In an audiometric test, the left ear shows no acoustic reflex with both contralateral and ipsilateral stimulation, while the right ear shows reflexes. What is the most likely cause?

In an audiometric test, the left ear shows no acoustic reflex with both contralateral and ipsilateral stimulation, while the right ear shows reflexes. What is the most likely cause?

• A. Conductive impairment in the right ear
• B. VIIIth-nerve pathology on the right side
• C. VIIIth-nerve pathology on the left side
• D. VIIth-nerve pathology on the left side

Answer: (D)

Acoustic reflex testing relies on a bilateral reflex arc: a sound travels via the cochlea and VIIIth nerve to the brainstem, and the reflex is carried back through the facial nerves (VII) to contract the stapedius muscle in both middle ears. Because the right ear shows reflexes, the afferent pathway from the left ear and the efferent pathway on the right side are functioning. The left ear, however, has no reflex with either ipsilateral or contralateral stimulation, which points to a problem in the left efferent limb that drives stapedius contraction. A lesion of the left facial nerve disrupts the left stapedius contraction for both reflex configurations, producing absent reflex in the left ear while the right ear can still show reflexes via the intact right facial nerve. If the issue were in the left VIII nerve, you'd expect the left ear to fail to evoke reflexes and the right ear reflex to be affected in at least some configurations, which isn’t the case here. So the finding is most consistent with a left facial (VII) nerve lesion.

Acoustic reflex testing relies on a bilateral reflex arc: a sound travels via the cochlea and VIIIth nerve to the brainstem, and the reflex is carried back through the facial nerves (VII) to contract the stapedius muscle in both middle ears. Because the right ear shows reflexes, the afferent pathway from the left ear and the efferent pathway on the right side are functioning. The left ear, however, has no reflex with either ipsilateral or contralateral stimulation, which points to a problem in the left efferent limb that drives stapedius contraction. A lesion of the left facial nerve disrupts the left stapedius contraction for both reflex configurations, producing absent reflex in the left ear while the right ear can still show reflexes via the intact right facial nerve. If the issue were in the left VIII nerve, you'd expect the left ear to fail to evoke reflexes and the right ear reflex to be affected in at least some configurations, which isn’t the case here. So the finding is most consistent with a left facial (VII) nerve lesion.