Introduction to VEMP Parameters

Vestibular Evoked Myogenic Potentials (VEMP) are a critical neurophysiological tool used to assess the function of the vestibular system, particularly the otolith organs (utricle and saccule). The accurate interpretation of VEMP tests relies on a thorough understanding of multiple VEMP parameters. These parameters provide insight into the integrity of the vestibular pathways, from the inner ear to the central nervous system.

Core VEMP Parameters

The primary VEMP parameters include latency, amplitude, and threshold. Each parameter carries distinct diagnostic value.

• Latency: Refers to the time (in milliseconds) from stimulus onset to the peak of the VEMP waveform (e.g., p13 and n23 for cervical VEMP). Prolonged latencies can indicate demyelinating conditions or central vestibular lesions.
• Amplitude: The size (in microvolts) of the response, often measured as peak-to-peak amplitude. Asymmetry in amplitude between ears is commonly seen in unilateral vestibular loss, such as in Meniere's disease or vestibular neuritis.
• Threshold: The lowest stimulus intensity (in dB nHL) at which a reproducible VEMP is observed. Elevated thresholds are a hallmark of superior canal dehiscence (SCD) or endolymphatic hydrops.

Key Parameters for Clinical Interpretation

Beyond basic measurements, several derived VEMP parameters enhance diagnostic accuracy:

• Asymmetry Ratio (AR): Calculated as (Left amplitude – Right amplitude) / (Left Right amplitude) × 100. An AR above 29% (for 500 Hz stimuli) suggests a significant unilateral lesion.
• Frequency Tuning: The frequency of the stimulus that elicits the largest amplitude (normally 500-750 Hz). A shift in tuning can indicate specific pathologies such as SCD (broadened tuning) or Meniere's (tuning shift to higher frequencies).
• Stimulus Type: Air-conducted sound (ACS) or bone-conducted vibration (BCV). The choice of stimulus influences which otolith organ is primarily activated and affects all VEMP parameter values.

Parameter Variability and Normal Values

Normal VEMP parameters vary with age, gender, muscle activation level (for cervical VEMP), and recording techniques. For example, older adults may show reduced amplitudes and prolonged latencies. It is essential that each laboratory establishes its own normative data for each VEMP parameter across appropriate age groups and stimulus conditions.

Conclusion

Mastering the interpretation of VEMP parameters is essential for clinicians diagnosing vestibular disorders. By carefully analyzing latency, amplitude, threshold, asymmetry, and frequency tuning, one can differentiate between peripheral and central pathologies, monitor disease progression, and guide treatment decisions. As VEMP technology evolves, standardized reporting of these parameters will remain the cornerstone of reliable vestibular assessment.