Four prism dioptre reflex test

A clinical test used in ophthalmology to assess binocular vision and detect microstrabismus.

The Four Prism Dioptre Reflex Test (4Δ test) is a clinical procedure used in ophthalmology to evaluate binocular vision and detect small-angle strabismus, particularly microstrabismus. This test is instrumental in identifying subtle ocular misalignments that may not be apparent during a standard eye examination.

Principle

The 4Δ test is based on the principle of prism adaptation and the Hering's law of equal innervation. When a prism is placed in front of one eye, it displaces the image seen by that eye. The brain compensates for this displacement by moving the eye to realign the visual axes, a process known as fusional vergence. In the presence of microstrabismus, this compensatory movement is either absent or abnormal.

Procedure

The test is typically performed as follows:

1. The patient is asked to fixate on a small target, such as a letter on a Snellen chart. 2. A 4-diopter prism is placed base-out in front of one eye. 3. The examiner observes the movement of both eyes.

Interpretation

- Normal Response: Both eyes make a version movement towards the apex of the prism, followed by a vergence movement to re-establish binocular single vision. - Abnormal Response: In the presence of microstrabismus, the eye behind the prism may not make the expected movement, indicating a disruption in normal binocular vision.

Clinical Significance

The 4Δ test is particularly useful in diagnosing microstrabismus, a condition where the eyes are misaligned by a very small angle, often less than 5 degrees. This condition can lead to amblyopia or "lazy eye" if not detected and treated early. The test is also valuable in assessing the quality of binocular vision in patients with known strabismus.

Limitations

While the 4Δ test is a valuable tool, it has limitations. It may not detect all forms of microstrabismus, particularly those with very subtle deviations. Additionally, the test requires a cooperative patient and a skilled examiner to interpret the results accurately.

Related pages

• Strabismus
• Binocular vision
• Amblyopia
• Prism (optics)

Gallery

Equipment used for the Four Prism Dioptre Reflex Test