Which semicircular canal is closest to the tympanic membrane and most affected by caloric testing?

Which semicircular canal is closest to the tympanic membrane and most affected by caloric testing?

• A. Anterior (superior) canal
• B. Posterior canal
• C. Lateral canal
• D. None of the above

Answer: (C)

Caloric testing works by changing the temperature in the external auditory canal to set up convection currents in the endolymph. The horizontal (lateral) semicircular canal is oriented in a way that these temperature-driven currents most effectively move its endolymph, and it is also the canal closest to the tympanic membrane, so the stimulus reaches it most directly. Because of this positioning and orientation, the lateral canal responds most strongly to caloric stimulation, producing the characteristic nystagmus used to assess vestibular function. The anterior and posterior canals, being in vertical planes, are less sensitive to this low-frequency, air/ water–driven stimulation.

Caloric testing works by changing the temperature in the external auditory canal to set up convection currents in the endolymph. The horizontal (lateral) semicircular canal is oriented in a way that these temperature-driven currents most effectively move its endolymph, and it is also the canal closest to the tympanic membrane, so the stimulus reaches it most directly. Because of this positioning and orientation, the lateral canal responds most strongly to caloric stimulation, producing the characteristic nystagmus used to assess vestibular function. The anterior and posterior canals, being in vertical planes, are less sensitive to this low-frequency, air/ water–driven stimulation.