the first 6
months of life. Previously, it was known as
congenital esotropia. It is characterised by fairly
large angle of squint (> 30o), alternate fixation in
primary gaze and crossed fixation in lateral gaze.
ii. Essential acquired or late onset esotropia. It is
a common variety of concomitant convergent
squint. It typically occurs during first few years
of life. It is of three types:
Basic type. In it the deviation is usually
equal at distance and near.
Convergence excess type. In it the deviation
is large for near and small or no deviation for
distance.
Divergence insufficiency type. It is
characterized by a greater deviation for
distance than near.
3. Secondary esotropia. It includes:
i. Sensory deprivation esotropia. It results from
monocular lesions (in childhood) which either
prevent the development of normal binocular
vision or interfere with its maintenance. Examples
of such lesions are: cataract, severe congenital
ptosis, aphakia, anisometropia, optic atrophy,
retinoblastoma, central chorioretinitis and
so on.
ii. Consecutive esotropia. It results from surgical
overcorrection of exotropia.
DIVERGENT SQUINT
Concomitant divergent squint (exotropia) is
characterised by outward deviation of one eye while
the other eye fixates.
Clinico-etiological types
It can be classified into following clinicoetiological
types:
1. Congenital exotropia. It is rare and almost always
present at birth. It is characterised by a fairly large
angle of squint, usually alternate with homonymous
fixation in lateral gaze, and no amblyopia.
2. Primary exotropia. It is a common variety of
exodeviation (unilateral or alternating). It presents
with variable features. It may be of:
Convergence insufficiency type (exotropia greater
for near than distance),
Divergence excess (exotropia greater for distance
than near) or
Basic non-specific type (exotropia equal for near
and distance).
It usually starts as intermittent exotropia at the
age of 2 years. It is associated with normal fusion
and no amblyopia. Stereopsis is usually absent.
Precipitating factors include bright light, fatigue, illhealth
and day-dreaming. If not treated in time it
decompensates to become constant exotropia (Fig.
13.15).
3. Secondary (sensory deprivation) exotropia. It is a
constant unilateral deviation which results from longstanding
monocular lesions (in adults), associated
with low vision in the affected eye. Common causes
include: traumatic cataract, corneal opacity, optic
atrophy, anisometropic amblyopia, retinal detachment
and organic macular lesions.
4. Consecutive exotropia. It is a constant unilateral
exotropia which results either due to surgical overcorrection
of esotropia, or spontaneous conversion
of small degree esotropia with amblyopia into
exotropia.