A squint (or strabismus) is a misalignment of the visual axis of the eyes.
- 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
- 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)
A squint can be classified as Paralytic or Non-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.
- Blowout fracture
- Aneurysm of the Posterior Communicating Artery (affects CN III)
- Cavernous Sinus Thrombosis
- Acoustic Neuroma (benign tumour of the Schwann Cells of the Vestibulocochlear Nerve
Raised Intracranial Pressure
In a non-paralytic squint there is full ocular movement, hence the angle of deviation is the same in all directions.
- High refractive error
- 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.
- Alignment of the visual axes: Use a pen torch to assess the corneal reflections. There will be deviation in a squinting eye
- Cover/Uncover test:
- Cover the squinting eye. The unaffected eye will not deviate
- Cover the unaffected eye. The squinting eye will move to take up fixation
- Since eye movements are equal and opposite then as the squint moves to take up fixation, the unaffected eye also moves.
- Remove the cover from the unaffected eye. The unaffected eye will resume fixation and the squint will return to it’s original position.
- Alternate Cover Test: Move the cover rapidly between the two eyes. This dissociates the eyes and will show if there is a latent squint.
- Assess ocular movements to determine if there is a paralytic squint.
- Fundoscopy: assess for cataracts, retinoblastoma, or papilloedema suggesting a raised intracranial pressure
- Determine any refractive error
- 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