Strabismus

Test

This is the initial test of choice. As this test requires a high degree of co-operation from the patient, it may not be appropriate in young children.

Both eyes fixate an object and one eye is covered. If the fellow eye makes a re-fixation movement, manifest strabismus is present, but if the fellow eye makes no movement, no manifest strabismus is present. The direction of the re-fixation movement is opposite to the direction of the deviation (e.g., if the fellow eye makes an outwards movement when the other eye is covered, it was deviated inwards, and an esotropia is diagnosed).

A positive cover test denotes the presence of a manifest strabismus, and a negative cover test denotes the absence of a manifest strabismus.

Test

If a manifest strabismus is detected on the cover test, an SPCT is carried out to measure the angle of the manifest strabismus.

A prism is introduced in front of the deviating eye and, simultaneously, the fixating eye is covered.

If the prism neutralises the strabismus, no re-fixation movement of the deviating eye is noted.

The strabismus is described according to its quality (e.g., esotropia or hypertropia) and size (prism dioptres in the US and degrees in Europe).

Test

If the cover test is negative (i.e., absence of a manifest strabismus), the presence of a latent strabismus is indicated by a positive UCT.

First the cover test is performed, and if this is negative, the cover is removed. If the uncovered eye makes a re-fixation movement, latent strabismus (also known as phoria) is present.

Test

Following the cover and uncover tests, the APCT is carried out to measure the angle of the manifest plus latent strabismus.

A cover is held in front of one eye and left there for several seconds while the patient fixates an object (e.g., reads letters on a chart). The cover is then moved swiftly to the fellow eye and left there for several seconds.

Re-fixation movements of the uncovered eye are noted and can be neutralised with prisms.

Test

In patients unable to co-operate with cover testing, the Hirschberg test can be carried out.

A light is shone on the eyes from an arm's length away while the patient looks into the light (fixates the light target). If the reflection of the light is symmetrically centred on the cornea in each eye, no manifest strabismus is present. If it is decentred in one eye, manifest strabismus is likely.

False-positive results occur if the visual axis does not coincide with the pupillary axis (e.g., in ectopia of the macula). False-negative results can be caused by lack of fixation.

This test can be used to estimate the angle of the strabismus. The angle of the strabismus is approximately 7 degrees or 15 prism dioptres for each 1 mm displacement of the light reflex relative to the corneal centre.

Test

Following the Hirschberg test, the Krimsky test is carried out to measure the angle of misalignment.

If the reflection of a fixation light is decentred on the cornea of one eye (i.e., the deviating eye), a prism is held over the fixating eye. This will induce a conjugate movement of both eyes (version) in the direction of the apex of the prism.

The correct prism strength is reached when the position of the corneal light reflex is symmetric between both the eyes.

Centring of the corneal light reflex with a prism over the fixating eye measures the angle of strabismus.

Test

Examines conjugate eye movements (versions). A target is moved in front of the eyes and the patient is instructed to follow it with both eyes. The patient is asked to report double vision during this test.

The eyes are observed for any incomplete excursions (e.g., incomplete abduction in abducens nerve [cranial nerve VI] palsy). Double vision noted by the patient when looking in a certain direction suggests misalignment.

Test

Examines the movements of each eye individually and is then carried out after version testing.

If the excursions of one eye are incomplete on version testing, the fellow eye is covered and the excursions are again examined.

If the abnormal finding persists, restrictive or paralytic strabismus is likely.

Test

Allows differentiation between mechanical and innervational (i.e., restrictive and paralytic strabismus) causes of abnormal duction.

In co-operative patients, this test can be undertaken in the office; otherwise anaesthesia is necessary as it is uncomfortable and painful. First, the ocular surface is anaesthetised with a topical anaesthetic. The eye is then grasped with forceps or a cotton-tip applicator (the latter method is more comfortable, but less precise) and pushed in the direction of gaze being tested. If the patient is awake, they are instructed to look in this direction and the examiner feels for any restriction, noting its intensity and location.

Test

Neuroimaging (MRI or CT of the brain and orbits) may be required for the evaluation of acquired paralytic and restrictive strabismus to identify the lesion and help define the underlying mechanism.[17]American College of Radiology. ACR appropriateness criteria: orbits, vision and visual loss [internet publication]. https://acsearch.acr.org/docs/69486/Narrative

In orbital trauma or Graves' disease, a CT scan of the orbit helps identify the cause of the strabismus (e.g., entrapment of an extraocular muscle or perimuscular orbital tissue in an orbital fracture, enlarged extraocular muscles in Graves' disease).

Test

Neuroimaging (MRI or CT of the brain and orbits) may be required for the evaluation of acquired paralytic and restrictive strabismus, to identify the lesion and help to define the mechanism of the restriction.[17]American College of Radiology. ACR appropriateness criteria: orbits, vision and visual loss [internet publication]. https://acsearch.acr.org/docs/69486/Narrative ​ If oculomotor nerve (cranial nerve III) palsy is diagnosed, an MRI/MRA (magnetic resonance angiogram) is needed to rule out an aneurysm of the posterior communicating artery (sometimes, a CT angiogram can be necessary).

In strabismus secondary to a cranial nerve palsy, a mass lesion must be ruled out with an MRI of the brain, depending on the associated clinical findings.

Test

In myasthenia gravis, a thymoma should be ruled out by obtaining a chest CT.