With the head upright, how is the lateral (horizontal) semicircular canal oriented relative to the true horizontal plane?

• A Exactly horizontal
• B Parallel to the petrous ridge of the temporal bone
• C Tilted about 45 degrees, anterior end lowered
• D Tilted about 30 degrees, anterior end raised ✓

Explanation

The lateral semicircular canal is tilted about 30 degrees above the horizontal plane with its anterior end elevated, so extending the head fully forward brings it into a truly horizontal position. The superior canal runs roughly parallel to the long axis of the petrous ridge, which makes option B describe the wrong canal. This 30 degree tilt explains the positioning used in caloric testing and the Dix-Hallpike manoeuvre.

Reference: Scott-Brown's Otorhinolaryngology and Head and Neck Surgery, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.