H test of eye movements: muscle and nerve
In the H test, whether the eye is turned in or out and whether it looks up or down show which muscle, and so which nerve, is chiefly tested.
Remember
The oblique muscles, which reach the eyeball at an angle, do the vertical work when the eye is turned in: looking up chiefly tests the inferior oblique (cranial nerve III) and looking down chiefly tests the superior oblique (IV). When the eye is turned out, the superior and inferior rectus muscles, which run straight to the eyeball, are tested: looking up chiefly tests the superior rectus (III) and looking down chiefly tests the inferior rectus (III).
Mnemonic
LR6 SO4 R3
- L lateral rectus
- S superior oblique
- R rest
It links each muscle to its nerve: the lateral rectus is supplied by VI, the superior oblique by IV, and the rest by III.
The H test is a bedside check of eye movements. The patient keeps the head still and follows a pen or finger that traces the shape of the letter H, saying whenever the image splits into two or blurs. Each eye muscle is the main driver of a particular direction of gaze, so a gap in the pattern points to the muscle or its nerve supply, and further assessment is needed to decide where along the pathway the problem lies.
The rectus muscles run straight to the eyeball. The remaining muscles, the superior oblique and the inferior oblique, reach the eyeball at an angle (oblique means slanting). A muscle's vertical action depends on where the eye is pointing, so each position lets one muscle show its action most clearly.
With the eye turned in towards the nose (adduction), the oblique muscles do the vertical work, and their names are the opposite of their actions. Looking up chiefly tests the inferior oblique, supplied by the oculomotor nerve (cranial nerve III). Looking down chiefly tests the superior oblique, supplied by the trochlear nerve (cranial nerve IV). The trochlear nerve is named after the trochlea, the pulley through which the muscle's tendon passes.
With the eye turned outwards (abduction), the superior and inferior rectus muscles are tested, and their names match their actions. Looking up chiefly tests the superior rectus (III), and looking down chiefly tests the inferior rectus (III).
Moving the target to the side of the eye being tested chiefly tests the lateral rectus, which turns the eye outwards and is supplied by the abducens nerve (VI). The medial rectus, supplied by the oculomotor nerve (III), turns the eye towards the nose and is tested when the target crosses to the other side of the face.
Key facts
- Eye turned in, looking up
- chiefly tests the inferior oblique, supplied by the oculomotor nerve (III).
- Eye turned in, looking down
- chiefly tests the superior oblique, supplied by the trochlear nerve (IV).
- Eye turned out, looking up
- chiefly tests the superior rectus, supplied by the oculomotor nerve (III).
- Eye turned out, looking down
- chiefly tests the inferior rectus, supplied by the oculomotor nerve (III).
- Sideways step
- moving the target to the side of the eye being tested chiefly tests the lateral rectus and so the abducens nerve (VI).
- Weak superior oblique
- the eye cannot be lowered properly when turned inwards, which gives vertical double vision when reading or on stairs.
Exam traps
- The superior rectus is supplied by the oculomotor nerve and does lift the eye, but it does so best when the eye is turned outwards, so it is not the main muscle tested by looking up with the eye turned inwards.
- The inferior rectus also lowers the eye, but the H test isolates it by looking down with the eye turned outwards, so a failure to look down with the eye turned inwards points to the superior oblique instead.
- The inferior oblique is also supplied by the oculomotor nerve and lifts the eye, but it is isolated by looking up with the eye turned in towards the nose, so a failure to look up with the eye turned outwards points to the superior rectus.
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