The Parks-Bielschowsky Test uses 3 steps to identify which single ocular muscle is weak, thereby causing a hypertropia.
3 Step Test:
(1) In or near primary gaze, determine whether there is a right or left hypertropia.
(2) Determine which lateral gaze direction that worsens the hypertropia.
(3) Determine the side to which tilting the head increases the hypertropia.
Step
1
2
3
Affected Muscle
R
L
R
right superior oblique
L
R
L
left superior oblique
L
L
L
right inferior oblique
R
R
R
left inferior oblique
L
L
R
left inferior rectus
R
R
L
right inferior rectus
L
R
R
right superior rectus
R
L
L
left superior rectus muscle
(Figure 10.7, Rubin, 6: 10.5)
After the most likely muscle has been identified, weakness in that muscle should be confirmed to exclude possible mimicking conditions such as myasthenia gravis or thyroid disease.
• If the suspected muscle is the superior oblique muscle, then hypertropia is greater in downgaze than in upgaze.
• If the suspected muscle is the inferior oblique muscle, then hypertropia is greater in upgaze than in downgaze.
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