Synonym
– squint, turned eye, cross eyes.
 
Definition
– a condition bin which there is a disturbance of the relative position of the optical axes of the eye so that one fovea is deviated from the object of regard.

Incidence
– 3 –5 % of normal children are strabismus.  But more than 50% of Cerebral Palsy children have strabismus.  Similarly the incidence is high in hydrocephalus.

TYPES OF STRABISMUS



1. Incomitant

The angle of deviation varies in different directions of gaze.  This occurs most commonly where there is paralysis of one or more extraocular muscles.

Causes:

a. Neurological - due to lesions of 3rd, 4th, or 6th cranial nerves caused by:

  • Trauma- Tumour – (Intracranial)
  • Infection
  • Raised Intra-Cranial Pressure (6th Nerve Palsy)


b. Muscular – Direct involvement of the extraocular muscles by

  • Trauma
  • Tumour of the orbital or periorbital tissue
  • Infection
  • Muscular Anomaly – Dystrophy etc.

c. Neuro Muscular – Myasthenia Gravis

d. Congenital Conditions:

  • Duane’s Retraction Syndrome
  • Browns Syndrome
  • Moebius Syndrome

In all these states the angle of squint varies, becoming maximal when an attempt is made to gaze in the direction of action of the paralysed muscle.  In children the eyes may fail to realign following temporary paralysis and a permanent comitant squint may occur.

Duane’s Syndrome
– The involved eye is unable to abduct and on adduction is retracted into the orbit.

Brown’s Syndrome 
- The inability to elevate the eye in adduction

Moebius Syndrome
– Combined 6th and 7th nerve palsies.



2.Comitant Strabismus

 The angle of squint is equal in all directions of gaze.  There is no abnormality of function of extra ocular muscles but instead, incorrect co- ordination of binocular muscle function is present.

 Types Of Concomitant Strabismus
- Esotropia (Convergent Squint) optical axes converge  (A convergent squint)
- Exotropia (Divergent Squint) optical axes diverge (A Divergent squint)
- Hypertropia – (Vertical Squint) – One optical axis is deviated vertically.
- Latent Strabismus becomes apparent only on dissociation of the vision of the eyes (eg on covering one eye) and is termed a phoria (exophoria, esophoria, hyperphoria).     This may become overt with fatigue, illness, or with lack of attention.

Causes of Comitant Strabismus

Hereditary :
a familial predisposition to develop strabismus may be inherited as an autosomal dominant trait.

Sensory Deprivation
A blind eye has no incentive to remain aligned.  Therefore any condition which results in markedly reduced vision may cause a squint.  E.g. Corneal Scarring, Cataract, Opacity of the Refractive Media, Retinal Lesion e.g. – Retinoblastoma, Retinal Detachment, Optic Atrophy, High Refractive Error.

Secondary To Paralytic Squint
Children have a tenuous hold on single binocular vision therefore they frequently do not manage to redevelop this following a paralytic squint.

Accommodative
The close anatomical and physiological link between accommodation and convergence causes the frequent excessive convergence (esotropia) in children who are hypermetropic i.e. because these children need to accommodate excessively to obtain clear vision they often break down and develop a convergent squint.

Unknown
No cause for the occurrence of strabismus will be found in many cases.

WHY IS STRABISMUS STRESSED BY OPHTHALMOLOGISTS?



The reasons are multiple:

1.  The Incidence Of Strabismus
- a common condition
- 3-5% in the normal population
- > 50% in Cerebral Palsy and Hydrocephalic patients

2.  Common Misconceptions:
- “The child will grow out of his squint”
- “ Nothing can be done before the age of 2,3 or even 4 years”
- “ Learning difficulties may be cause by a squint”
- That the child only needs “some eye exercises”

THE TRUTH


Children rarely grow out of squints! The misunderstanding arises because an apparent squint due to eyelid shape (e.g. prominent epicanthic folds) frequently becomes less obvious with age, thus confusing parents and perpetuating the myth. Epicanthic fold & Squint (The child on the left of this picture has a true squint -see the light reflections- but the other child has straight eyes)

Squints can be adequately measured, assessed and corrected at any age!  The importance of this early management is clear when one considers that loss of vision caused by a turned eye may become permanent!

Some of the organic causes of squint (eg optic atrophy and retinoblastoma) may require early diagnosis to prevent catastrophe!click picture to enlarge     

Squint and Learning Disability – Major studies have shown incidence of eye movement disorders is not increased in children with learning problems.  There appears to be no correlation between squint and learning disabilities!

Eye exercises play a very minor role in the management of strabismus.

3. THE IMPORTANCE OF STRABISMUS
 
What causes a squint?
Sensory deprivation must always be excluded as a cause of strabismus.  Potentially fatal conditions such as Optic Atrophy – resulting from an intracranial space occupying lesion (Brain Tumour) – or Retinoblastoma frequently present as a squint.  As may Retinal Detachment, Lens opacity, Vitreous Haemorrhage or Toxoplasmosis Retinopathy.

All children with a unilateral squint must be fully examined to exclude these organic disorders if catastrophe is to be avoided!  The possibility that a squint may be secondary to other neurological disorders (e.g. raised intracranial pressure) must also be excluded.

What effect does does a squint Have?
When one eye deviates  immediate brain confusion results.  There is then immediate and absolute suppression of foveal vision of the deviated eye (The eye is turned off).  The object of regard projects to the wrong point on the retina – thus double vision occurs.

After visual maturity is reached (age 7 years) this diplopia is permanent.  But young children are able to ignore this second image – by suppressing it – and they avoid diplopia.

They pay a price for this. Because they are not using the eye correctly in this formative period this causes loss of vision which becomes permanent if not eliminated while the patient is visually immature.  i.e. Amblyopia.

4.  AMBLYOPIA
-. diminished visual acuity not correctable with spectacles, in the absence of organic pathology.

 Incidence of Amblyopia
– 3-5% in the general population.

Amblyopia is caused by a disturbance of foveal visual stimuli – either displacement of the image in strabismus i.e. Strabismic amblyopia or a defocused image with media opacities (vitreous, lens, or cornea) – i.e. Deprivational amblyopia or unilateral refractive error – i.e. anisometropic amblyopia.

Amblyopia is amenable to therapy provided such therapy is prompt, early and continuous until visual maturity is attained.  It is rarely successful after the age of 7 years.

The earlier the onset of amblyopia the denser and the more rapid is its occurrence.

The earlier its management the more rapid and more complete its resolution.

Thus the early diagnosis and correction of strabismus will result in better prevention and control of permanent visual loss  through amblyopia.

MANAGEMENT OF STRABISMUS

 
This involves
1. Confirming the presence of the Squint
2. Confirming the type of squint
3. Establishment of the cause of the squint
4. Eliminating the cause if possible      eg – organic , refractive
5. Treat Amblyopia if present
6. Straighten the eye

The clinician requires a history and examination

History :
- What do parents see?
- Duration and frequency of the squint
- Is it constant?
- Is it greater for near or distance?
- Is it always in the same eye?
- Is there a family history of strabismus?
- Is there a family history of neurological disorder?

This history will help eliminate pseudo squint of epicanthic folds.

A convergent squint seen principally when reading will suggest an accommodative squint.

An intermittent divergent squint is frequently worse for distance.

An alternating squint suggests that there is no amblyopia present and makes an organic cause unlikely.

Examination (A.) Initial Medical Assessment
- Visual acuity – assessed as discussed.
- Extra Ocular Movements -  The ability of the eyes to move to the Cardinal Points of Gaze is assessed to demonstrate paralytic strabismus.
- Corneal Reflections – The symmetry of the reflections, on the cornea, of a narrow light beam is assessed.  Each millimetre of deviation from the centre of the cornea is equivalent to 7 degrees deviation of the eye.
- Cover/Uncover Test -  This test is used to detect the present of a tropia or a phoria.  It may be used together with a prism to measure the angle of deviation.  The test is performed with the patient fixating a detailed object at distance (6m) and near (35cm). When the non – deviated eye is covered the other eye will move to take up fixation i.e. moves in, If Exotropic or moves out, if Esotropic. If the deviating eye is covered no movement will be seen.  If a phoria exists no movement will be seen on covering either eye but the covered eye deviates while occluded and recovers when uncovered.
- Ophthalmoscopy – To exclude organic ocular disorder which may result in a sensory    defect and secondary strabismus.

Direct ophthalmoscopy
At this stage of the examination it is possible to determine whether a squint is present.

If strabismus is diagnosed or if uncertainty exists than referral to an Ophthalmologist should be undertaken IMMEDIATELY.

 Examination (B).  Ophthalmic Assessment
Includes examination as discussed above.  The following examinations are then carried out.
- Refraction - test for glasses (following Cycloplegic Drops) This will demonstrate any refractive error
- Ophthalmoscopy including Indirect Ophthalmoscopy (while pupils are dilated) To exclude intra-ocular disorders, as discussed

Treatment :
If significant refractive error is present (particularly hypermetropia) spectacles are prescribed.  Spectacles may fully correct an accommodative squint.
If amblyopia is present this is treated by occluding the fixating eye until visual acuity is equal in each eye.

Surgery:
This is undertaken, when steps 1 & 2 are completed. To correct any residual angle of squint.  The extra-ocular muscles are shortened or lengthened surgically to correct the angle of strabismus.

Follow up:
Until the possibility of recurrence of strabismus or amblyopia is assured

When to operate:
When the angle of squint can be accurately measured – anytime after 5 months of age.  There is no advantage in delaying the procedure further.  For if an infant is to gain binocular fusion this is only possible prior to the age of two years.  The surgery is routine, complications are rare and minor.  More than one surgical procedure may be required.