Introduction

Dizziness is one of the most common complaints in outpatient neurology and otolaryngology clinics. Accurate assessment of vestibular function is critical for diagnosis. The vestibular system, responsible for balance and spatial orientation, can be evaluated using several specialized tests, with caloric testing being one of the most established and widely utilized methods. However, a variety of drugs can significantly alter test results by suppressing or irritating the vestibular end organs, central pathways, or by changing the patient’s overall arousal state. Understanding these interactions is essential to avoid misdiagnosis.

What is Caloric Vestibular Testing?

The caloric test involves irrigating the external ear canal with warm or cool water (or air) to induce convection currents in the horizontal semicircular canal. This elicits nystagmus (involuntary eye movements) and a sensation of dizziness in a healthy individual. The magnitude and direction of the response are recorded to assess the function of each labyrinth independently. It is a key component of the electronystagmography (ENG) or videonystagmography (VNG) test battery.

How Drugs Affect Vestibular Responses

Many classes of drugs can dampen or confound the results of caloric testing. The primary mechanisms include central suppression of the vestibular nuclei, direct effects on hair cells in the labyrinth, and alterations in efferent vestibular pathways. Patients are typically advised to withhold certain medications for 24 to 48 hours before testing, though this must be balanced with clinical safety (e.g., anticonvulsants or benzodiazepines should not be abruptly withdrawn).

Common Drug Classes and Their Impact

• Benzodiazepines (e.g., diazepam, lorazepam): Potent suppressants of central vestibular processing. They reduce the gain of the vestibular-ocular reflex (VOR) and can markedly diminish or abolish caloric-induced nystagmus. This can falsely suggest a peripheral deficit.
• Antihistamines (e.g., meclizine, diphenhydramine): Commonly prescribed for motion sickness and acute dizziness. They have central anticholinergic and antihistaminic effects that suppress vestibular responses. Their use often leads to reduced slow-phase velocity of nystagmus during caloric stimulation.
• Anticholinergics (e.g., scopolamine): Reduce muscarinic activity in the vestibular nuclei and cerebellum. The result is a marked reduction in the duration and intensity of post-caloric nystagmus.
• Anticonvulsants (e.g., carbamazepine, phenytoin): Can affect cerebellar and brainstem function. They may produce gaze-evoked nystagmus and alter central integration, thereby affecting vestibular test results.
• Alcohol: Acute alcohol intoxication causes a transient reduction in VOR gain and can induce positional dizziness (positional alcohol nystagmus). It also directly affects the cupula, rendering caloric results unreliable.
• Sedatives and hypnotics (e.g., zolpidem, barbiturates): Similar to benzodiazepines, they cause central nervous system depression and attenuate caloric-induced nystagmus.
• Aminoglycoside antibiotics (e.g., gentamicin, streptomycin): These can be directly ototoxic, damaging hair cells in the vestibular labyrinth. This may cause chronic bilateral vestibular loss, leading to absent or severely reduced caloric responses even in the absence of any acute suppression.

Practical Recommendations for Clinicians

Before performing caloric or other vestibular testing, a thorough medication history is mandatory. Key steps include:

• List all prescription, over-the-counter, and recreational drugs.
• Withhold CNS suppressants (e.g., benzodiazepines, antihistamines, muscle relaxants) for at least 48 hours if medically safe.
• Document any recent use of ototoxic drugs (especially aminoglycosides, loop diuretics, cisplatin).
• Be aware that the patient's dizziness itself may be drug-induced (e.g., from ototoxicity or orthostatic hypotension).

Conclusion

The interpretation of vestibular tests, particularly the caloric test, is highly dependent on concurrent medication use. Clinicians must remain vigilant about the confounding effects of drugs that suppress or alter vestibular responses. Failure to account for these factors can lead to erroneous diagnosis of peripheral or central vestibular disorders. A standardized pre-test medication washout protocol, when safe, will enhance the accuracy of dizziness evaluations and improve patient outcomes.