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 3rd,4th and 6th 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/TNear 8XOPPD = 60mmAC/A= 6 + (-8-(-18))/2.5AC/A = 10∆/DAdd 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 zoneThe fusional reserve should be twice the heterophoria amplitude

Sheards prism = 2(Phoria) – fusional reserveFixation 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

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 SignificantAdult 20 cycles 64 seconds >90 secondsChild 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