The slow-phase velocity (SPV) of nystagmus is the most useful measurement variable for quantifying the intensity of nystagmus [10]. The SPV of nystagmus induced by the caloric test was measured from electronystagmography (ENG) recordings. The visual impact susceptibilities (VIS) of the loupe and Frenzel goggles were calculated from the SPV and subsequently compared. Additionally, the utility of the loupe for observing nystagmus that is inhibited by the naked eye under bright conditions was investigated.

The loupe comprised a thin Fresnel plastic lens and storage cover (Fig. 1a, b). When the lens is placed in front of one eye and the other eye is closed using the storage cover, the observer can visualize the participant’s eye magnified through the Fresnel lens; however, the participant’s vision remains suppressed. The focal length of the loupe is 50 mm, and the magnification is 2x. The loupe is 10-mm thick with the lens and weighs only 24 g, making it portable [9]. Its storage size is appropriate for covering one eye in the test of skew (Fig. 1c). The loupe is used in normally illuminated areas, such as examination rooms and wards with ceiling lights.

Portable loupe based on the Fresnel lens. (a) The size of the loupe when the lens is in the storage cover is 90 × 90 × 10 mm. (b) The dimensions of the loupe when in use are 160 × 90 × 10 mm. (c) Its storage size is appropriate for covering one eye in the test of skew

Fifteen healthy participants with no history of visual impairment or dizziness were enrolled. Written informed consent was obtained from all participants after providing a written explanation of the purpose and methods of the study. This study was approved by the Medical Research Ethics Committee of Mejiro University (Approval number: 23 medicine-006). All procedures were performed in accordance with the tenets of the Declaration of Helsinki and its later amendments.

Nystagmus was induced by the caloric test and recorded using ENG (Daiichi Medical Co. Ltd., FNG-1004, Japan). The caloric test stimulus was cold air at 10 °C at a rate of 6 L/min (Daiichi Medical Co. Ltd., FAC-700, Japan). Cold air was injected into the ear canal for 60 s. All procedures are similar to the routine caloric test performed at our institute.

In our clinical practice, the caloric and the visual suppression tests [11] are performed consecutively to examine lateral semicircular canal function and cerebellar function. Lateral semicircular canal function is assessed using the maximum SPV of the caloric test in darkness, which reaches its maximum value approximately 30 s after the end of the stimulus. Therefore, the visual suppression test is performed 30 s after the end of the stimulus by instructing the patient to gaze at the target point for 15 s in a bright condition. The SPV during the visual suppression test in normal subjects is less than half the maximum SPV. After the visual suppression test, nystagmus recovers with darkness, but gradually decays spontaneously about 60 s after the end of the stimulus. Based on the above our clinical experience, the active phase of caloric nystagmus was defined as the period from the end of the stimulus to 45 s, and the decay phase was defined as the period after 60 s from the end of the stimulus in this study. The first 30 s of the active phase was conducted in complete darkness, and subsequently, the eyes were covered with a tool, i.e., Frenzel goggles or the loupe. When using Frenzel googles, the room remained darkened, and the lights inside the goggles were turned on (Fig. 2a). When using the loupe, the right eye was covered with a Fresnel lens, and the left eye with a storage cover, with the ceiling-light on (Fig. 2b). Each participant underwent caloric test for both ears. Different devices were used on the participant’s left and right ears, and the order was randomized. During the decay phase, the ceiling light was turned on, and the participant’s eyes were uncovered to achieve the naked eye condition for the first 15 s. Subsequently, the loupe was used as mentioned above.

Participants’ eyes when using the nystagmus observation tools. (a) When using Frenzel goggles, the room remains darkened, and the lights inside the goggles are turned on. (b) When using the loupe, the ceiling light is turned on, the right eye is covered by the lens and the left eye is covered with the storage cover

Using ENG recordings, we measured the average SPV (°/s) over a 5-s period under different conditions and expressed it as mean SPV. The mean SPV in complete darkness was measured during the 25–30 s in the active phase. The mean SPV when using the loupe or Frenzel goggles during the activity phase was measured for 5–10 s after the change of conditions, because blinking increased and eye position fluctuated (Fig. 3).

Electronystagmography during the active phase of caloric nystagmus. Black arrow, 30 s after the end of the stimulus. In the complete darkness condition, the mean slow-phase velocity (SPV) of nystagmus was measured 25–30 s after the end of the stimulus. (a) For the Frenzel goggles (Frenzel), the mean SPV was measured 5–10 s after applying the Frenzel goggles. (b) For the loupe, mean SPV was measured 5–10 s after applying the loupe

When the loupe or Frenzel goggles are used, nystagmus is weaker than in complete darkness due to visual impact. To assess the visual impact of the tools, the ratio of mean SPV when using the tools to mean SPV in complete darkness was calculated for each ear as visual impact susceptibility (VIS) using the following formulas:

VIS of Loupe (%) = (mean SPV of the loupe) / (mean SPV of complete darkness immediately before) × 100.

VIS of Frenzel (%) = (mean SPV of Frenzel goggles) / (mean SPV of complete darkness immediately before) × 100.

The larger the VIS, the easier the observation of spontaneous nystagmus because visual fixation is reduced.

During the decay phase, the intensities of nystagmus in the naked eye condition and using the loupe were compared with the mean SPV for 5–10 s after the change of conditions (Fig. 4).

Electronystagmography during the decay phase of caloric nystagmus. Black arrow, 60 s after the end of the stimulus. The ceiling light was kept on and recording was performed in the naked eye condition for 15 s, followed by recording during the use of the loupe. (a) The mean slow-phase velocity (SPV) with the use of the loupe is greater than that of the naked eye condition. (b) No nystagmus was recorded with the naked eye condition; however, nystagmus reappeared when using the loupe

Data with a mean SPV of ˂10 °/s under complete darkness were excluded.

Concomitant with the ENG recording, nystagmus was recorded on video with a camera fixed 50 cm in front of the eyes in eight participants. In the active phase, eight videos were evaluated for the loupe and Frenzel goggles each. In the decay phase, eight videos were evaluated, because one side of the participants was randomly selected. Six clinical staff members who were well-trained in nystagmus observation (two nurses, two laboratory technicians, and two physical therapists) independently assessed nystagmus visibility in the video files. Each staff members gave a rating score from 0 to 2 points: Clearly visible nystagmus direction: 2 points, unclear direction: 1 point, no nystagmus: 0 point. The sum of the scores by the six staff members was used as the score for each video (0–12 points).

The Shapiro–Wilk test was used to determine whether the data were normally distributed. If the p-value of the Shapiro–Wilk test was ≥ 0.05, the data were considered to follow a normal distribution and parametric tests were applied. Conversely, if p < 0.05, the data were considered to deviate from normality and a non-parametric test was employed.

The main outcome was the VIS of Loupe compared with VIS of Frenzel; the 95% confidence interval (CI) for the mean difference in VIS between the loupe and Frenzel was calculated. As a secondary outcome, the mean SPV under the naked eye was compared with the mean SPV when using a loupe during the decay phase using a paired t-test or the Wilcoxon signed rank test (one-sided). Finally, for each video, the scores of the naked eye and loupe assessments in the decay phase were compared using the Wilcoxon signed-rank test (one-sided). The p-value < 0.05 was considered significant (Bell Curve for Excel Ver. 3).