Spontaneous visual exploration during locomotion in patients with phobic postural vertigo

Abstract

Background: Earlier studies on stance and gait with posturographic and EMG-recordings and automatic gait analysis in patients with phobic postural vertigo (PPV) or visual height intolerance (vHI) revealed similar patterns of body stiffening with muscle co-contraction and a slow, cautious gait. Visual exploration in vHI patients was characterized by a freezing of gaze-in-space when standing and reduced horizontal eye and head movements during locomotion.

Objective: Based on the findings in vHI patients, the current study was performed with a focus on visual control of locomotion in patients with PPV while walking along a crowded hospital hallway.

Methods: Twelve patients with PPV and eleven controls were recruited. Participants wore a mobile infrared video eye-tracking system that continuously measured eye-in-head movements in the horizontal and vertical planes and head orientation and motion in the yaw, pitch, and roll planes. Visual exploration behavior of participants was recorded at the individually preferred speed for a total walking distance of 200 m. Gaze-in-space directions were determined by combining eye-in-head and head-in-space orientation. Walking speeds were calculated based on the trial duration and the total distance traversed. Participants were asked to rate their feelings of discomfort during the walk on a 4-point numeric rating scale. The examiners rated the crowdedness of the hospital hallway on a 4-point numeric rating scale.

Results: The major results of visual exploration behavior in patients with PPV in comparison to healthy controls were: eye and head positions were directed more downward in the vertical plane towards the ground ahead with increased frequency of large amplitude vertical orientation movements towards the destination, the end of the ground straight ahead. The self-adjusted speed of locomotion was significantly lower in PPV. Particularly those patients that reported high levels of discomfort exhibited a specific visual exploration of their horizontal surroundings. The durations of fixating targets in the visual surroundings were significantly shorter as compared to controls.

Conclusion: Gaze control of locomotion in patients with PPV is characterized by a preferred deviation of gaze more downward and by horizontal explorations for suitable auxiliary means for potential postural support in order to prevent impending falls. These eye movements have shorter durations of fixation as compared to healthy controls and patients with vHI. Finally, the pathological alterations in eye-head coordination during locomotion correlate with a higher level of discomfort and anxiety about falling.

Keywords: Eye movements; Functional dizziness; Head movements; Phobic postural vertigo; Visual exploration.

Fig. 1

a Participants walked at a self-paced speed on a hospital hallway for a total distance of 200 m separated by 4 recording blocks with intermittent breaks. b Visual exploration behavior (i.e., head and eye-in-head orientation and motion) was recorded by a mobile infrared video eye-tracking system, consisting of goggles, a head-fixed camera, a head-fixed inertial measurement unit, and a backpack with a recording laptop

Fig. 2

a Exemplary view of the visual surroundings during an experimental trial. Comparison of b the level of crowdedness of the corridor during individual trials, c walking speed, and d levels of discomfort between patients with PPV (red bars) and healthy controls (gray bars) on a 4-point numeric rating scale. Despite equally crowded scenery on average across individual recordings, patients walked considerably slower and frequently reported feelings of discomfort, imbalance, or dizziness

Fig. 3

Comparison of a mean and b range of head orientations, c mean head velocity, d mean and e range of eye-in-head orientations, and f fixation duration during walking between patients with PPV (red dots, bars) and healthy controls (gray dots, bars). During walking, eye and head positions of patients were directed more downward in the vertical plane towards the ground ahead with increased frequency of large amplitude vertical orientation movements towards the destination, the end of the floor straight ahead and an overall shorter fixation duration

Fig. 4

Comparison of gaze-in-space behavior in a healthy controls and patients with PPV (b). Density plots represent fixations of environmental structures with combined head and eye-in-head movements during walking. The number of participants (N; coded by color) fixating identical targets (resolution of 1° horizontally and vertically) centered around the central perspective vanishing point of the walkway (white cross) are depicted. Patients directed their gaze more along the vertical plane with a preference for downward orientations towards the ground ahead

Fig. 5

Exemplary gaze-in-space behavior in a a patient with PPV reporting no discomfort (patient 9/+) compared to b a patient with PPV reporting intense discomfort (patient 6/*) during the experiment. Density plots represent fixations of environmental structures with combined head and eye-in-head movements during walking. The number of fixations (N; coded by color) focusing on identical targets (resolution of 1° horizontally and vertically) centered around the central perspective vanishing point of the walkway (white cross) are depicted. c Correlation between individual levels of subjective discomfort on a 4-point numeric rating scale and the range of horizontal eye and head orientations in patients with PPV. Particularly anxious patients exhibited a more or less pronounced nervous visual exploration of their horizontal surroundings to a degree that depended on their level of subjectively reported discomfort