Exploration and Application of Multiluminance Mobility Test in Bietti Crystalline Dystrophy

Abstract

Purpose

This study evaluated the multiluminance mobility test (MLMT) as a functional vision assessment tool for Bietti crystalline dystrophy (BCD) and explored its relationship with structural and visual function parameters.

Methods

A retrospective study of 34 BCD patients (67 eyes) incorporated 2 additional low light levels (0.1 and 0.5 lux) into the MLMT. Visual function assessments included best-corrected visual acuity, visual field (VF), and microperimetry (MP). Macular structure was evaluated via spectral-domain optical coherence tomography. MLMT performance was correlated with these parameters using Spearman's rank correlation tests.

Results

Despite severe global photoreceptor dysfunction, a notable proportion of participants (38.8%) passed the MLMT at 1 lux or less, with 23.9% reaching the lowest luminance (0.1 lux). MLMT lux scores correlated significantly with best-corrected visual acuity (ρ = −0.5322; P = 0.0012), mean defect of VF (ρ = −0.5919; P = 0.0005), and mean macular sensitivity of MP (MS-MP; ρ = 0.7175; P < 0.0001). Subgroup analysis revealed that subjects with preserved foveal ellipsoid zone had significantly higher MLMT lux scores and better MS-MP (both P < 0.05) than those with ellipsoid zone atrophy. Although visual function parameters were markedly impaired in certain patients, their MLMT performance remained relatively favorable.

Conclusions

The standardized MLMT has a limited range for assessing advanced BCD. Preservation of central macular structure, rather than visual acuity alone, appears to be critical for mobility performance in BCD patients. Disease-specific adaptations of functional end points are needed for BCD clinical trials.

Translational Relevance

To assess functional vision for BCD, the MLMT requires adaptation, and clinical trials should incorporate macular structural parameters.

Introduction

Bietti crystalline dystrophy (BCD), caused by mutations in CYP4V2,^1,2 is a rare inherited retinal disease (IRD) characterized by the deposition of yellowish–white crystalline substances in the retina. Clinically, BCD manifests as progressive vision loss, night blindness, and visual field (VF) constriction frequently reported in mid- to late- disease stages.³ Most patients experience nyctalopia during their second and third decades of life. According to a natural history study, the best-corrected visual acuity (BCVA) of patients with BCD more than 40 years old experienced a yearly increase of 0.09 units.⁴ Another history study revealed BCVA increased by 0.09 logarithm of the minimum angle of resolution (logMAR) per year in patients with BCVA of 0.5 logMAR or greater and disease duration of more than 10 years.⁵ Currently, no effective treatments are available. Thus, there is an urgent need for new treatments.⁶

Recently, our team conducted an investigator-initiated trial of the gene therapy drug rAAV8-hCYP4V2 for BCD. In this exploratory study, most treated eyes demonstrated significant visual improvements,⁷ consistent with findings from other CYP4V2 gene replacement therapies.⁸ These promising results provide new hope for patients facing the severe therapeutic challenges of BCD. Moving forward, research will likely focus on evaluating the long-term efficacy and safety of gene therapy. However, a critical challenge remains: the lack of robust, validated functional end points to sensitively and accurately measure therapeutic outcomes in BCD, particularly for gene replacement therapy.⁹

The multiluminance mobility test (MLMT), developed with support from the U.S. Food and Drug Administration (FDA), is a functional vision assessment method specifically designed for IRD patients with low vision. By integrating visual parameters such as visual acuity, VF, and light sensitivity into a mobility task under varying illumination.¹⁰ This standardized and quantifiable evaluation method has been widely used in clinical trials,^11–13 including studies for RPE65-associated Leber congenital amaurosis (LCA), where 71% of patients completing a 3-year follow-up successfully passed MLMT at the lowest light level.¹⁴ Importantly, MLMT metrics have shown strong correlations with improvements in functional vision, making it a valuable tool for assessing therapeutic outcomes. In these rod-dominant disorders, the MLMT effectively captures deficits in dim light mobility.¹⁰

However, our previous investigations identified a key observation: certain BCD patients, despite having severely reduced visual acuity or constricted VF, were able to pass MLMT at its lowest light level. Notably, the full-field electroretinography (ffERG) responses were unrecordable in most of these patients—yet they still succeeded at the standardized lowest luminance level of the test. This finding challenges the direct applicability of the standardized seven-level MLMT to BCD and raises fundamental questions about the relative contributions of cones versus rods to mobility performance in BCD. Unlike LCA, which is characterized by an early onset and relatively uniform disease progression,¹⁵ BCD exhibits later onset and irregular progression patterns.^16,17

These disease-specific differences suggest that the MLMT, as currently standardized, may not fully capture the functional deficits or potential improvements in BCD. Therefore, a dedicated investigation into the utility and potential adaptation of the MLMT for BCD is warranted.

To address this limitation, we incorporated two additional lower light levels (0.5 lux and 0.1 lux) to refine the MLMT for BCD patients. Additionally, we analyzed the relationships between MLMT performance, BCVA, VF, and MP to better characterize the functional vision of these patients and assess the applicability of MLMT in evaluating the therapeutic outcomes of BCD patients.

Methods

Trial Design and Participants

This cross-sectional, observational study enrolled thirty-four subjects. The inclusion criteria were as follows: (1) male or female patients aged 18 years or older, (2) a characteristic fundus phenotype of BCD with confirmed genetic mutations in the CYP4V2 gene, with no restrictions on visual acuity and VF, (3) the ability to comply with study testing requirements and demonstrate sufficient attention span to navigate the mobility courses, and (4) the capacity to provide signed informed consent for study participation. Exclusion criteria included individuals with physical disabilities (physical, neurological, or cognitive impairments preventing safe, independent navigation of the course), attentional limitations (inability to understand or follow test instructions during a standardized pretest practice session), concurrent retinal disorders (diabetic retinopathy, age-related macular degeneration, or glaucoma), or a history of intraocular surgeries. Examinations were conducted after obtaining written informed consent and in accordance with the principles of the Declaration of Helsinki. The procedures used in this study were approved by the local ethics committee of Southwest Hospital, Army Medical University, Chongqing, China (approval number: (A)KY2023007).

Functional Vision: MLMT

Before the examination, a brief training session was conducted for both examiners and participants to ensure adherence to standardized test protocols. Each mobility course was recorded using video and audio equipment. The final MLMT lux score was determined based on the minimum illuminance level (in lux) required to achieve the target accuracy score (defined as committing a maximum of three errors) and time score (defined as the total time to complete the course plus time penalties, with a cumulative duration of less than 180 seconds).⁹

Although light levels of less than 1 lux were deemed less relevant to activities of daily living, two additional illuminance levels (0.1 lux and 0.5 lux) were introduced to account for the greater variability in light detection ability observed in BCD patients compared with those with LCA, who typically exhibit severe rod cell dysfunction (Table 1). The remaining lux levels and corresponding real-world examples were consistent with those used in the phase 3 study of voretigene neparvovec.¹⁰ Participants unable to pass the 400-lux threshold were assigned a score of −1.

Table 1.

Additional Lux Level and Corresponding Real-World Examples

Illuminance (Lux)	Luminance (cd/m2)	Corresponding Environment
0.1	0.03 mesopic vision	Indoor without lighting, or desert under starlight
0.5	0.16 mesopic vision	Weak moonlight or with cloud cover

Other lux levels were identical to the version used in phase 3 study of voretigene neparvovec. Light meters were used to provide examples and to set/verify specified light levels used.

Clinical Investigations

Visual function was evaluated through measurements of BCVA, VF, and MP, and ffERG. Macular structure was evaluated using spectral-domain optical coherence tomography (SD-OCT) on a Spectralis OCT platform (Heidelberg Engineering, Heidelberg, Germany).

BCVA was assessed using the Early Treatment Diabetic Retinopathy Study (ETDRS) chart, with results converted to the logMAR via the formula: logMAR = −0.02 × ETDRS + 1.7). Counting fingers, hand movement, and light perception were represented by extrapolated logMAR values of 2.6, 2.7, and 2.8, respectively.¹⁸ Visual impairment severity was graded according to the World Health Organization classification into three categories: no or mild, moderate, and severe or blindness.¹⁹

VF testing was conducted using the Humphrey Static Perimeter. A full-threshold strategy with a size V test stimulus was used, consisting of 76 points within a central 30° circle. The background luminance was set at 31.5 apostilbs. Tests with reliability factor scores exceeding 25% were excluded to minimize positive or negative bias. The mean defects (MDs) were used to quantify the extent of VF loss.

Central retinal sensitivity was measured monocularly using the MP-3 microperimeter system (NIDEK, Gamagori, Japan). Tests were performed on a 10-2 grid, comprising 45 test loci covering the central 12°. We used a Goldman size III light on a 31.4 asb background. A 4-2 bracketing strategy was applied, and a 1° red circle served as the central fixation target. Examinations with fixation losses of greater than 30% were considered unreliable and excluded from analysis. Average macular sensitivities (MSs) were used for MP analysis.

ffERG (Diagnosys LLC, Lowell, MA) was recorded in accordance with the international standards of the International Society for Clinical Electrophysiology of Vision. Macular structure was analyzed using SD-OCT. Based on the integrity of the ellipsoid zone (EZ) at the fovea, eyes were categorized into two subgroups: an EZ-preserved group (characterized by a continuous EZ band at fovea) and an EZ-atrophy group (defined as disruption or absence of the EZ at the fovea).

Statistical Analysis

Continuous variables, such as age, BCVA, and VF-MD, are presented as medians with interquartile ranges (IQRs). Categorical variables, such as onset symptoms, are summarized using frequencies and percentages. All statistical analyses were conducted using SPSS version 27.0 (SPSS Inc, Chicago, IL). Spearman's rank correlation tests were used to evaluate the pairwise relationships between MLMT lux scores and each of the following three visual metrics: BCVA, VF-MD, and mean MS-MP. Data analysis and graphical representations were performed using GraphPad Prism (version 8.2.1; GraphPad Software, La Jolla, CA).

Results

Demographics

A total of 67 eyes from 34 BCD patients were recruited, excluding one eye with a history of vitrectomy. The 67 eyes were categorized according to the World Health Organization visual impairment criteria, resulting in 18 eyes with no or mild impairment, 28 with moderate impairment, and 21 with severe impairment or blindness. The cohort had the median age of 43 years (IQR, 13 years), median BCVA of 0.70 logMAR (IQR, 0.90 logMAR), median VF-MD of 18.68 dB (IQR, 11.34 dB), and a median MS-MP of 1.20 dB (IQR, 2.90 dB) (Table 2).

Table 2.

Demographics and Clinical Characteristics of Patients With BCD

Variables	Value
Agea, years		43 (13)
Onset ageb, years		34 (12)
Sexc
Female		20 (58.82)
Male		14 (41.18)
Onset symptomsd
Nyctalopia		15 (53.57)
Vision declined		4 (14.29)
VF defect		1 (3.57)
Vision declined and nyctalopia		8 (28.57)
Visual Impairmente
No or mild		18 (26.9)
Moderate		28 (41.8)
Severe or blindness		21 (31.3)
ffERGf
Waveform extinguished		25 (83.3)
Waveform nonextinguished		5 (16.7)
	Both Eyes	Right Eye	Left Eye
BCVAg/logMAR units	0.70 (0.90)	0.65 (0.62)	0.80 (1.05)
VF-MDh/dB	18.68 (11.34)	19.26 (12.17)	17.82 (11.08)
MS-MPi/dB	1.20 (2.90)	1.60 (4.50)	0.60 (2.90)

Continuous variables are reported as medians with IQR. Categorical variables are described with frequencies and percentages.

^aAge, available in 34 subjects.

^bOnset age, available in 24 subjects.

^cSex and available in 34 subjects.

^dOnset symptoms available in 28 subjects.

^eVisual Impairment obtained from 67 eyes.

^fffERG, obtained from 60 eyes.

^gBCVA, obtained from 67 eyes.

^hVF-MD, obtained from 62 eyes.

ⁱMS-MP were obtained from 61 eyes.

ffERGs were performed in 30 patients; 25 patients (83.3%) showed extinguished waveforms in both light-adapted and dark-adapted responses. Of the remaining five patients, one had normal responses in the right eye (the left eye was excluded owing to a history of vitrectomy), one showed severely decreased rod and cone responses, and three presented with well-preserved cone responses but moderately decreased rod responses (Table 2). Approximately 38.8% of BCD patients were able to pass the MLMT at light intensities of 1 lux or less, with 23.9% achieving a score at 0.1 lux (Fig. 1).

Figure 1.

The distribution of MLMT lux scores, single-eye assessment for 34 BCD patients (67 eyes).

OCT-based Subgroup Analysis: Functional Differences by EZ Integrity

SD-OCT images were obtained in all patients. The EZ-preserved group was significantly younger (mean age, 39.40 ± 5.63 years) than the EZ-atrophy group (mean age, 49.37 ± 7.75 years; t = −4.18; P < 0.001). The median MLMT lux score was significantly higher in the EZ-preserved group (6.00; IQR, 4.00) compared with the EZ-atrophy group (3.50; IQR, 4.00; P = 0.03). In addition, the VF-MD showed a trend toward a difference between the EZ-preserved group (14.30 ± 7.68 dB) and the EZ-atrophy group (19.04 ± 4.93 dB; P = 0.05). MS-MP was significantly higher in the EZ-preserved group (2.70; IQR, 4.10) than that in the EZ-atrophy group (1.20 (IQR, 2.1; P = 0.01) (Fig. 2).

Figure 2.

Comparison of MLMT lux score, BCVA, VF-MD, and MS-MP between EZ-preserved and EZ-atrophy groups. Participants were categorized by the presence or absence of RPE or EZ integrity at the fovea and parafovea. Those with preserved RPE or EZ structure showed notably higher MLMT lux scores and better MP-derived MS than subjects without these structural features (both P < 0.05), while BCVA and VF-MD did not differ significantly between the two groups.

Correlations Between MLMT Lux Score and Visual Function Assessments

Spearman's rank correlation tests were analyzed to further investigate the correlation between the MLMT lux score and visual acuity, VF-MD. BCVA (logMAR) of right eyes were shown to moderately correlate with MLMT lux score (ρ = −0.5322; 95% confidence interval [CI], −0.7423 to −0.2268; P = 0.0012; Fig. 3A), as well as between the MLMT lux score and the VF-MD (ρ = −0.5919; 95% CI, −0.7864 to −0.2906; P = 0.0005) (Fig. 3B).We also incorporated the MP to predict functional vision performance, finding a stronger correlation between MLMT lux scores and mean sensitivity on MP (ρ = 0.7175; 95% CI, 0.4785 to 0.8575; P < 0.0001) (Fig. 3C; Supplementary Fig. S1) compared with traditional VF assessments. This finding suggests that MP may be a better predictor of functional vision. However, mobility assessment remains complex, influenced by multiple factors beyond visual parameters. Despite some meaningful correlations among the visual measurements, a few BCD patients with poorer visual function, such as patient 8 and patient 32, exhibited higher bilateral MLMT lux scores of 8 and 7, respectively (Fig. 4). This finding indicates that the mobility test may be influenced by other unmeasured factors, underscoring the need for a more comprehensive approach to patient evaluation.

Figure 3.

Scatter plots of the correlations between visual functional parameters and MLMT lux score for each patient with BCD. Two separate analyses were conducted using data from the right and left eyes of patients, respectively, to examine the correlation between each visual function parameter (x axis) and MLMT lux scores (y axis). Visual function parameters include BCVA (A), VF-MD (B), and MS-MP (C).

Figure 4.

Fundus photography, fundus autofluorescence (FAF), OCT, VF, and MP images of two cases of BCD with poor visual function. (A) Patient 32 fundus photography: stage 3; FAF: extensive AF atrophy; OCT: complete outer retinal atrophy involving the central fovea; MLMT lux scores of 5, 7, and 7 for the right eye, left eye, and both eyes, respectively; BCVA of 1.1 and 0.6 in the logMAR unit for the right and left eyes, respectively; VF-MD of 24.43 and 24.09 dB for the right and left eyes, respectively; MS-MP of 0.1 and 0.2 dB for the right and left eyes, respectively. (B) Patient 8: fundus photography, FAF, and OCT of patient 8 exhibit similar manifestations as patient 32. MLMT lux scores of 6, 8, and 8 for the right eye, left eye, and both eyes, respectively; BCVA of 1.5 and 0.6 in the logMAR unit for the right and left eyes, respectively; VF-MD of 20.30 and 20.03 dB for the right and left eyes, respectively; MS-MP of 0 and 1.3 dB for the right and left eyes, respectively. Although patients 32 and 8 have similar BCVA and VF-MD, patient 8 demonstrates superior performance in the MLMT lux score, attributed to relatively better MS-MP result of the left eye.

In mesopic vision and photopic vision, MLMT lux scores decreased as visual acuity worsened, showing a clear negative trend. BCVA and MLMT lux scores were linked in mesopic vision (Y = −1.402X + 7.263; R² = 0.1720; P = 0.0078) (Fig. 5A) and in photopic vision (Y = −1.243X + 2.588; R² = 0.4438; P = 0.0001) (Fig. 5A).

Figure 5.

Relationship between VF defects/BCVA/MP and MLMT score. Blue circles (mesopic vision): MLMT lux scores under low luminance conditions. Orange triangles (photopic vision): MLMT lux scores under bright luminance conditions. Solid lines (regression): Trendlines for mesopic and photopic vision. (A) The x axis is BCVA (greater values = poorer). (B) The x axis is the VF-MD (greater values = more severe). (C) The x axis is the mean MS-MP (greater values = better). All y axes represent the MLMT lux score.

MLMT lux scores decreased as VF defects increased, demonstrating a negative correlation in mesopic vision (Y = −0.1335X + 8.343; R² = 0.3021; P = 0.0004) (Fig. 5B). This decrease showed some variability, reflecting individual differences in mesopic performance. In photopic vision, MLMT lux scores also dropped as VF defects increased (Y = −0.2038X + 5.266; R² = 0.3722; P = 0.0012) (Fig. 5B). This finding indicates that photopic vision may be more consistently impacted by VF defects.

A tendency of increase in the MLMT lux score with MS-MP was found in both mesopic and photopic vision (Y = 0.7414X + 0.9594 [R² = 0.0830; P = 0.1827]; mesopic vision, Y = 0.1410X + 5.759 [R² = 0.1526; P = 0.0153]) (Fig. 5C). However, the impact was more pronounced in photopic conditions, with a steeper improvement observed compared with mesopic vision. These results indicate that both mesopic and photopic MLMT lux scores are impacted by VF defects, visual acuity, and MS, but the specific effects vary depending on the condition. Photopic vision shows a more uniform decrease with VF defects and is more dependent on central visual acuity compared with mesopic vision.

In addition, for reason that the untreated eye can serve as the control for the treated eye, we conducted interocular symmetry analysis of visual function in BCD patients. This analysis showed significant interocular correlations for BCVA, VF-MD, MS-MP, and MLMT lux scores (ρ = 0.4106, 0.9316, 0.8051, and 0.8816, respectively) (Supplementary Fig. S2).

Discussion

To the best of our knowledge, this study is the first to evaluate the applicability of MLMT in BCD patients. The MLMT has demonstrated sensitivity to a 2-lux change, which is considered clinically meaningful in IRD trials.²⁰ Although several clinical trials have advanced to phase III and demonstrated sustained improvement of MLMT lux scores in RPE65 mutation-associated IRD, no prior studies have specifically investigated the performance of BCD patients in this test.

In the present study, 25 of 30 patients exhibited extinguished ffERGs, indicating severe global photoreceptor dysfunction, 30 of 34 were able to pass the MLMT. This discrepancy raises two critical questions: first, whether the FDA's two-level improvement criterion may inadequately reflect treatment efficacy in BCD, where achieving a clinically meaningful improvement appears to be more challenging; and second, what factors determine mobility performance in patients with apparently extinguished electrophysiological responses. To address the first question, we conducted two lower light levels to refine the MLMT for BCD patients. We found that 38.8% patients were able to pass the MLMT at light intensities of 1 lux or lower, with 23.9% achieving a score at 0.1 lux. This result suggests that the standard MLMT has a limited dynamic range for assessing residual functional vision in certain BCD patients.

Regarding the second question, we performed a subgroup analysis based on macular structural integrity at the fovea. Patients were classified according to preservation of the EZ, revealing that patients had relatively EZ structure at fovea were associated with significantly higher MLMT lux scores and better MS-MP (both P < 0.05). There was no significant difference in BCVA and VF-MD(P > 0.05). This finding indicates that, in advanced BCD, although most patients exhibit severe BCVA loss and VF defects, the integrity of the central macular architecture plays a crucial role in supporting performance on a structured mobility task.

These findings contrast with the performance of MLMT in LCA2, where it exhibits a strong correlation with functional deterioration. The different results of MLMT between LCA and BCD may stem from their distinct pathological origins. RPE65-associated LCA primarily with onset in infancy or by age 5 affects rod function, causing severe early loss of night vision and peripheral field, although central acuity is often relatively spared initially. As a low-luminance mobility test, the MLMT may elicit invisible-in-the-dark behavior among patients with LCA. In contrast, BCD with onset of second to third decade, predominantly affects the macular early in the disease course,³ leading to significant central vision loss, whereas peripheral retinal function may remain relatively intact compared with RPE65-associated LCA. Therefore, the primary challenge for BCD patients during MLMT is often reduced clarity or contrast sensitivity in perceiving path markers, rather than a complete inability to visualize targets under dim light conditions.

Recently, two clinical trails, including ours, reported the outcomes of gene therapy for BCD, showing average improvements of 13.9 ± 13.1 letters for 11 study eyes⁷ and 11.0 ± 10.6 letters for 4 treated eyes⁸ at the 1-year follow-up. Despite significant improvements in BCVA observed, they did not meet the FDA's criterion of a 15-letter change.²⁰ In this research, although moderate to strong correlations between BCVA, VF-MD, MS-MP, and MLMT scores were observed, discrepancies existed. For instance, some patients with better visual function performed worse on the MLMT, whereas others with poorer visual function performed better. Notably, BCD eyes with BCVA ranging from −0.1 to 0.8 logMAR and VF-MD from 0.71 to 20.7 dB achieved the lowest light level score. This finding underscored the importance of carefully selecting end points and patient recruitment criteria for future phase III trials.

Our study did, however, have some limitations. First, the small sample size of 34 patients and an inability to recruit more BCD patients at mild-to-moderate disease stages limited us to investigating the differences in functional vision among BCD patients. Second, a notable objective shortcoming of the present study is the lack of rod-specific functional evaluations (e.g., full-field sensitivity threshold), which emphasizes the need to integrate these assessments into subsequent studies. Third, although we introduced lower luminance levels within the limits of existing techniques, the ceiling effect associated with this assessment could not be avoided.

In summary, our data suggest that, in advanced BCD, where rod-mediated function is widely compromised, the relative preservation of the EZ structure in the macular area may provide critical support for patients to navigate, leading to better MLMT performance. However, the direct translation of the MLMT into routine clinical practice faces significant challenges: as a research-based tool, it requires specialized equipment, trained personnel, and controlled settings, making it costly, and it is further limited by its known ceiling effect. When designing trials for other IRDs, it is important to select functional end points of each specific condition.