Otol Neurotol. 2016 Feb;37(2):171-8. doi: 10.1097/MAO.0000000000000947.

Age-related Refixating Saccades in the Three-Dimensional Video-Head-Impulse Test: Source and Dissociation From Unilateral Vestibular Failure.

Rambold HA1.
  • 1Department of Neurology, County Hospitals of Altötting and Burghausen, Altötting Department of Neurology, University of Regensburg, Regensburg, Germany.

 

Abstract

OBJECTIVE: To examine “refixating saccades” in the three-dimensional video-head impulse test (vHIT) depending on the age and compare them to refixating saccades in pathological vHIT.

DESIGN: Retrospective database study.

SETTING: County hospital, specialized vertigo center.

PATIENTS: Eight hundred ninety-nine patients without a peripheral vestibular hypofunction were tested with the three-dimensional vHIT and compared with 135 patients with unilateral vestibular hypofunction (UVF).

MAIN OUTCOME MEASURES: Occurrence and peak velocity of refixating saccades (covert and overt) during the video-head-impulse test (vHIT) in three age groups (0-30, 30-60, and 60-100 yr) and in UVF.

RESULTS: Overt saccade frequency of refixating saccades increased with increasing age, especially in the horizontal and posterior vHIT. Saccades were mostly directed opposite to the head movement. The aging effect was not explained by the VOR-gain decrease. Refixating saccades in normal vHIT were less frequent and slower than in UVF.

CONCLUSION: We conclude that refixating saccades increase with higher age and could be differentiated from those in UVF by frequency and peak velocity. Saccades are not caused by a deficient VOR but might be caused by a deficient suppression of saccades to novel targets.

PMID: 26719965

 

Supplement:

The video-head-impulse test (vHIT) is an important modern method to measure the rotational vestibulo-ocular reflex (VOR) (1-3). This reflex stabilize vision during body and head movements by rotating the eyes in the opposite direction of the head. The sensors for head movements of this reflex are the six semicircular canals (SSC) located in the vestibulum.

During the test the patient fixates a target at one meter straight ahead while the examiner performs brief duration, small amplitude and high acceleration passive head movements. The direction of the head movements defines the SCC stimulated. Rotating the head around an earth vertical axis to the right or left tests the right or left lateral SCC, rotating around an earth horizontal axis (forward and backward) while the head is constantly turned by 45° to the right or left around an earth vertical axis, activates either the right anterior (RALP) or left posterior or the left anterior or right posterior SCC (LARP). Accordingly, all six SCCs, the anterior (AC), posterior (PC) and horizontal SCC (HC) could be selectively examined on both sides of the head (s. Fig.1).

 

 

figure 1Fig 1. Schematic view on the head from above during the vHIT examination. The location of the different SCCs is indicated by black lines (AC: anterior, PC: posterior, HC: horizontal SCC). The arrows indicate the stimulation direction around the indicated axis. The black arrow in the top of the figure indicates the fixation direction of the eyes. RALP: indicates the stimulation direction right anterior -left posterior, HC: horizontally and LARP: left anterior – right posterior.

 

In case of a SCC hypofunction, the eye is not compensating the head movement anymore and a refixating saccade (RFS) is elicited in direction opposite to the head movement (Fig.2A). RFS might occur during the head impulse or after the end of the head impulse and are referred to as covert and overt RFS. The vHIT is commercially available test and measures eye and head movements with a small high resolution video camera and accelerometer mounted in a light weighted goggle frame. Results of a vHIT are presented as eye and head velocity traces. Typical result is shown in Fig.2. This vHIT superior to the well known the clinical bed-side HIT (4,5). RFS are often observed in the vHIT with a normal VOR gain (Fig.2 C). Especially in elderly without any vestibular failure in the horizontal vHIT an increasing number of such RFS has been observed (6).

 

 

fig2

Fig 2. Results of the vHIT presented as overlaid velocity traces. Traces for eye velocity (black lines) are inverted for better comparison to the head velocity (gray lines). In (A) the pattern of a right unilateral vestibular failure (UVF) is shown. Initially the eye velocity is less than head velocity indicating a decreased gain (*). Afterwards RFS in the covert (x) and overt (o) time window are observed. For better comparison a normal response of a healthy young subject is shown (B). The velocity profiles of eye and head nicely overlap and no RFS are observed. In (C) the results of the vHIT of an elderly healthy subject are shown. Note, traces of eye and head velocity nicely overlap but small RFS are observed.

 

In this paper the following questions were addressed: Is there an increase in RFS with increasing age for all six SCCs? Is this observation explained by the VOR-gain, the ratio of eye to head velocity?  How could we dissociate RFS in ‘vestibular healthy subjects’ d from those with a unilateral vestibular failure (UVF)?

To address this question we performed a retrospective study of RFS in 899 healthy controls (aged 56±18 years; range: 14 to 92 years) without vestibular dysfunction and without nystagmus and compared them to 135 patients with a UVF (aged 58±15 years, range: 29 to 93 years) using the three-dimensional vHIT of all six SCCs. We carefully excluded all artefacts, e.g., google-bump artefacts, slippage- or lid artefacts which might contaminate the results.

 

Results of this study showed the following new findings: 1) With increasing age there is an increase in RFS frequency for overt saccades in the HC- and PC-vHIT and less for AC-vHIT; 2) The RFS frequency is not dependent on the VOR-gain and there was no major aging effect on the VOR gain itself; 3) Compared to UVF the RFS frequency and peak velocity is reduced in healthy subjects for HC- and PC-vHIT and AC-vHIT; 4) The RFS in UVF could be dissociated from the RFS in healthy subjects by the frequency and peak velocity (higher than 110°/s for HC-vHIT, 90°/s for AC-vHIT and 100°/s for PC-vHIT).

 

This study could not answer the question of the source of the RFS in healthy subjects, but exclude the VOR-gain, nystagmus and fixation instabilities as a possible source. Other mechanisms probably explaining the effect are VOR-adaptation caused by optic corrections and the inability to suppress saccades to novel stimuli. While optic correction in higher age shift to plus lenses which have a visual magnifying factor. The persistence of such adaptation state during testing without googles would lead to RFS but in the opposite direction. Therefore, we hypothesize that the saccades might be caused by a failed saccadic inhibition to a new stimulus, probably caused by the goggles frames of the vHIT as discussed in detail in the original paper. A proof is missing so far.

 

Important in this study is that RFS in the vHIT not necessarily indicate a vestibular hypofunction. RFS and a decreased VOR-gain are necessary to diagnose a vestibular hypofunction. To be more specific, in vestibular hypofunction the RFS frequency is higher and peak velocity over about 100°/s in vHIT. These findings should help improving diagnosing UVF in clinical routine with the vHIT and elucidate the importance not to rely on RFS only but additionally use the VOR-gain. It is important to point out that this is only possible with technical systems as the vHIT and not with the clinical bed-side HIT with could easily mislead a clinician.

 

References:

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  5. Mahringer A, Rambold HA. Caloric test and video-head-impulse: a study of vertigo/dizziness patients in a community hospital. Eur Arch Otorhinolaryngol 2013.
  6. Matino-Soler E, Esteller-More E, Martin-Sanchez JCet al. Normative data on angular vestibulo-ocular responses in the yaw axis measured using the video head impulse test. Otology & neurotology : official publication of the American Otological Society, American Neurotology Society [and] European Academy of Otology and Neurotology 2015;36:466-71.

 

 

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