Advanced Clinical Test | Parks-Bielschowsky Three-Step Test: How To Use It

David B. Sabin
4 days ago
3 min read

The Parks three-step test is used for acquired vertical diplopia to help identify a paretic cyclovertical muscle. It is classically useful for suspected CN IV palsy, especially superior oblique palsy. EyeWiki describes it as the standard diagnostic test for cyclovertical muscle palsy, while also noting that newer studies have questioned its sensitivity in some cases.

Doctor and patient discuss the Parks 3-Step Test for eye muscle palsies in a clinic. A monitor displays test steps amid calming decor. — Doctor and patient discuss the Parks 3-Step Test for eye muscle palsies in a clinic. A monitor displays test steps

When to Use the Parks three-step

Use the Parks three-step test when the patient has:

Binocular vertical or oblique diplopia
A measurable hypertropia
No obvious restrictive pattern
No clear orbital disease
No strong suspicion for skew deviation or myasthenia as the primary explanation

Use caution when the pattern is comitant, variable, bilateral, restrictive, traumatic, or neurologically complex.

Doctor and patient in an eye exam room discussing hypertropia. Step 1. — Step 1. Doctor and patient in an eye exam room discussing hypertropia.

Step 1: Identify Which Eye Is Hypertropic in Primary Gaze

Perform cover testing in primary gaze. Determine whether there is a right hypertropia or left hypertropia.

Example: Right Hypertropia

If the right eye is higher, the weak muscle is either:

A depressor of the right eye:

Right superior oblique
Right inferior rectus

An elevator of the left eye:

Left superior rectus
Left inferior oblique

So after Step 1, your possible muscles are:

A doctor explains eye test steps to a patient in an office. Step 2. — Step 2. A doctor explains eye test steps to a patient in an office.

Step 2: Determine Whether the Hypertropia Worsens in Right Gaze or Left Gaze

Now measure the hypertropia in right gaze and left gaze.

Using the same example, if the patient has a right hypertropia, and it worsens in left gaze, that narrows the list.

Why? In left gaze, the main vertical movers are:

Right superior oblique
Right inferior oblique
Left superior rectus
Left inferior rectus

From the remaining Step 1 possibilities, the muscles that fit are:

Right superior oblique
Left superior rectus

So after Step 2, your possible muscles are:

Step 3: Determine Whether the Hypertropia Worsens With Right Head Tilt or Left Head Tilt

Now perform the Bielschowsky head tilt portion. Tilt the patient’s head right and left, then measure which tilt worsens the hypertropia.

Using the same example:

Right hypertropia in primary gaze
Worse in left gaze
Worse with right head tilt

This pattern localizes to a right superior oblique palsy.

Why? With right head tilt, the right eye should intort using the right superior oblique and right superior rectus. If the right superior oblique is weak, the right superior rectus acts relatively unopposed as an elevator, increasing the right hypertropia.

Classic Example: Right Superior Oblique Palsy

A right superior oblique palsy typically presents with:

Right hypertropia in primary gaze
Worse in left gaze
Worse with right head tilt
Often excyclotorsion
Often contralateral head tilt posture
Symptoms worse reading, downgaze, or stairs

EyeWiki notes that CN IV palsy causes ipsilateral hypertropia and excyclotorsion, with diagnosis commonly made through the Parks-Bielschowsky-Helveston three-step test.

Doctor and patient discussing an eye chart in an office. Steps 3 for right hypertropia. — Steps 3 for right hypertropia. Doctor and patient discussing an eye chart in an office.

Doctor and patient discussing an eye chart in an office. Steps 3 for left hypertropia. — Steps 3 for left hypertropia. Doctor and patient discussing an eye chart in an office.

Quick Parks 3-Step Cheat Sheet

If Right Hypertropia

Possible muscles after Step 1:

If worse in left gaze, narrow to:

If worse with right head tilt, diagnosis favors:

Right superior oblique palsy

If worse with left head tilt, diagnosis favors:

Left superior rectus palsy

If Left Hypertropia

Possible muscles after Step 1:

If worse in right gaze, narrow to:

If worse with left head tilt, diagnosis favors:

Left superior oblique palsy

If worse with right head tilt, diagnosis favors:

Right superior rectus palsy

Practical Tip

The Parks test is most helpful when the finding is a clean, acquired hypertropia. If the result localizes to something unusual, does not match the motility pattern, or the patient has neurologic/orbital signs, broaden the differential. EyeWiki’s skew deviation discussion specifically warns that if the three-step test localizes away from the superior oblique, alternatives such as skew deviation, thyroid eye disease, and myasthenia should be considered.