Strabismus and Extraocular Muscles

Study Notes

Strabismus is a misalignment of the eyes, so the image of an object does not fall on the fovea of both eyes simultaneously.
The image of the object formed by each eye should normally lie at its fovea (Bifoveal Imaging).
Six extraocular muscles insert in the sclera of each eye. These muscles allow for the coordinated movement of the eye.
2-5% of the population are affected by strabismus.

Superior Rectus: Oculomotor nerve innervates this muscle, responsible for:
- Elevation (in adduction)
- Intorsion
- Adduction
Inferior Rectus: Oculomotor nerve innervates this muscle, responsible for:
- Depression (in adduction)
- Extorsion
- Adduction
Medial Rectus: Oculomotor nerve innervates this muscle, responsible for:
- Adduction
Lateral Rectus: Abducens nerve innervates this muscle responsible for:
- Abduction
Superior Oblique: Trochlear nerve innervates this muscle, responsible for:
- Depression (in abduction)
- Intorsion
- Abduction
Inferior Oblique: Oculomotor nerve innervates this muscle, responsible for:
- Elevation (in abduction)
- Extorsion
- Abduction

Strabismus is a common problem.
Significant cause of visual impairment. It may be caused by or associated with:
- Hypermetropia (farsightedness)
- Optic Nerve Hypoplasia
- Retinoblastoma
Strabismus can have a significant cosmetic and social impact.

Direction of Deviation:
- Eso Deviation - Convergent squint, eye turns inward
- Exo Deviation - Divergent squint, eye turns outward
- Hyper Deviation - Upward deviation of one eye
- Hypo Deviation - Downward deviation of one eye
Severity:
- Tropia - Apparent squint, misalignment is always present.
- Phoria - Latent squint, misalignment only present when one eye is covered.
- Example: Right Esotropia - apparent convergent squint of the right eye.
Pathological:
- Concomitant Squint - Angle of deviation of the squinting eye is constant in any direction.
  - Early childhood onset without correction can lead to amblyopia and loss of binocular vision.
  - Examples: Congenital Esotropia, Accommodative Esotropia
- Paralytic Squint (Incomitant) - Caused by complete or partial palsy of one of the nerves that supply the extraocular muscles.
  - Angle of deviation is maximal in the direction of action of the weak muscle.
  - Examples:
    - Oculomotor Palsy
    - Abducens Palsy
    - Trochlear Palsy

History:
- Birth: Prematurity, method of delivery
- Medical problems, developmental abnormalities
- Age of onset
- Direction of deviation
- Constant or intermittent misalignment
- Association with fatigue
- Family history
Examination:
- General Appearance:
  - Craniofacial abnormalities
  - Abnormal head posture (may be to avoid diplopia)
  - Epicanthal fold (a fold of skin at the inner corner of the eye)
- Visual Assessment:
  - Different techniques used to assess vision
  - Assessment for amblyopia (reduced vision in one eye)
- Corneal Reflection Test: Using a pen torch, assessing distance, symmetry, and position of the light reflection.
  - Example: If one reflection is nasally positioned, the eye is turned outward.
- Cover Test: Performed at distance and near. Cover one eye and look for movement of the other to take up fixation. This helps determine the type and severity of strabismus.
- Ocular Movements: Tested in different directions of gaze. Helps assess for muscle weakness, paralysis, or restriction.
- Cycloplegic Refraction: This involves dilating the pupils with eyedrops to measure the refractive error more accurately. This is important for diagnosing and treating accommodative esotropia.
- Dilated Fundus Examination: Examining the back of the eye, including the retina, optic nerve, and blood vessels. This is important for identifying any underlying pathology that may be causing the strabismus.

Full cycloplegic correction of refractive errors: This is important for correcting the refractive error, which can help to reduce or resolve the strabismus.
Treatment of Amblyopia: This is done by occluding the "good" eye to force the brain to use the weaker eye and improve visual function.
Surgery: This is often needed to restore alignment and achieve binocular vision. This involves strengthening or weakening procedures of the extraocular muscles.
- Example: Esotropia (convergent squint) requires medial rectus recession and lateral rectus resection, which weakens the inward pulling muscles and strengthens the outward pulling muscles.

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