Childhood Strabismus

Strabismus is a condition in which the two eyes are not parallel but one is deviated withrespect to the other, horizontally and/or vertically. The cause of childhood strabismus is generally unknown, although weakness of one or several of the eye muscles may be present as will be shown in the case presentation below. The most common type of strabismus, without any obvious dysfunction of the eye muscles, is called concomitant strabismus, implying that the angle or size of the strabismus is more or less constant, irrespective of the direction of gaze. This is in distinction to strabismus due to muscle weakness, called incomitant strabismus where the angle varies with the direction of gaze.
population. In this type of strabismus the eyes are constantly deviated in relation to each
other. Latent concomitant strabismus is even more common and seen in more than half the
population. In latent strabismus the eyes stay straight most of the time due to activation of the
eye muscles over the visual system to keep the eye aligned in binocular single vision.
After the presentation of a case of complicated strabismus, the effects of strabismus on the
visual functions will be described, and the connections between strabismus and eye muscle
function and eye proprioception explored.
Case report
The following case report exemplifies recent advances in the management of a difficult case
of incomitant strabismus. An infant boy presented with horizontal convergent strabismus of a
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very large angle, as a part of a so called Möbius syndrome, a congenital disorder, involving
weakness of both the outer (lateral) eye muscles and of many of the facial muscles (Fig. 1,
left). Ordinary surgery for convergent strabismus failed due to the complete absence of
function of both lateral eye muscles. In order to restore some ability to deviate the eyes to the
sides and obtain an acceptable eye position, the tendons of the upper and lower vertical eye
muscles were transposed to the insertion of the non-functioning lateral eye muscles. A
weakening was also done of the inner (medial) horizontal eye muscles of both eyes by
injection of Botulinum toxin into the medial muscles. This combination of surgical and
pharmacological procedures resulted in fairly straight eyes (Fig. 1, right), a limited range of
horizontal movements and almost normal vertical eye motility. Vision was normal in both
eyes due to alternate fixation and well performed occlusion therapy during the first years of
life (see Chapter I). Binocular vision was abnormal as a result of the early onset strabismus.
Figure 1. (Left) Boy 18 months old with Möbius’ syndrome, prominent convergent
strabismus and slight vertical strabismus. (Right) Same boy at 9 years of age after strabismus
surgery supplemented with Botulinum toxin. Much improved appearance, although some
vertical strabismus remains.
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Visual functions
In manifest strabismus where one of the eyes is constantly out of line, the child will
experience double vision. In childhood the visual system is very flexible and double vision
may be quite easily compensated and shifted into single vision by suppressing the image of
one eye at a time. However, suppression may impair the visual development of the eye that is
out of line most of the time, i.e. the image of the object fixated is falling outside the fovea
centralis. In order to prevent reduced vision of the deviated eye treatment is instituted. It
consists of occlusion of the better eye with a patch for a period each day. In most cases
glasses are also prescribed, producing sharp images on the retina of the two eyes and
supplementing the occlusion therapy. The therapy has to be started at an early age, and
continued and supervised up to about age 10-12, during the so called plastic period of visual
development (see also Chapter I). Suppression leads to disrupted binocular vision, the main
part being loss of stereopsis, but this is a visual handicap of very limited consequences for the
individual with strabismus. Reduced vision due to strabismus is uncommon in our country as
a result of the extensive scheme for visual screening of young children and early treatment of
children with impaired vision in one or both eyes (see also Chapter VI). Thus, we now know
very much of the visual dysfunctions that are a result of manifest strabismus and we have
reliable means to detect and treat them.
Motor functions
Some types of convergent strabismus are connected with refractive errors of the child’s eye,
the most common being hyperopia (far-sightedness). In order to see clearly without glasses,
the hyperopic eye has to accommodate and the motor activity of the ciliary (focusing) muscle
inside the eye produces an impulse to converge the eyes, which may result in a manifest
convergent strabismus. This type of strabismus may be cured by proper glasses to compensate
the hyperopia.
In concomitant strabismus there exist only small abnormalities of eye muscle function. The
eye muscles that move the eyes and control the gaze positions are known to be very fast and
strong with respect to their size, and also very fatigue resistant. Fast movements are used in
repositioning the eyes and delicate but long-lasting muscle activity is needed in steady
fixation with both eyes directed to the same point in space. The muscle components for steady
fixation have been shown to develop earlier than those for swift eye movements, showing that
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steady control of the eye position is needed for proper development of visual acuity and
binocular functions. Function of the muscle components can be adjusted in proportion to
visual activity, and in strabismic eye muscles the components for steady control do not
develop to a full extent when the need for steady control in binocular vision is reduced. In
some of the divergent types of strabismus the eyes have to converge constantly in order to
overcome an inherent outward deviation and subsequently the medial eye muscles become
stronger in order to keep the eyes straight.
Control of eye position is mediated also by connective tissue structures outside the eye
muscles proper. These structures can control the muscle tendon directions and therefore
influence the actions of the muscle on position and movements the eye globe. The connective
tissue structures are under influence of the eye muscles themselves, possibly independent of
the main eye muscle activity. This would imply that eye position and eye movements are
controlled by two muscle systems, one directly acting on the eye globe and the other
indirectly by adjusting the sideway forces on the muscle tendon. The control of the connective
components around the tendon could be of importance for high precision eye movement
activities in e. g. prolonged fixations for near, but the clinical significance of this motor
system and how it works in strabismus has to be further investigated.
Eye muscle proprioception
The disturbances that strabismus exerts on vision are treated with glasses and occlusion as
described in Chapter I. There is as yet no effective treatment for binocular dysfunction. Even
if the eyes seem properly aligned by surgery at an early age, and weak binocularity may be
restored, the stereoscopic vision will always remain abnormal.
Eye muscle surgery is the common mode of treatment and it is generally performed at an age
when the child can cooperate in mea in creating an acceptable eye position and facial
appearance, so important for the self-esteem and social interaction of the child. Surgery also
often creates conditions for some binocular vision although abnormal.
In addition to surgery, treatment with eye muscle injection of Botulinum toxin A has been
recently introduced. Such injection can reduce small angle strabismus without surgery and has
been used in childhood strabismus of both manifest and latent types. In may be useful also in
the treatment incomitant strabismus as shown in the Case report.
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Treatment of strabismus
The disturbances that strabismus exerts on vision are treated with glasses and occlusion as
described in Chapter I. There is as yet no effective treatment for binocular dysfunction. Even
if the eyes seem properly aligned by surgery at an early age, and weak binocularity may be
restored, the stereoscopic vision will always remain abnormal.
Eye muscle surgery is the common mode of treatment .It is generally performed at an age
when the child can cooperate in the measurements needed for surgery. The aim of surgery is
to create an acceptable eye position and facial appearance, so important for the self-esteem
and social interaction of the child. Surgery also often creates conditions for some binocular
vision although abnormal.
In addition to surgery, treatment with eye muscle injection of Botulinum toxin A has been
recently introduced. Such injection can reduce small angle strabismus without surgery and has
been used in childhood strabismus of both manifest and latent types. In may be useful also in
the treatment incomitant strabismus as shown in the Case report.
Conclusions
Concomitant childhood strabismus is a common eye disorder and about 2% of the population
is affected by the manifest type and at least 50% by the latent type. The cause of strabismus is
most likely a deficiency in the visual system leading to a misalignment of the eyes. Manifest
strabismus is accompanied by reduced vision, sometimes manifested as monocularly reduced
visual acuity and always as reduced binocular vision. Latent strabismus may cause visual
discomfort and headache due to eye muscle fatigue in keeping the eye aligned. The ocular
motor dysfunction is considered secondary to the primary visual deficits. In order to gain
further understanding of the underlying basic mechanisms of strabismus more studies are
needed of the visual and ocular motor development.
References
Lennerstrand G, Tian S & Han Y. (2000).: Functional properties of eye muscles: motor and
sensory adaptation in strabismus. In Lennerstrand G & Ygge J (eds) Advances in Strabismus
Research: Basic and Clinical Aspects”, Wenner-Gren International Series, vol. 78, Portland
Press, London, pp 3-15.
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Lennerstrand G (2007): Strabismus and eye muscle function. A review. Acta Ophthalmol
Scand, 85: 711 – 723
Lennerstrand G, Bolzani R, Schiavi C, Tian S & Benassi M (2009): Isometric force
development in human horizontal eye muscles and pulleys during saccadic eye movements.
Acta Ophthalmologica, 87: 837 – 842.