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

The Parks Three-Step Test for Acquired Vertical Diplopia: A Comprehensive Guide

The Parks three-step test is a crucial tool for diagnosing acquired vertical diplopia. It helps identify a paretic cyclovertical muscle, particularly in cases of suspected CN IV palsy, especially superior oblique palsy. EyeWiki describes it as the standard diagnostic test for cyclovertical muscle palsy. However, newer studies have raised questions about its sensitivity in certain situations.

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 Test

You should use the Parks three-step test when the patient exhibits:

• 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

  
  

Exercise caution if the pattern is comitant, variable, bilateral, restrictive, traumatic, or neurologically complex.

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

Step 1: Identify Which Eye Is Hypertropic in Primary Gaze

Begin with cover testing in primary gaze. Determine whether there is right hypertropia or left hypertropia.

Example: Right Hypertropia

If the right eye is higher, the weak muscle could be:

A depressor of the right eye:

• Right superior oblique

• Right inferior rectus

  
  

or

An elevator of the left eye:

• Left superior rectus

• Left inferior oblique

  
  

After Step 1, your possible muscles are:

• RSO

• RIR

• LSR

• LIO

  
  

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

Next, measure the hypertropia in both right gaze and left gaze.

If the patient has a right hypertropia that worsens in left gaze, this narrows your list. 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

  
  

After Step 2, your possible muscles are:

• RSO

• LSR

  
  

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 worsen with reading, downgaze, or stairs

  
  

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

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

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:

• RSO

• RIR

• LSR

• LIO

  
  

If worse in left gaze, narrow to:

• RSO

• LSR

  
  

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:

• LSO

• LIR

• RSR

• RIO

  
  

If worse in right gaze, narrow to:

• LSO

• RSR

  
  

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

Conclusion: Importance of Accurate Diagnosis

Accurate diagnosis of vertical diplopia is essential for effective treatment. The Parks three-step test provides a structured approach to identify the underlying causes. By following these steps, I can ensure that patients receive the appropriate care they need. For comprehensive and personalized eye care, I recommend considering the services offered by OPT-ISM.