Heat radiation during caloric vestibular test: thermographic demonstration in temporal bone experiments

A Pilot Study to Investigate the Relationship Between Interaural Differences in Temporal Bone Anatomy and Normal Variations in Caloric Asymmetry

PURPOSE

This study assesses interaural differences in temporal bone anatomy in subjects with normal caloric findings.

METHOD

Eligible patients included those referred to the Duke University Medical Center otology clinic complaining of dizziness, with a head computed tomography scan and caloric stimulation results within normal ranges (inter-ear difference ≤ 10% or < 25% unilateral weakness). Three-dimensional reconstructions of computed tomography scans in 11 patients were used to calculate the surface area and volume of lateral semicircular canals (LSCCs), mastoid airspaces, mastoid bones, and internal auditory canal diameter and circumference. Percent differences in interaural temporal bone anatomy (i.e., left-to-right asymmetry) were analyzed and correlated with warm caloric inter-ear difference (WCD) and clinically indicated caloric predictor asymmetry.

RESULTS

A multivariate model predicting WCD from 9 interaural anatomic variables demonstrated a Pearson's coefficient of 0.999. A similarly constructed model of the clinically indicated caloric predictor demonstrated a Pearson's coefficient of 0.999. The univariate correlation was strongest for WCD versus Proctor internal auditory canal diameter (r = 0.476; p = .139) and WCD versus lateral semicircular canal surface-area-to-volume ratio (r = -0.474; p = .141).

CONCLUSIONS

This pilot study provides multivariate models that predict caloric asymmetry in subjects without vestibular pathologic findings per caloric testing, based on interaural differences across variables of the temporal bone anatomy.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.5895988.

What does the head impulse test versus caloric dissociation reveal about vestibular dysfunction in Ménière's disease?

In patients with Ménière's disease (MD), caloric testing can show, depending on the stage and activity of the disease, a variety of results. Between attacks, many, or perhaps even most, patients with unilateral early or mild MD have normal caloric tests; late MD can show abnormalities ranging from mild to severe unilateral canal paresis with or without directional preponderance. The explanation of canal paresis in MD is not clear. The most obvious explanation, severe loss of lateral canal hair cells, is not likely to be correct because hair cell loss will not explain the fluctuating canal paresis to caloric stimulation. In contrast, the published evidence is that rotational testing of semicircular canal function in MD patients typically shows little reduction in function and even enhancement of vestibulo-ocular reflex gain, at least in the early stages of the disease. Here, we offer a novel explanation for this dissociation. We propose that hydropic expansion of the lateral canal membranous labyrinth permits convective recirculation within the duct that allows dissipation of the hydrostatic force that would normally cause cupular displacement and nystagmus in the caloric test.