Marcello Cherchi, M.D., Ph.D • Page last modified: April 3, 2021
Cherchi and Yacovino recently published a paper entititled "Histology and neuroanatomy suggest a unified mechanism to explain the distribution of lesion patterns in acute vestibular neuropathy" (2021)
Human temporal bone studies have described the distribution of afferent fibers from each of the five organelles in the laby- rinth. Data from vestibular tests in patients with vestibular neuritis can be abnormal in almost any pattern. We propose a unified explanation for these patterns, based on histological and neuroanatomical factors.
Each inner ear has five "organelles" that detect acceleration — two otolith organs (saccule, utricle) and three semicircular canals (posterior [pSCC], anterior [aSCC] and horizontal [hSCC]) — and the afferent fibers from each of these structures take up different proportions relative to the total number of fibers in the vestibular nerve. The study by Lee et al (1990) reported that, in order of increasing number of fibers, the findings showed: hSCC 12% < aSCC 13% < pSCC 15% < saccule 26% < utricle 34%.
Histology: Proportion of fibers from each organelle in ascending order
The arrows reflect that:
• The afferent pathways from the posterior semicircular canal are more resistant to lesion damage because of dual innervation (posterior ampullary nerve, accessory posterior ampullary nerve).
• The afferent pathways from the saccule are more resistant to lesion damage because of dual innervation (a portion of the fibers pass from the inferior division to the superior division through Voit's nerve).
In cases of vestibular neuritis, the vestibular tests of each organelle are abnormal in a variety of patterns, but these patterns do not appear random. Averaging over several clinical lesion studies shows that, in order of order of decreasing frequency of affectation, hSCC 96% > aSCC 77% > utricle 70% > pSCC 36% > saccule 35%.
Comparing the fiber proportions of afferent pathways with the frequency of affectation shows an almost inverse correlation, namely: the greater then number of fibers, the smaller the likelihood that the test of that pathway's function will be abnormal. However, as shown in the table, this inverse correlation is approximate; the utricle has the greatest proportion of afferent fibers, but only an intermediate frequency of affectation — it is affected more frequently than the pSCC and saccule. We suggest that these latter two structures (pSCC and saccule) are less vulnerable than the utricle because the pSCC and saccule each has dual innervation, thereby dispersing their pathways more widely.
In trying to make sense of the distribution of pattterns of vestibular test result abnormalities in AVN, we propose that there are two considerations, one of which is histo- logical, and the other of which is neuroanatomical in nature. The first factor is the proportion of afferent fibers from a given organelle (compared to the entire vestibular nerve); the greater the proportion of fibers, the less likely the ves tibular test of that organelle will detect an abnormality. The second factor is how widely (spatially) distributed the fibers from a given end-organelle are; the more widely distributed the fibers, the less likely the vestibular test of that organelle will detect an abnormality. Ultimately, these histologic and neuroanatomical factors come down to just one principle: the more broadly spatially distributed an organelle's afferent fibers are (whether by virtue of there being a large number or fibers, or by virtue of those fibers being dispersed through anatomically separate pathways), the less likely it is that a test of that organelle's function will be affected by AVN