Preferential Impairment of the Contralesional Posterior Semicircular Canal in Internuclear Ophthalmoplegia

Abstract

Background: The vertical vestibulo-ocular reflex (VOR) may be impaired in internuclear ophthalmoplegia (INO) as the medial longitudinal fasciculus (MLF) conveys VOR-signals from the vertical semicircular canals. It has been proposed that signals from the contralesional posterior semicircular canal (PSC) are exclusively transmitted through the MLF, while for the contralesional anterior canal other pathways exist.

Objective: Here, we aimed to characterize dysfunction in individual canals in INO-patients using the video-head-impulse test (vHIT) and to test the hypothesis of dissociated vertical canal impairment in INO.

Methods: Video-head-impulse testing and magnetic resonance imaging were obtained in 21 consecutive patients with unilateral (n = 16) or bilateral (n = 5) INO and 42 controls. VOR-gains and compensatory catch-up saccades were analyzed and the overall function (normal vs. impaired) of each semicircular canal was rated.

Results: In unilateral INO, largest VOR-gain reductions were noted in the contralesional PSC (0.55 ± 0.11 vs. 0.89 ± 0.08, p < 0.001), while in bilateral INO both posterior (0.43 ± 0.11 vs. 0.89 ± 0.08, p < 0.001) and anterior (0.58 ± 0.19 vs. 0.88 ± 0.09, p < 0.001) canals showed marked drops. Small, but significant VOR-gain reductions were also found in the other canals in unilateral and bilateral INO-patients. Impairment of overall canal function was restricted to the contralesional posterior canal in 60% of unilateral INO-patients, while isolated involvement of the posterior canal was rare in bilateral INO-patients (20%). Reviewers correctly identified the INO-pattern in 15/21 (71%) patients and in all controls (sensitivity = 84.2% [95%-CI = 0.59.5-95.8]; specificity = 95.5% [95%-CI = 83.3-99.2]).

Conclusion: Using a vHIT based overall rating of canal function, the correct INO-pattern could be identified with high accuracy. The predominant and often selective impairment of the contralesional posterior canal in unilateral INO further supports the role of the MLF in transmitting posterior canal signals. In patients with acute dizziness and abnormal vHIT-results, central pathologies such as INO should be considered as well, especially when the posterior canal is involved.

Keywords: compensatory saccade; head impulse test; internuclear ophthalmoplegia; medial longitudinal fasciculus; vestibulo-ocular reflex.

Figure 1

Magnetic resonance imaging (MRI) in two illustrative cases with internuclear ophthalmoplegia (INO). (A) Axial diffusion-weighted imaging–MRI in a patient with unilateral INO (patient 7) demonstrating a right-sided paramedian dorsal pontine lesion (black arrow). (B) Axial fluid-attenuated inversion recovery-image in a patient with bilateral INO (patient 19) showing a hyperintense bilateral paramedian dorsal pontine lesion (white arrow).

Figure 2

Video-head-impulse testing in two patients with internuclear ophthalmoplegia (INO). Eye velocity traces (in green) and head velocity traces (in red for testing the right vestibular organ and in blue for assessing the left vestibular organ) are plotted against time. Summary plots in the center illustrate average individual vestibulo-ocular reflex (VOR)-gains ± 1SD for all six canals. (A) Significant VOR-gain reduction accompanied by overt catch-up saccades in the contralesional (left) posterior semicircular canal (PSC) in a patient with right-sided INO (patient 7; see also MR-image in Figure 1A). Note that all other canals showed normal responses. (B) Significant VOR-gain reduction and catch-up saccades in both PSCs and both HSCs in a patient with bilateral INO (patient 19; see also MR-image in Figure 1B).

Figure 3

Frequency of compensatory catch-up saccades in patients with unilateral internuclear ophthalmoplegia (n = 16). For each semicircular canal the fraction of patients with either no catch-up saccades (grade 0, in dark gray), catch-up saccades in less than 50% of traces (grade 1, in light gray) and catch-up saccades in more than 50% of traces (grade 2, in black) is shown in a box plot. Abbreviations: Contra-ASC, contralesional anterior semicircular canal; Contra-HSC, contralesional horizontal semicircular canal; Contra-PSC, contralesional posterior semicircular canal; Ipsi-ASC, ipsilesional anterior semicircular canal; Ipsi-HSC, ipsilesional horizontal semicircular canal; Ipsi-PSC, ipsilesional posterior semicircular canal.

Figure 4

Frequency of compensatory catch-up saccades in patients with bilateral internuclear ophthalmoplegia (n = 5). For each semicircular canal the fraction of patients with either no catch-up saccades (grade 0, in dark gray), catch-up saccades in less than 50% of traces (grade 1, in light gray) and catch-up saccades in more than 50% of traces (grade 2, in black) is shown in a box plot. For explanation of abbreviations see figure legend of Figure 3.