Introduction

A squint (or strabismus) is a misalignment of the visual axis of the eyes.

Eye Movements

  • Superior Rectus: Abduction and elevation
  • Lateral Rectus: Abduction
  • Inferior Rectus Abduction and depression
  • Inferior Oblique: Adduction and elevation
  • Medial Rectus: Adduction
  • Superior Oblique: Adduction and depression

Describing a Squint

Persistence

  • Manifest Squint: Present all the time and is referred to as a “tropia”
  • Latent Squint: On present on dissociation of the eyes and is referred to as a “phoria”

Direction of deviation

  • Exotropia: Divergent squint (affected eye looks outwards)
  • Esotropia: Convergent squint (affected eye looks inwards)
  • Hypertropia: Upwards vertical squint (affected eye looks up)
  • Hypotropia: Downwards vertical squint (affected eye looks down)

Classification

A squint can be classified as Paralytic or Non-Paralytic.

Paralytic

Paralytic squints are due to paralysis of one or more of the extra-ocular muscles. The angle of deviation varies according to the direction of gaze, and the squint is greatest when looking in the direction of the action of the paralysed muscle.

Causes

Trauma

Vascular

  • Diabetes
  • Hypertension
  • Aneurysm of the Posterior Communicating Artery (affects CN III)
  • Cavernous Sinus Thrombosis

Neoplasia

  • Acoustic Neuroma (benign tumour of the Schwann Cells of the Vestibulocochlear Nerve
  • Glioma

Inflammatory

Raised Intracranial Pressure

Non-Paralytic Squint

In a non-paralytic squint there is full ocular movement, hence the angle of deviation is the same in all directions.

Causes

  • High refractive error
  • Cataracts
  • Retinoblastoma

Examination

  1. Visual Acuity: In children squinting affects normal visual development because the visual cortex receives a misaligned image from one eye. This causes it to suppress the visual information from the affected eye, leading to a reduction in the visual acuity known as Amblyopia. This can be corrected, but only before the age of seven.
  2. Alignment of the visual axes: Use a pen torch to assess the corneal reflections. There will be deviation in a squinting eye
  3. Cover/Uncover test:
    1. Cover the squinting eye. The unaffected eye will not deviate
    2. Cover the unaffected eye. The squinting eye will move to take up fixation
    3. Since eye movements are equal and opposite then as the squint moves to take up fixation, the unaffected eye also moves.
    4. Remove the cover from the unaffected eye. The unaffected eye will resume fixation and the squint will return to it’s original position.
  4. Alternate Cover Test: Move the cover rapidly between the two eyes. This dissociates the eyes and will show if there is a latent squint.
  5. Assess ocular movements to determine if there is a paralytic squint.
  6. Fundoscopy: assess for cataracts, retinoblastoma, or papilloedema suggesting a raised intracranial pressure
  7. Determine any refractive error

Management

  • Correct any refractive errors with glasses
  • Use an eye patch in children with amblyopia to stimulate the visual acuity in the amblyopic eye. The eye patch should be worn around 6 hours a day.
  • If it is a paralytic squint consider surgery (muscles are weakened by recessing them, and strengthened by shortening them)
  • Use prisms for any diplopia secondary to the squint

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