REVIEW OF STRABISMUS

Incidence of Strabismus
Caucasians = 3-4% - more prevalent in girls
Blacks (USA) = 0.6%
Esotropia vs Exotropia
Europe 5:1
In Japan exotropia is more prevalent

Parental influence
Risk of strabismus is much greater if either of the parents has a strabismus
advise any parent to have a child screened if there is a history of strabismus in the family
60% of children with a strabismus have a close relative with strabismus
unknown how the strabismus trait is transmitted.
strabismus itself is not inherited but some other factor is that predisposes the patient to strabismus.
inheritance pattern is probably multifactorial.

Comitant Strabismus
Infantile esotropia
Esotropia of late onset
Exodeviations
Microtropia

Infantile esotropia :
an early onset (up to 6 months)
large and relatively stable angle of strabismus
Patients are usually emmetropic and rarely have more than a low degree of hypertropia or astigmatism

Esotropia of late onset
often familial
starting at 6 months of age and being most prevalent at 1-2 years
Accommodative factors have an important role together with hyperopia and anisometropia
AC/A ratio is abnormal
Binocularity can be restored if the eyes can be re-aligned by optical, prismatic or orthoptic means.

Exodeviations
onset variable
Progress from latent stage to intermittent and finally a constant angle
Convergence insufficiency and divergence excess are examples that readily respond to orthoptic treatment.

Microtropia
small angle strabismus with HARC, eccentric fixation, amblyopia,
a low or anisometropic refractive error
peripheral fusion and stereopsis recorded on a Titmus fly test.

Treatment of comitant strabismus
Correction of refractive error (prisms)
Orthoptics
Pharmacological treatment
Surgery

Incomitant Strabismus
Myogenic
Neurogenic

Myogenic types
Includes Myasthenia Gravis
different forms of myogenic dystrophies and myopathies
Maldevelopment or malformation of the eye muscles such as the superior oblique tendon sheath syndrome of Brown, Daunes retraction syndrome
effects of muscle entrapment in orbital fractures are included in this group.

Neurogenic types
Strabismus caused by lesions of the 3 rd ,4 th and 6 th nerves
congenital or acquired
Other types of incomitancy will be caused by disruption of supranuclear control of ocular motor function (gaze palsies)

Treatment of Incomitant Strabismus
All new incomitancies must be referred
Once the correct diagnosis with reference to the CNS disease or neuromuscular function has been established it may be possible to relieve the diplopia
Usually surgery followed by prism is necessary to relieve the diplopia
In lesions restricted to localised muscles or nerves the prognosis for control is better than that for supra nuclear lesions

The role of accommodation and refraction in comitant strabismus
The synkinetic relationship between accommodation and convergence is generally attributed to the observations of Donders
Fusion is usually sufficient to maintain binocular vision but the phoria can breakdown in the presence of insufficient fusional amplitude and a tropia results
Correction of the refractive component can restore the phoric conditions in many cases
The presence of esotropia in the absence of refractive error indicates that the aetiology of strabismus cannot be solely attributed to refraction.

General Pathophysiology of Strabismus
The causes of manifest strabismus are known only to a limited extent
Abnormalities of the fusion mechanism
Brain Damage
Neuromuscular Anomalies
Reflexological theories

Abnormalities of the fusion mechanism
If poor fusion exists then the influence of precipitating factors such as hyperopia, anisometropia, trauma and illness may cause the eye to become strabismic
Loss of fusion in childhood leads to an esodeviation and in adulthood exodeviations
differences probably due to differences in the tonus of the muscles.

Brain Damage
A higher incidence of strabismus is present in patients with general brain disease
Downs Syndrome
Cerebral palsy
Hydrocephalus
Depending on the condition 40-60% of patients present with strabismus
Children suffering from general disease (heart lesions) have 4-6 times higher frequency

Neuromuscular Anomalies
Pathophysiological factors connected with
orbital mechanism
eye muscle function,
brain stem
cerebral function
accommodation and convergence coupling
the eye movement system
development of oculomotor function.

Reflexological theories
suggested that strabismus is due to a disturbance of the oculomotor reflexes
Eye position during feotal life depends upon subcortical reflexes initiated by stimulation of the eye muscle proprioceptors (induce monocular duction reflex) and the vestibular organs (induce a binocular reflex for versions)
After birth the light stimulation initiates the development of the oculomotor reflexes which supersede the older subcortical reflexes.

would depend upon an abnormal development of the oculomotor reflexes and consist of a predominance of the monocular adduction reflexes over those for conjugate movements and abduction.
However, disturbances in oculometric responses would be secondary to changes in the sensory systems and need not be the primary cause of strabismus
Esotropia

Motor Factors related to Strabismus
Disruption of fusion with the subsequent onset of strabimus can be caused by several motor and sensory mechanisms in isolation or in combination.

Mechanical factors in the orbit
Anomalies of the check ligaments that connect the muscles and surrounding tissue have been proposed as important in the aetiology of strabismus
More recently anatomical variations in the insertions of the EOM on the globe have been proposed as being more significant
Mechanical factors are the most likely cause of strabismus in cranio-facial malformations (eg Browns Syndrome).

Mechanical factors in the orbit

Extra-Ocular Muscles (EOM)
The orbital singly innervated muscle fibre may have a prominent role in ocular motility
This fibre type in highly oxidative and fatigue resistant
both factors depend upon its extensive capillary network
It is the last of the 6 basic morphological fibre types to develop its adult features and is most susceptible to alterations in its innervation both neural and vascular.
The changes in length/tension that are characteristic of strabismus may be attributable to variations from normal in the singly-innervated muscle fibre and its associated microvasculature under the influence of neural and environmental factors.

Brain Stem Control of Eye Movements
Specific lesions of the brain stem can be identified with respect to strabismus
Internuclear ophthalmoplegia is caused by a lesion in the MLF, the pathways for the axons of the abducens internuclear neurons to the oculomotor complex.
Abnormal vestibulo-ocular function has been reported in patients with early onset strabismus
Slight abnormalities of balance and gait have been shown in children with esotropia but not exotropia
These disturbances might represent signs of dysfunction in the cerbellopontine control of gait of postural control
Duane’s and Moebius’ syndromes – abducens palsy due to hypoplasia or aplasia of the adducens nucleus
There are many other examples

Conclusion
The aetiology of strabismus is poorly understood and is likely to be the result of interactions between many control systems
The adaptive systems are most likely to be involved with the cause of the strabismus rather than the short lasting phasic components

Strabismus – Treatment
When evaluating a certain disorder it is important to consider both functional and cosmetic results
Rarely does a strabismic patient complain about binocular status if the strabismus has been longstanding
Clinical objectives should be realistic in relation to the initial binocular status
Alternative treatments available

Surgery
15% of strabismics have no refractive component
surgery necessary
If only a small component of the strabismus in refractive referral is probably necessary
Not necessarily the size of the strabismus that is important but the overall cosmesis

2 main goals when surgery is performed
Functional binocularity (Approx. 10%)
Improvement of astenopic or diplopic symptoms when prism does not work
Restoration of binocular alignment following injury or disease
Development of binocular vision by providing alignment or by improving comitance
Prevention of sensory anomalies
Correction of head turns or tilts
Cosmetic Improvement
Large angle strabismus are cosmetically unacceptable

Timing of Surgery

Infantile strabismus
Early surgery advantageous as ocular alignment important
Later if there is a limited prospect of binocular function

Acquired Childhood Strabismus
A child up to 6 who had binocular fusion but later develops manifest strabismus has an important backlog of fusion.
Immediate restoration of BV is essential.

Acquired adult strabismus
Usually results from disease or strabismus (also a breakdown of childhood strabismus)
If incomitant an immediate referral is required to determine the aetiology.
Comitant disorders should receive vergence training or prismatic correction.
If these alternatives are unsatisfactory and a cosmetically unacceptable strabismus is present referral for surgery should be considered.

Non-Surgical Treatment

Pharmocological Treatment

Miotics
can be used to reduce innervation to the vergence system as less accommodative drive is required.
Produces an accommodation that is peripheral to the vergence system – less accommodative/convergence is required at near.
Will reduce AC/A ratio – reduce SOT to SOP and maybe then use exercises in conjunction.

Diagnostic Trial :
Use of miotics helps distinguish between an accommodative SOT and a non-accommodative SOT in infants.
After miotic instillation if the SOT is reduced then the origin is either accommodative or related to the AC/A ratio.
No change = non-refractive strabismus and surgery is indicated

Therapy
but bifocals are preferred
Local Side Effects : reduced VA is reported due to miosis and increased accommodative spasm

Atropine
Can use atropine for long-term use in control of accommodative esotropia
not advised due to the toxicity of the drug, the use of bifocals to read and photophobia.
Cyclopentolate has some short-term use

Botulinum A Toxin
Potent neurotoxin
Affects specific nerve receptors in striated muscles when used in minute doses
Denervates the muscle
Target muscle paralysis is transient the resulting change in static eye position can be permanent
Weakens target overacting muscel to balane with its counterpart

Optical Correction

Refractive Correction
Correcting the refractive error provides a clear retinal image (preventing amblyopia)
creates the correct balance between accommodation and convergence.
Cycloplegic refraction should always be considered in young patients to determine the full extent of the underlying problem.
If the refractive correction achieves binocular vision then consider prescribing the full amount.

Manipulation of the AC/A ratio using modified presentation
Over-correct myopes to control XOP/T
It is not practical to under correct myopes as VA is reduced
Consider maximum plus if BV is achieved, supplementary exercises can be given to improve fusional reserves.

Bifocals
Useful in non-refractive conditions with abnormal AC/A ratios
Convergence excess – extra plus at near
Convergence insufficiency – extra minus at near
Divergence excess – increase minus at distance
Divergence insufficiency – increase plus at distance (not very practical)
Use a large bifocal fitted to lower pupil margin

To calculate add lens power a theoretical estimate can be made by measuring the AC/A ratio
Distance 18XOP/T Near 8XOP PD = 60mm AC/A= 6 + (-8-(-18))/2.5 AC/A = 10∆/D Add lens for distance = 1.75D
This value provides a starting point for your investigation.
Can use the lens in the short-term while building up fusional reserves

Prismatic Correction
approach is used in small comitant vertical deviations and can also be useful in small horizontal phorias that are not ammenable to refractive correction or orthoptic exercises.
Sheards criterion – placing oculomotor system in the middle third zone The fusional reserve should be twice the heterophoria amplitude Sheards prism = 2(Phoria) – fusional reserve Fixation vergence

Fixation Disparity
Only give prism to patients with symptoms
Many patients have a fixation disparity but have no difficulty
The amount of prism to neutralize the disparity is not always related to the angular subtense of the error as stability plays a key role in the stability of the oculomotor system.

Orthoptic Therapy
Preferred to surgery as it provides the best possibility of a functional cure.
The aim is to give stable, comfortable, functional binocular vision
Orthoptic therapy is often used in conjunction with the above treatments.

Three basic elements can be trained
Sensory fusion
Disparity vergence
Accommodation One general principle is to establish sensory fusion prior to training motor fusion. Both can be trained simultaneously but priority should be given to anti-suppression training.

Techniques used in orthoptic therapy
Fusional reserve exercises
Free-space techniques
Near to far tracking
Step Vergence
Sliding vergence
Near to far jumps

Free Space Techniques

Fusional reserve exercises
the treatment of choice for exo-deviations up to 20  .
In exo-deviations the ability to converge at the relevant distance is trained (positive fusional reserves)
Training negative fusional reserves is more difficult but can produce good results.

Free-space techniques
involve the fusion of two stereo pairs by over-converging or under-converging in free space
must demonstrate physiological diplopia to the patient using two fairly large and obvious targets (eg two pencils)
If the patient has difficulty seeing the two pencils then gross suppression is indicated (the suppression can usually be broken down fairly easily in heterophoria)
Once the principle of physiological diplopia has been demonstrated to the patient they can progress to exercises such as the three cats
This exercise can be used in both exo- and eso-deviations.
Ensure check markers are used
Helps to break down suppression
If BV present at one fixation then in principle should be able to achieve it at all distances

Near to far tracking
Pen to nose exercises
Care is required as fusion is not always achieved but patient does not report diplopia due to well developed suppression

Step Vergence
Introduce prism before the eyes while patient fixates a target
This introduces disparity and the vergence system is stimulated to retain fusion
The prism is then removed when fusion is established
A series of repetitions are performed to strengthen fusion.

Sliding vergence
A target is fixated using a VPS or a Risley prism and the amount of prism is increased to blur/break/recovery
Repeated trials can improve the fusional amplitude.

Near to far jumps
Exercises accommodation and vergence together.
Must maintain clear single vision.

Facility Training
Prism flippers
two pairs of prisms mounted on a horizontal bar with base in prism on top of the bar and base out on the bottom
Another method that is useful in optometric practice is to glaze prisms into an old spectacle frame that has the sides taken off
The idea then is to flip the amount of prism while the patient is viewing at a certain distance.

Training Accommodation
Accommodation insufficiency is often reported in puberty and is often associated with convergence insufficiency
Signs
Low amplitude of accommodation
Gets lower on repeated measures
Patients report target alternately blurred and clear when performing a task
Reduced accommodation facility

Norms for Accommodation Facility
Average Clinically Significant Adult 20 cycles 64 seconds >90 seconds Child 10 cycles 52 seconds >75 seconds

Symptoms
Discomfort
Intermittent blur
Headaches

Treatment
Reading spectacles or bifocals with associated base-in prism to relieve the stress on the vergence system
Alternative orthoptic treatment has been shown to be successful
Push-up or physiological diplopia
Jump focus – lens flippers
Monocular and binocular accommodative rock
If no improvement it may be necessary to refer

Overview of Treatment
Diagnosis
Identify cause
Consequences if treatment not performed
Prognosis
Alternatives

Non-surgical management
Poor prognosis
Small angle longstanding strabismus
ARC – stable
Inconsisitency
Nystagmus
Deep amblyopia,
Deep suppression

REVIEW OF STRABISMUS

by Hossein Mirzaie

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