In order for the eyes to move fully and together, there has to be correct functioning of the extraocular muscles, cranial nerves III, IV and VI, and the higher cortical centres, which control the speed of eye movements.
Problems in the function of any of these can lead to the development of strabismus or a squint.
Strabismus describes the appearance of the eyes when the visual axis fails to meet at the fixation point. A squint can be convergent or divergent.
1. Congenital squint
Congenital squints occur in those under six months old.
Although squints are common in neonates, the eyes should be fully aligned by around three months of age.
2. Non-paralytic squint
A non-paralytic or concomitant squint is when the squint occurs in all directions of gaze. Double vision does not usually occur. The eye that does not fixate usually has amblyopia. A non-paralytic squint is usually more obvious (or sometimes only noticed) at certain times, for example when the patient is tired.
A non-paralytic squint affects about 5 per cent of five-year-olds and around 3 per cent of 13 to 24-year-olds. The vast majority of children with non-paralytic squints have no associated conditions.
3. Diagnosis of a squint
For many patients, the squint is obvious on examination. A simple screening test for strabismus is the Hirschberg test. This can also be used to give an estimate of the degree of strabismus. A pen torch is shone in front of the patient's eyes and the light reflection should be in the same spot bilaterally. In the presence of a squint it is in different areas of each eye.
The cover/uncover test is mainly used to assess non-paralytic squint and also to recognise latent squint. The patient is asked to focus on an object and one eye is covered quickly.
If a squint is present the uncovered eye moves to fixate on that point. The test is then repeated for the other eye.
The dominant, fixing eye will not move when the other, squinting, amblyopic eye is covered or uncovered.
4. Management of a non-paralytic squint
A child with a suspected squint should be referred to an eye clinic (usually to an orthoptist) for further assessment. Prompt referral is important as the earlier the referral, the better the chance of avoiding amblyopia.
The treatment depends on the nature of the squint and on the patient's age. Correction of any refractive errors is vital. In patients under eight years old, any concurrent amblyopia should also be treated with either eye patching or by using miotic drops in the dominant eye. Most poor results of the treatment of a lazy eye are due to a lack of compliance from either the parent, carer or child.
More advanced strabismus is usually treated with prisms and/or surgery. Surgery is performed to align the eyes by shortening, lengthening or changing the position of one or more of the extraocular eye muscles and is frequently the only way to achieve cosmetic improvement.1
Surgery may under or over correct, which leads to further surgery being required.
The presence of strabismus has been shown in many studies to adversely affect many aspects of patients' lives including finding a partner, job prospects and interaction with peers.
Surgical correction has been shown to have hugely positive effects on these psychosocial aspects of having a squint.
The outcome for children with squints depends on the nature and degree of the squint and whether there are any associated underlying problems.
Generally, early intervention produces good alignment and also limits any amblyopia. However, perfect stereopsis (3D vision) is rarely achieved.
6. Paralytic squint
Paralytic or incomitant squint occurs when there is an acquired defect of movement of an eye. The squint (and double vision) is maximally demonstrated by looking in the direction of action of the weakened muscle.
Paralytic squints occur due to disease of the III, IV and VI cranial nerves. A nerve palsy may be isolated or there may be multiple nerves involved. Each nerve may be affected at any point along its course from the brainstem to the orbit.
Cranial nerve III palsy may present with external ophthalmoplegia where there are partial or complete motility problems resulting in varying degrees of squint. A ptosis may also occur.
Pupil-sparing causes tend to relate to ischaemic microvascular disease (and rarely, cavernous sinus syndrome). Pupil-involving palsies usually arise as a result of an aneurysm but can also occur as a result of other conditions, such as a tumour, trauma or herpes zoster.
Cranial nerve VI palsy may present with binocular vertical diplopia, difficulty in reading and the sense that things appear to be tilted. Causes include trauma, vasculopathy (usually related to diabetes or cardiovascular disease) and MS.
Cranial nerve VI palsy presents with horizontal diplopia which is worse for distance than near vision and most pronounced on lateral gaze on the affected side. This usually occurs due to a vasculopathy (usually diabetic or cardiovascular disease) or trauma. It can also be idiopathic.
A combination of these cranial nerve palsies can also occur due to numerous conditions and syndromes. These include an arteriovenous fistula, cavernous sinus tumours or thrombosis, intracavernous aneurysm and herpes zoster.
Patients with paralytic squints should be referred to the local ophthalmologist or neurologist, depending upon the underlying clinical condition.
If no cause is found, or while the underlying problem is being corrected, patients may have prisms fixed onto their glasses to alleviate the diplopia.
- Dr Newson is a GP in the West Midlands
1. Simon JW. Curr Opin Ophthalmol 2010; 21: 361-6.
2. Durnian JM, Noonan CP, Marsh IB. Br J Ophthalmol 2010; doi: 10.1136/bjo. 2010. 188425