The study of the Vestibulo-Ocular Reflex (VOR) is considered a fundamental tool in the identification of diseases of the vestibular system. The Caloric Test (CT) is the most used method for assessing VOR. In case of partial impairment of afferent impulses from the VIII nerve, warm (excitatory) stimuli cannot excite the motoneurons.1 The deafferented nucleus has a reduced response due to reduced peripheral stimuli; furthermore, it is inhibited by the opposite (healthy) nucleus through the inter-commissural pathway. However, cold (inhibitory) stimuli inhibit the already compromised peripheral—input and the basal activity of the nucleus on the injured side. Based on this physiological knowledge, an increased cold/warm ratio for the same ear is suggestive of tone asymmetry between the vestibular nuclei. This asymmetry is called Post-Caloric Recruitment (PCR).2–5 PCR is an objective phenomenon that reflects peripheral input impairment and shows a state of equilibrium between the vestibular nuclei and the central compensation phase.6,7

The video Head Impulse Test (vHIT) is useful in differentiating central and peripheral disease in acute vertigo and as a complement to the CT in the assessment of chronic dizziness. vHIT analysis considers two parameters: VOR gain and the presence of saccades. There are two types of saccades in uncompensated acute peripheral lesions. Overt saccades occur after the end of head movement and show VOR incompetence in maintaining the target on the retina. Anti-compensatory Quick Eye Movements (AQEM) or anticompensatory saccades occur toward head movement, in the contralateral ear, and indicate vestibular tone asymmetry of peripheral origin. Its function is to reposition the eye to maintain ocular fixation.8

As vestibular recruitment indicates a phase of vestibular system compensation after a peripheral lesion, it would be possible to find overt and AQEM in patients who present with PCR. The vHIT can provide information regarding the current state of the disease and of the compensation, and provides effective therapeutic strategies; furthermore, its advantages include being quick and more comfortable for patients.

The objectives of this study are to evaluate vHIT results in patients with vestibular recruitment in CT, to compare these patients with a control group, and to calculate the sensitivity and specificity of vHIT in the diagnosis of PCR.

This cross-sectional study was approved by the Ethics Committee for the Analysis of Research Projects, and in accordance with the 1975 Declaration of Helsinki.

The SG consisted of 250 patients (89 men and 161 women; mean age, 54.84 years) who met all the inclusion criteria. The CG consisted of 35 participants, 18 men and 17 women, with a mean age of 40.42 years. An analysis of covariance verified sex and age similarities and showed no relationship with the vestibular tests of any of the groups (Table 1).

The Table 2 show the gain and saccade frequency distribution in recruiting and non-recruiting ears in the study group and in the control group.

The term vestibular recruitment was used to characterise a lesion that originates in the peripheral vestibular organ.7 CT shows the involvement of the peripheral organ by verifying the hyperexcitability of the vestibular nucleus contralateral to the lesion. Increased vestibular tone can be identified by a more intense caloric response to inhibitory (cold) stimulation compared to direct (warm) stimulation of the lateral canal. Thus, CT can identify the compromised ear responsible for recruiting the vestibular pathway. In our clinical practice, we observed that “recruiting” patients usually present good progression after adequate treatment of the etiological factors causing the symptoms. Thus, in addition to aiding in making the right diagnosis, the PCRI improves the clinical management and prognosis of the lesion without additional costs to routine tests. Therefore, we decided to evaluate the usefulness of vHIT as a possible predictor of PCR due to its easy and increasingly frequent use.

Our sample presented low gain in 36 ears (7%) in the SG, which was not observed in the CG. This result was already predicted since the eye movement generated by the impulse does not reach the range of head movement in vestibular hypofunction, reducing gain. High gain was present in 91 (18%) ears in the SG and in 16 (23%) ears in the CG, with no statistical difference between the groups. Several factors can affect neural impulse transmission, such as metabolic and hormonal factors and anxiety. These situations affect VOR suppression by the cerebellum, especially by the flocculus.10,11 We believe that these situations may justify the presence of hyperactive VOR even in individuals without dizziness. In our clinical practice, we frequently observe high gains in patients with migraine, especially when anxiety is present. Other authors have described increased gain in endolymphatic hydrops,12 although we have not encountered such. As already mentioned, recruitment shows a phase of asymmetry between vestibular nucleus discharges resulting from the lack of afferents from the peripheral vestibular organ. In this phase, the role of the cerebellum in vestibular nucleus hyperactivity has not been completely established. In this case, we could find some changes in the gain in the recruiting subjects, a situation that has been attributed to the lack of cerebellar action. However, we found no gain and PCR differences between the SG and the CG.

Available literature provides controversial details regarding the presence of saccades as a sign of VOR hypofunction, whether overt or covert. Our CG had overt saccades in 3% of ears and covert saccades in 11% of ears. These results corroborate the study by Salmito et al.,13 who also reported corrective saccades in asymptomatic individuals. Patients who successfully compensate for a vestibular insult may experience saccadic correction before the end of head movement. Therefore, covert saccades may be present in normal subjects who previously had a vestibular dysfunction, even if asymptomatic.14 Other authors suggest that the VOR is a hypometric system and saccades are part of a normal correction of the reflex; thus, the presence of corrective saccades shows no VOR dysfunction.15,16 Considering these factors, the saccades observed in the CG may be due to normal VOR physiology.

Since vestibular recruitment depicts a phase of decompensation, our first hypothesis predicted the presence of overt saccades in vHIT in the presence of PCR. This happens because overt saccades appear in the acute phase of peripheral dysfunction; furthermore, as vestibular compensation occurs, they approach head movement and end up becoming covert. However, overt saccades were found in 98 (20%) ears in the SG, of which only 21 (4%) were RE. These observations invalidate the initial hypothesis and rule out overt saccades as evidence of PCR.

An interesting indicator in this study would be the AQEM, which indicate vestibular tone asymmetry of peripheral origin as a result of a lesion in the contralateral ear.8 Our previous hypothesis was that these saccades could function as a predictor of recruitment in the contralateral ear. They were present in 94 (19%) ears in the SG, but only 26 (5%) of them had a contralateral RE. On the other hand, they were present in 68 (14%) contralateral NRE. We found no interaction between recruitment and anti-compensatory saccades in none of the groups.

Disagreements between CT and vHIT have been discussed by several authors.7,26 The explanation may be based on the VOR anatomy itself. The angular VOR receptor is the crista ampullaris, composed of type I and type II hair cells. Type I hair cells populate the central part of the crista ampullaris and encode head movements at high frequency and high acceleration. Irregular afferent fibres carry the output from these cells toward second-order excitatory type I neurons located in the vestibular nucleus. Type II hair cells populate the periphery of the crista ampullaris and encode low frequency and low acceleration movements. They originate regular afferent fibres that synapse in the vestibular nucleus with second-order inhibitory type II neurons.18,19,21 Regarding vestibular recruitment, it is important to highlight the difference between vHIT and PC afferent pathways. In the first, ampullaris signals are transmitted directly to effector eye muscles, while the second is responsible for commissural inhibition, which inhibits the contralateral nucleus. This pathway depends on GABAergic type II neurons, the only inhibitory cells in the VOR circuit. PCR depends on these GABAergic neurons, which are responsible for commissural inhibition.6 Therefore, recruitment is established in CT because it assesses the commissural pathway, while vHIT responses depend on the direct VOR activation pathway. Thus, further explains the absence of relevant vHIT results in predicting PCR.

In this study, PCR-related vHIT sensitivity and specificity remained around 50%. Clinically, sensitivity 51% means that 51% of recruiting subjects may have changed vHIT. On the other hand, 49% of them may have had normal vHIT. A specificity of 50% indicates that only half of the vHIT may be changed in cases without PCR. Therefore, the test has negligible sensitivity or specificity in the diagnosis of PCR.

In conclusion, vHIT did not prove to be a good predictor of PCR. Our results are in agreement with those of previous studies that concluded that CT and vHIT assess semi-circular canals in different segments and different VOR pathways.17–26

We thank José Ferreira de Carvalho, PhD, for the careful statistical analysis and Suelen Cesaroni, MS, for support in carrying out the exams.

This study did not receive external funding. The authors have no financial relationship to disclose.

The authors declare no conflicts of interest.

Informed consent was obtained from each participant included in the study.