To maintain single vision, fine co-ordination of eye movement of both eyes is necessary. Abnormality of the co-ordinated movement is called squint. Other names for squint: strabismus; tropia. Exotropia is divergent (one eye turned out) squint: esotropia is (one eye turned in) convergent squint. Prominent epicanthic folds (diagram) may produce pseudosquint.
Non-paralytic squints
These usually start in childhood. The range of eye movements is full. Squints may be constant or not. All squints need ophthalmological assessment as vision may be damaged if not treated.
Diagnosis
Difficult, eg in uncooperative children. Screening tests:
- Corneal reflection: reflection from a bright light falls centrally and symmetrically on each cornea if no squint, asymmetrically if squint present.
- Cover test: movement of the uncovered eye to take up fixation as the other eye is covered demonstrates manifest squint; latent squint is revealed by movement of the covered eye as the cover is removed.
Convergent squint (esotropia)
This is the commonest type in children. There may be no cause, or it may be due to hypermetropia. In strabismic amblyopia the brain suppresses the deviated image, and the visual pathway does not develop normally and the eye may be blind in later life.
Divergent squint (exotropia)
These tend to occur in older children and are often intermittent. Amblyopia is less commonly a problem.
Management
Remember 3 O's: Optical; Orthoptic; Operation. Treatment starts as soon as the squint is noticed.
Optical
Assess the refractive state after cyclopentolate 1% drops; the cycloplegia allows objective determination of the refractive state; the mydriasis allows a good view into the eye to exclude abnormality, eg cataract, macular scarring, retinoblastoma, optic atrophy. Spectacles are then provided to correct refractive errors.
Orthoptic
Patching the good eye encourages use of the one which squints.
Operations (eg resection and recession of rectus muscles)
These help alignment and give good cosmetic results. NB: use of botulinum toxin has helped some patients with esotropia.
Paralytic squint
Diplopia is most on looking in the direction of pull of the paralysed muscle. When the separation between the two images is greatest the image from the paralysed eye is furthest from the midline and faintest.
Third nerve palsy (oculomotor)
Ptosis, proptosis (as recti tone ), fixed pupil dilatation, with the eye looking down and out.
Fourth nerve palsy (trochlear)
There is diplopia and the patient may hold his head tilted (ocular torticollis). The eye looks upward, in adduction and cannot look down and in (superior oblique paralysed). Causes: trauma 30%, diabetes 30%, tumour, idiopathic.
Sixth nerve palsy (abducens)
There is diplopia in the horizontal plane. The eye is medially deviated and cannot move laterally from midline, as the lateral rectus is paralysed.
Causes
Tumour causing intracranial pressure (compresses the nerve on the edge of the petrous temporal bone), trauma to base of skull, vascular, or multiple sclerosis.
[prescription take]
Botulinum toxin can eliminate need for strabismus surgery in selected VI palsies.
Medial rectus
Look at your nose (adduction).
Lateral rectus
Look away from your nose.
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| fig 1: Superior oblique |
Superior rectus (fig 1) primarily moves the gaze upward and secondarily rotates the top of the eye towards the nose (intorsion). Note its eccentric attachment.
Inferior rectus primarily moves the gaze down (Secondary action: rotation of the bottom of the globe towards the nose.)
Superior oblique primarily rotates the top of the globe towards the nose; it secondarily depresses gaze. Note its eccentric attachment.
Inferior oblique primarily rotates the bottom of the globe towards the nose1 and secondarily moves gaze upward. Tertiary action of each obique: abduction.
Best results are achieved in childhood strabismus by:
- Early detection of amblyopia. If >7yrs old, amblyopia may be permanent.
- Conscientious and disciplined amblyopia treatment.
- Optimal glasses (especially full plus in esotropia).
- Having the child see as straight as possible as soon as possible after amblyopia treatment is optimized.
Gobin's principles: Evaluate all aspects of the strabismus (horizontal, vertical and oblique); search for the obstacles to ocular movements which cause alteration of binocular vision; remove them.
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