Parks–Bielschowsky three-step test

Parks–Bielschowsky Three-Step Test is a diagnostic procedure used in ophthalmology and neurology to determine the cause of acquired vertical diplopia (double vision). It is named after Marshall M. Parks, an American ophthalmologist, and Alfred Bielschowsky, a German neuro-ophthalmologist. The test is designed to isolate the paretic extraocular muscle in cases of trochlear nerve palsy or other ocular motility disorders.

Overview

The Parks–Bielschowsky Three-Step Test is based on the principle that the direction and magnitude of diplopia can help localize the affected extraocular muscle. It is particularly useful in diagnosing fourth nerve palsies, which affect the superior oblique muscle, one of the muscles responsible for eye movement.

Procedure

The test consists of three steps:

1. Determine the eye with the hypertropia: The patient is asked to fixate on an object, and the examiner observes which eye is higher than the other. The eye that is higher is presumed to have the paretic muscle.
2. Determine if the hypertropia increases in ipsilateral or contralateral gaze: The patient is then asked to look to the right and left. An increase in the height of the hypertropic eye in either gaze indicates that the muscle involved is either an elevator or depressor that is most active in that direction of gaze.
3. Determine if the hypertropia increases on head tilt to the right or left: Finally, the patient tilts their head to the right and left shoulders. An increase in hypertropia on head tilt indicates involvement of the oblique muscles, which are intorsors or extorsors of the eye.

Interpretation

The results of the Parks–Bielschowsky Three-Step Test can help identify the paretic muscle. For example, if the hypertropia increases on tilting the head to the right, it suggests a weakness in the right superior oblique muscle or the left inferior oblique muscle. The test is highly specific for diagnosing fourth nerve palsy but can also indicate other ocular motility disorders.

Limitations

The Parks–Bielschowsky Three-Step Test has some limitations. It assumes that the only problem is with one muscle and does not account for complex cases where multiple muscles may be involved. Additionally, the test's accuracy can be affected by previous ocular surgery, orbital trauma, or restrictive strabismus.

Conclusion

The Parks–Bielschowsky Three-Step Test is a valuable tool in the diagnostic evaluation of vertical diplopia. It provides a systematic approach to identifying the paretic muscle involved in ocular motility disorders, particularly fourth nerve palsy. However, clinicians should be aware of its limitations and consider other diagnostic tests and clinical findings in complex cases.