The Role of Videonystagmography (VNG)
Many decades ago, vestibular assessment consisted entirely of electronystagmography (ENG). Although ENG still serves as the “gold standard” for unilateral peripheral vestibular system impairment, ENG is inadequate for assessment and diagnosis of bilateral peripheral vestibular system impairment.
McCaslin and Jacobson (2009) note that the current balance function assessment includes ENG/videonystagmography (ENG/VNG) to assess vertigo, dizziness, and imbalance. Additionally, vestibular-evoked myogenic potentials (VEMP), rotational testing, and computerized dynamic posturography (CDP) are very useful in evaluating multi-sensory integration of vision, somesthesia, and vestibular system function.
VNG offers a multitude of advantages over the traditional ENG protocols For example, VNG goggles require just a few seconds to place on the patient, the VNG recording system has a very low noise floor, and VNG systems allow the entire session to be recorded for later analysis.
ENG/VNG can be used to document evidence of retinal impairment, can identify eye movement disorders consistent with CNS disease, can be used to document spontaneous nystagmus, can identify BPPV, and can be used to identify unilateral peripheral vestibular impairment.
McCaslin and Jacobson report that four subsystems govern eye movement: (1) saccade, (2) smooth pursuit, (3) gaze, (4) and optokinetics. They note that positional testing is performed to identify the presence of spontaneous nystagmus (SN), which must be differentiated from gaze-evoked nystagmus (GEN). The authors note that gazing causes GEN and GEN is rarely present in the vision-denied conditions. SN is “direction-fixed” and follows Alexander’s Law that states nystagmus intensity is greatest when gaze is toward the fast phase of nystagmus and reduced when gaze is directed toward the slow phase. McCaslin and Jacobson note that a bilaterally absent caloric response does not necessarily mean reduced or absent function across the complete operating range of the vestibular system, although it may indicate exactly that with respect to the lowest frequency operating range of the vestibular system.
The authors reinforce their conviction that when specific criteria are met, the monaural warm caloric test may replace the alternate binaural bithermal test.