|

Strabismus

Last updated:

Overview

Also known as squint, strabismus describes the misalignment of the visual axis, meaning the eyes are not directed at an object at the same time. The other eye may:

  • Deviate inwards – esotropia
  • Deviate outwards – exotropia
  • Deviate upwards – hypertropia
  • Deviate downwards – hypotropia

The misalignment may be persistent or intermittent.

If uncorrected, strabismus may lead to amblyopia, where the brain does not process inputs from one eye and favours the other. 

Types

Strabismus can broadly be classed as:

  • Concomitant (non-paralytic) – make up most childhood squints
    • Ocular deviation is seen in all directions of gaze by an equal amount and is not affected by the direction of gaze
    • There is no paralysis/limitation of eye movements, but the eyes are not balanced
  • Incomitant (paralytic)
    • Ocular deviation varies with the direction of gaze
    • This suggests an acquired neurological or muscular disorder leading to paresis/paralysis of one or more extraocular muscles, leading to eye movement limitations

Epidemiology

  • Esotropias are the most common type of strabismus in children
  • Around 2-3% of children develop strabismus
  • Normal binocular coordination is attained at around 3 months

Causes

Disorders affecting extraocular muscles cause incomitant strabismus, which suggests a serious underlying cause.

Risk Factors

  • Low birth weight
  • Prematurity
  • Maternal smoking during pregnancy
  • Anisometropia – eyes have unequal refractive power causing them to focus unevenly
  • Hypermetropia
  • Family history
  • Pseudo-squint – where it looks like there is strabismus but one is not actually present
    • Prominent epicanthic folds can partially cover the sclera on the nasal side leading to the appearance of strabismus
  • Neurodevelopmental disorders

Presentation

Patients present with:

  • An obvious squint – parents may present with concerns about their child having a ‘turning eye’
    • This should be taken seriously, even if it cannot be replicated in the consultation as strabismus may be intermittent
  • Older patients and adults may present with diplopia
  • Some patients may present with amblyopia

Screening

Overview

In the UK, routine eye checks are offered to children:

  • Within 72 hours of birth as part of the newborn physical examination
  • Between 6-8 weeks to identify any problems not picked up soon after birth
  • At school entry (around 4-5 years) to identify strabismus and refractive errors

Assessment

Overview

  • General eye inspection:
    • To screen for asymmetry in eye position and eye abnormalities including pupil asymmetry and ptosis
    • To screen for eye movement problems (e.g. impaired abduction in esotropia)
    • To screen for nystagmus
  • Corneal light reflex test (Hirschberg test):
    • Have the patient fixate on an object in the distance
    • Hold a light source around 30 cm in front of the patient’s eye
    • Look for where the light shines back on the patient’s corneas – they should be at the same place in both eyes and asymmetry suggests strabismus
  • Cover test – to look for manifest strabismus (where squint is present when the eyes are open and being used):
    • Ask the patient to focus on an object 33 cm away from their eyes
    • Cover one eye with a piece of card
    • As the cover goes in front of the eye, watch the uncovered eye for movement and repeat with the other eye
    • The squinting eye moves to align and fixate on the object and this is the movement suggesting manifest strabismus. If this is not present, look for a latent strabismus
  • Cover/uncover test – to look for latent strabismus (where the eye turns only when covered or shut):
    • Ask the patient to fixate on an object and cover an eye for about 3 seconds then quickly remove the cover and look for any eye movement in the covered eye. Repeat in the other eye
    • In latent strabismus, the eye drifts under the cover and then straightens to regain binocular vision after the cover is removed.
  • Check the range of eye movements to check whether it is concomitant/incomitant strabismus:
    • Limitations in eye movements and varying degrees of strabismus with eye movement suggest incomitant strabismus, which is more likely to be associated with more serious causes.
    • Perform a general clinical assessment to identify serious underlying causes

Referral

Routinely refer any child with suspected or confirmed strabismus to the paediatric eye service. They will assess the type and severity of strabismus and initiate treatment which may involve:

  • Corrective glasses
  • Occlusion therapy to treaty amblyopia using eye patches
  • Penalisation therapy – deliberately blurring the unaffected eye with atropine to force the patient to use the amblyopic eye
  • Eye exercises
  • Surgery

Urgently refer any patient with a suspected serious underlying condition. Red flags include:

  • Limited abduction
  • Double vision
  • Headaches
  • Nystagmus

Complications

  • Amblyopia (lazy eye)
    • The visual cortex may suppress images from the squinting eye to prevent double vision in children. This is because their visual pathways are still developing.
    • As a general rule of thumb, vision develops until around 7 years old.

Prognosis

  • Early intervention and treatment are associated with good outcomes.

Author