JRSM >
Volume 96, Number 1 >
Pp. 3-6
Strabismus and Paediatric Service, Moorfields Eye Hospital, 162 City Road, London EC1V 2PD, UK
Correspondence to: G G W Adams
 | INTRODUCTION |
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 TOP INTRODUCTION STRABISMUSAMBLYOPIACONCLUSIONREFERENCES |
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Squint and amblyopia are common conditions: about one in fifty children have a squint5, and up to 5% of the population have an amblyopic or lazy eye6,7. In view of their lifelong impact on visual function and physical appearance, with consequences for education, jobs and psychological wellbeing8, good management offers substantial long-term benefits. Recent work favours early diagnosis and treatment, and there has been increasing effort to treat children as soon as possible. There is also renewed interest in the treatment of adults. This article reviews these areas.
| STRABISMUS |
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TOPINTRODUCTIONSTRABISMUSAMBLYOPIACONCLUSIONREFERENCES |
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Neonatal deviations and infantile esotropias
Many small babies are thought by their parents to have crossed eyes and the
traditional advice has been to wait and see if this settles and to advise
ophthalmological referral at about three to four months of age if it does not.
In North America some surgeons have been recommending very early squint
surgery, sometimes within a few months of birth, to obtain the best chance of
stereopsis10,11.
Support for an initial wait-and-watch policy has come from Horwood, who has
shown in a prospective longitudinal study of 1150 normal children that
neonatal misalignments are extremely
common12. However,
the 89 children (7.7%) with very frequent neonatal misalignment (more than 15%
of the day) were significantly more likely to develop a convergent squint and
to have associated refractive errors. Interestingly, children without any
evidence of neonatal deviations are at greater risk of astigmatism. In later
life the deviations are commonest at about seven weeks of age, with a decline
in prevalence by fifteen weeks. They tend to be associated with near fixation,
and are thought to be due to immaturity of vergence
mechanisms13. Two
American studies, analysing the histories of children seen with early-onset
squints, have confirmed the variability of early eye alignment and have helped
to define the risk of development of a true
squint14,15.
Whereas a small-angle intermittent squint will frequently resolve, a child
presenting after ten weeks of age with a constant deviation of more than 40
dioptres on two examinations is very unlikely to recover spontaneously. These
data will allow early decisions as to which children will need squint
surgery.
Recent studies have suggested that both the duration of ocular misalignment and the age at which the eyes are straightened are of prognostic importance: they correlate not only with improved stereoacuity outcome but also with improved stability of the long-term eye position and a lesser need for corrective surgery at an older age10,16. Stereoacuity develops between three and five months of age and matures to a near adult level during the first two years of life2. Surgery before the age of two years—possibly within the first six to twelve months of life—is now believed to maximize the chance of a good outcome16. Obviously, other points need to be considered such as the technical difficulty of operating on a small eye. If it becomes established that very early surgery yields the best outcomes, this will have important practical implications for early referral, rapid assessment and availability of surgeons and anaesthetists experienced in operating on and anaesthetizing small children.
Muscle pulleys
Studies of the functional anatomy of the extraocular muscles have
demonstrated the importance of connective-tissue sleeves or pulleys
surrounding the rectus muscles just posterior to the equator of the
eye17. These act as
the functional origins of the muscles, rather than the origin at the orbital
apex, and prevent sideways slipping of the muscles during eye movements. MRI
work has shown that malposition or heterotopia of the pulleys can be a major
factor in incomitant squint and can mimic, for example, inferior oblique
overaction or Brown's
syndrome18,19.
Damage to these pulleys during surgery may be a further cause of incomitance
in a squint.
Squint surgery in older patients
Strabismus surgery can be valuable not only in children but also in older
patients. A US analysis of Medicare claims showed that more than 6500
strabismus procedures were performed on patients over 65 years of
age20, whilst one
UK surgeon reported that 7% of his surgical strabismus workload was in the
over-60s21. The
range of diagnoses was varied but 48% of elderly patients had diplopia, which
is a significant functional disorder that requires correction. No serious
surgical or anaesthetic complications were reported. With the increase in the
aged population the strabismus surgeon will be dealing increasingly with adult
strabismus. If there are no anaesthetic contraindications, age should not be a
barrier to ocular realignment.
| AMBLYOPIA |
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TOPINTRODUCTIONSTRABISMUSAMBLYOPIACONCLUSIONREFERENCES |
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Disability due to amblyopia
Patients with amblyopia may be debarred from undertaking certain jobs
because they fail the required visual standards (in particular they may not be
able to hold a class II professional driving licence) and additionally they
may be at risk of visual handicap if they should damage or lose the vision of
the fellow eye8.
A national population-based epidemiological study undertaken through the British Ophthalmological Surveillance Unit examined the risk, causes and outcomes of visual impairment after loss of vision in the non-amblyopic eye, and the likelihood and factors predictive of improvement in the amblyopic eye25,26. The results indicate that both the risk of permanent visual impairment and its consequences, for the individual and for society in general, are greater than previously assumed, with limited scope for secondary and tertiary prevention of visual impairment in this context. Clinically useful improvement in vision in the amblyopic eye is uncommon, but is associated with greater severity of visual loss in the non-amblyopic eye (i.e. the worse the sight loss in the ‘good’ eye, the better the chance of improvement in the amblyopic eye), age, previous acuity in the amblyopic eye and new optical treatment26. These studies have emphasized the importance of providing children with good vision in each eye, and also undermined the long-held belief that the amblyopic eye will improve if the non-amblyopic eye is damaged.
Screening and treatment of amblyopia
In Sweden and
Israel27,28
the prevalence and severity of amblyopia have declined substantially where
screening programmes are in place. Moreover, vision does not improve in
patients not complying with treatment, and can subsequently be improved with
supervised
compliance29.
The mainstay of amblyopic treatment remains refractive correction with spectacles, and occlusion therapy30. Treatment with spectacles alone may be enough to improve vision in some patients with late-onset amblyopia. Compliance with treatment is the major factor in response, and work with occlusion dose monitors has shown that in many cases compliance is not what is reported. In these circumstances admission to hospital may be effective in securing the necessary compliance29. Most of the improvement in vision is obtained in the first four hundred hours of occlusion therapy31.
Drug treatment for amblyopia has been tried, with levodopa and carbidopa. In the small number of trials reported, usually including patching as well, vision has improved modestly32. This treatment is as yet unproven, and many clinicians are wary of using pharmacological treatments. In adults the side-effects of these drugs are well documented33; long-term side effects in children, particularly on behaviour, may be difficult to detect.
Assessment of vision defects in children with amblyopia
The accurate assessment of vision in children is particularly important in
the detection of amblyopia and in the assessment of response to treatment. The
standard method of assessing vision in older children and adults has been the
Snellen-based letter chart, but this is not appropriate for very young
children. For these, picture-based tests or single letter tests are used.
However, it is important to realize that these can give an apparently better
acuity than a test with a line of letters, because of the effect of crowding.
This means that a child who is apparently doing well with a single letter test
can appear to deteriorate when he or she starts with a linear test—which
can be very disappointing to parents unless the reason is carefully explained.
Because the differences between lines in the Snellen chart are unequal, the
research standard for testing vision in adults is a logMAR based chart.
Similar logMAR based test cards have now been developed for
children34. They
seem to be much more sensitive for detecting interocular acuity
differences35,36,
and are gaining popularity for detecting and managing children with
amblyopia.
Treatment of adults with amblyopia
The success of amblyopia treatment declines steadily with age. For most
amblyopes it becomes ineffective around the age of eight, although some
straight-eyed anisometropes respond well to patching at much later ages. El
Mallah et
al.37 reported
that adult amblyopic patients had an increase in acuity in their amblyopic eye
after losing central vision from macular disease in their good eye. This
observation aroused considerable interest because it raised the possibility of
adult cortical plasticity. However, epidemiological evidence indicates that
such improvement is
uncommon26. Mosley
and Fielder38
suggest that a clinical trial is
needed38. In any
such trial it will be important to determine whether patients have anatomical
loss of retinal ganglion cells in their affected eye since such loss will
cause transneuronal degeneration in the central visual pathway related to the
non-amblyopic
eye39. This is
likely to increase the scope for recovery of vision in the amblyopic eye, as
indicated by the improvement seen in the amblyopic eye of one patient when he
had a retinal detachment in the fellow eye and then showed further improvement
when that eye had to be
enucleated40.
| CONCLUSION |
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TOPINTRODUCTIONSTRABISMUSAMBLYOPIACONCLUSIONREFERENCES |
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| REFERENCES |
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TOPINTRODUCTIONSTRABISMUSAMBLYOPIACONCLUSIONREFERENCES |
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