Strabismus (‘squint’)

• Patients with strabismus may suffer functional and psychosocial problems related to their condition.
• Early recognition and referral by GPs is important as visual pathways may become permanently impaired if not addressed early.
• All children with strabismus should be referred to an ophthalmologist for further assessment and visual rehabilitation.
• The new onset of strabismus may be a red flag for serious intracranial or intraocular pathology.

• is a common disorder of ocular alignment that affects 2–4% of children.
• untreated strabismus lead to:
  • medical
    • leads to development of amblyopia (permanent loss of best corrected visual acuity in a structurally healthy eye)
    • After the age of 9 years, these pathways may never be recovered even if normal visual function is restored
  • Social
    • lifelong cosmetic disability 
    • poor self-esteem
    • social prejudice
    • restricted career opportunities 
• usually present before school age (with an average onset at 1–4 years)

Common causes of strabismus
Primary causes of strabismus Risk factors:  – family history of strabismus – premature birth – low birth weight	Secondary causes of strabismus often associated with neurological pathology
Idiopathic strabismus Congenital syndromes	Cranial nerve palsies (CNIII, IV, VI)  Orbital fracture Intracranial bleed Intracranial/intraorbital/intraocular mass (benign or malignant) Intracranial infection Grave’s disease Myasthenia gravis Diabetes mellitus Amblyopia Toxins and heavy metal poisoning Post-vaccination

Types of strabismus

The most common type of strabismus involves horizontal misalignment of the eyes, although vertical misalignment also occurs.⁷ Ocular deviation may be manifest (tropia) or latent (phoria). Manifest ocular deviation can be present in all directions of gaze (comitant) or only present in specific directions of gaze (incomitant)

Strabismus terminology

Terminology

eso- = Nasal horizontal deviation (relative to fixing eye) exo- = Temporal horizontal deviation (relative to fixing eye) hyper- = Superior vertical deviation (relative to fixing eye) hypo- = Inferior vertical deviation (relative to fixing eye) tropia = Manifest disorder of ocular alignment phoria = Latent disorder of ocular alignment comitant = Ocular deviation present in all directions of gaze incomitant = Ocular deviation only present in specific directions of gaze amblyopia = Clinically defined as a 2-line difference from best corrected visual acuity in a structurally healthy eye

History

• screen for red flag features suggesting recent trauma or serious intracranial pathology
• explore include the obstetric and developmental history
• history of malignancy or autoimmune conditions, exposures, vaccinations, and whether the child has been generally well or if there has been unexplained illness. 
• Any history of trauma, particularly to the head, orbit or periorbital area should be elicited.
• In regards to vision, any previous visual testing and the outcome are relevant.  

Screening tests

• Tests
  • light reflex test
  • red reflex test
  • cover test
  • Any child diagnosed with strabismus should be referred to an ophthalmologist for further assessment.

The light reflex test

The child is placed on their parent’s lap. The doctor stands at a distance of 1 m in front of the child, holding a small light. The child’s attention is directed to the light. The position of the light’s reflection in each of the child’s eyes is noted and compared. Normal ocular alignment will generate an identical light reflection in each eye. Deflection of the light reflex indicates abnormal ocular alignment, with each 1 mm of deflection equivalent to 15–20 prism diopters deviation (Figure 1).

Figure 1. The light reflex test

The red reflex test

The doctor stands at a distance of 0.5 m in front of the child, holding a direct ophthalmoscope (set at 0). The child’s attention is directed to the ophthalmoscope and the doctor attempts to visualise the red reflex of both eyes simultaneously. Both red reflexes should be identical. Inequality in size, shape or colour is abnormal (Figure 2).

Figure 2. The red reflex test

The cover test 

The doctor stands in front of the child and directs the child’s attention to a target (eg. a light or a toy). The doctor covers one of the child’s eyes and closely observes the uncovered eye for corrective movement. When the fixating normal eye is covered, a manifest abnormal eye must move from its deviated position and take up correct fixation. Accordingly, movement indicates that manifest strabismus is present in the uncovered eye (a tropia). The cover test is repeated on each eye (Figure 3).

Figure 3. The cover test

The uncover test

The doctor covers one of the child’s eyes for 5 seconds and directs the child’s attention to a target. The cover is then quickly removed. The newly uncovered eye is closely observed for corrective movement. A latent abnormal eye will drift into a deviated position when covered. After it is uncovered, the abnormal eye must then return to correct fixation. Accordingly, movement indicates that latent strabismus is present in the newly uncovered eye (a phoria). The uncover test is repeated on each eye (Figure 4).

Figure 4. The uncover test

Management of strabismus

• timely referral to an ophthalmologist for a comprehensive assessment of visual function.
• Any patient with red flags on history or examination should be referred urgently for specialist investigation.
• Specific visual rehabilitation programs will depend on a number of factors, including the precise type of ocular deviation involved and whether the strabismus is primary or secondary.
•  Non-surgical interventions commonly include refractive error correction with spectacles or contact lenses, and amblyopia therapy with patching or atropine penalisation. 
• Other uncommon interventions include the use of prisms in glasses, behavioural eye exercises and the use of intramuscular botulinum A neurotoxin. 
•  Surgical correction of ocular alignment is commonly recommended and is well tolerated as a day procedure, with minimal post-operative discomfort.
• Treatment goals are primarily to prevent amblyopia and achieve binocular vision with functional depth perception (stereopsis), and secondarily to achieve better cosmesis.