Abstract
Purpose:
The retinal involvement of amyotrophic lateral sclerosis (ALS) is a novel idea about a possible correlation between retinal nerve fiber layer (RNFL) thickness in different spectra of ALS patients. Finding the association of RNFL with disease duration and severity will help identify a novel noninvasive biomarker.
Methods:
The study was designed as a cross-sectional study and was conducted with a suitable proforma. We included the ALS cases based on the revised El Escorial criteria. Healthy controls were age and gender matched. We used the revised ALS functional rating scale (ALSFRS-R) to assess the operational status of the patients. We measured RNFL thickness in the four quadrants with spectral-domain optical coherence tomography (OCT) and analyzed it.
Results:
We included 30 cases (60 eyes) and 10 healthy controls (20 eyes) having a mean (standard deviation [SD]) age of 49.5 (11.1) years with a median of 50 years, and a majority of them (65%) were middle aged (between 41 and 60 years). We found statistically significant differences in RNFL thicknesses between ALS patients and healthy controls. On segmental analysis, the right eye superior and nasal quadrants and the left eye superior, inferior, and nasal quadrants were significantly affected, along with a gross asymmetry found between the left and right eyes among ALS patients. There was a significant decrease in average RNFL thickness in definite ALS patients than probable ALS patients, with significantly reduced average RNFL thickness in moderate to severe ALS patients. On correlation analysis, disease duration showed a good negative correlation with bilateral average RNFL thickness, and the ALSFRS-R score demonstrated a good positive correlation with bilateral average RNFL thickness, which was statistically significant. Thus, a reduced bilateral RNFL thickness is associated with a decreased ALSFRS-R score.
Conclusion:
The retinal changes can serve as a marker for diagnosing and monitoring patients with ALS.
Keywords: Amyotrophic lateral sclerosis, optical coherence tomography, retinal nerve fiber layer
The dominant feature of amyotrophic lateral sclerosis (ALS) is motor neuron degeneration, and the ophthalmic complaints were not considered a specific symptom of ALS. Still, postmortem findings in human retinal tissues in thinning, axonal degeneration, and inclusion of bodies similar to the spinal cord were present in subjects with ALS.[1] Retina shares an ontogenic relationship with the brain, and many genes are associated with both neurodegeneration of the brain and retina.[1] Therefore, the anterograde degeneration caused by ganglion cell death in the retina and retrograde degeneration caused by neurodegeneration in the cerebral cortex might be two major credible processes in this disease group.[2] ALS leads to retina neuronal damage by activating the mouse model’s brain and spinal cord proinflammatory microglia or immune cells. The retinal thinning in ALS is associated with the loss of retinal neurons called ganglionic cells, which convey visual information from other retinal neurons to the brain.
Optical coherence tomography (OCT) is a noninvasive, quick, and painless imaging technique that produces high-resolution cross-sectional retinal images with high temporal accuracy. It uses low coherence light to capture 2D and 3D images of the retina, allowing detailed pictures of the retinal nerve fiber thickness, ganglion cell layer, and macular thickness. Early changes could be detected at a minute level.
In this study, we aim to assess possible structural changes of the retina in the spectrum of ALS patients and to evaluate any possible correlation between the retinal changes with ALS severity and disease duration. In addition, due to the lack of reliable biologic markers for early diagnosis and progression, retinal structural changes could help us assess the entity, so that early neurointervention can delay mortality and morbidity.
Methods
This cross-sectional study was conducted with a suitable proforma in a medical college hospital in Kolkata, India, from October 2020 to April 2022. Institutional ethical committee approval was taken. The diagnosis of ALS was made based on the revised El Escorial criteria 2015,[3] and we included all ALS patients randomly on the basis of these criteria. We analyzed 30 ALS cases (60 eyes) and compared them with age- and gender-matched 10 healthy controls (20 eyes) to minimize bias. As the coronavirus disease (COVID-19) pandemic was rampant in India during the study period and healthy people avoided to attend hospitals, the number of healthy controls in our study remained less.
They were examined properly after written consent was obtained from them. The revised ALS functional rating scale (ALSFRS-R) was used to assess the operational status of the patients. We evaluated the retinal structure using a spectral-domain optical coherence tomography (SD-OCT) device (Heidelberg). B-scans were made in circles (with a radius of 1.7 mm/diameter of 3.4 mm) from the center of the optic disk. Retinal nerve fiber layer (RNFL) thickness in the upper, lower, nasal, and lateral quadrants and the average were measured along with visual evoked potential (VEP) and magnetic resonance imaging MRI.
Results
Distribution of cases according to spectrum, symptom at onset, duration of diseases, and severity of ALS
Background sociodemographic and clinical parameters of the ALS patients are presented in Table 1.
Table 1.
Background sociodemographic and clinical parameters of ALS patients
| Parameter | Category | Number (percentage) |
|---|---|---|
| Age (years) | ≤40 | 9 (22.5) |
| 41-60 | 26 (65) | |
| >60 | 5 (12.5) | |
| Gender | Male | 33 (82.5) |
| Spectrum of ALS | Probable ALS | 16 (53.3) |
| Definite ALS | 14 (46.7) | |
| Onset of symptoms | Spinal | 24 (80) |
| Bulbar | 6 (20) | |
| Duration of disease | <6 | 0 (0) |
| 6-24 | 14 (46.7) | |
| >24 | 16 (53.3) | |
| Severity of ALS (ALSFRS-R score) | Up to 30 | 11 (36.7) |
| 31-40 | 8 (26.6) | |
| >40 | 11 (36.7) |
ALS=amyotrophic lateral sclerosis, ALSFRS-R=revised amyotrophic lateral sclerosis functional rating scale
More than half of the cases (53.3%) were diagnosed as probable ALS and the rest, 46.7%, were definite ALS. Spinal-onset ALS was 80% and the rest, 20%, had bulbar-onset disease. More than half of the cases, that is, 53.3%, had more than 24 months of illness, with 46.7% having 6–24 months of disease duration. The mean duration was 29.6 ± 14.5 months, ranging from 10 to 59 months with a median of 26 months. According to the ALSFRS-R score, 36.7% had a score of ≤30, 36% had a score of >40, and the rest, 26.6%, had scores between 30 and 40. Their mean ALSFRS-R score was 36.1 ± 7.8, ranging from 23 to 48 with a median of 34.
Comparative analysis of RNFL thickness and different parameters between cases and controls
The segmental analysis of RNFL thickness showed a significant difference in the mean RNFL thickness in right eyes between cases and controls in average, superior, and nasal quadrants, but not in inferior or temporal quadrants. Similarly, the superior, inferior, and nasal quadrants were significantly affected in the left eye of ALS cases, sparing the temporal quadrant [Table 2].
Table 2.
Comparison of RNFL thickness in different quadrants in both eyes among the study subjects
| RNFL thickness (µm) in eye quadrants | Parameters | Cases (n=30) | Controls (n=10) | Total (n=40) | P |
|---|---|---|---|---|---|
| Left eye | |||||
| Average | Mean (SD) | 111.2 (7.3) | 119.2 (1.3) | 113.2 (7.3) | 0.001 |
| Median (IQR) | 111.3 (105-117.8) | 119.3 (118.8-119.8) | 115.8 (108.3-119.3) | ||
| Range | 97-123.4 | 116.5-121.5 | 97-123.4 | ||
| Superior | Mean (SD) | 135 (13) | 152 (4) | 140 (14) | <0.001 |
| Median (IQR) | 135 (124-149) | 152 (149-155) | 145 (129-151) | ||
| Range | 112-159 | 145-159 | 112-159 | ||
| Inferior | Mean (SD) | 152 (11) | 161 (3) | 154 (11) | 0.007 |
| Median (IQR) | 151 (143-158) | 161 (159-163) | 157 (147-163) | ||
| Range | 123-175 | 157-168 | 123-175 | ||
| Nasal | Mean (SD) | 77 (7) | 83 (4) | 77 (6) | 0.002 |
| Median (IQR) | 78 (73-80) | 82 (81-86) | 79 (74-81) | ||
| Range | 60-93 | 78-89 | 60-93 | ||
| Temporal | Mean (SD) | 81 (6) | 84 (4) | 82 (6) | 0.177 |
| Median (IQR) | 81 (78-86) | 84 (82-87) | 82 (78-87) | ||
| Range | 67-95 | 76-89 | 67-95 | ||
| Right eye | |||||
| Average | Mean (SD) | 113.5 (6.3) | 119.8 (1.5) | 115.1 (6.1) | <0.001 |
| Median (IQR) | 114.6 (108.5-118.3) | 119.8 (118.5-121) | 116.6 (110.9-119.2) | ||
| Range | 100.2-127.8 | 117.8-121.8 | 100.2-127.8 | ||
| Superior | Mean (SD) | 138 (11) | 154 (6) | 142 (12) | <0.001 |
| Median (IQR) | 139 (129-148) | 151 (150-159) | 146 (132-150) | ||
| Range | 116-154 | 148-168 | 116-168 | ||
| Inferior | Mean (SD) | 155 (10) | 160 (4) | 156 (9) | 0.072 |
| Median (IQR) | 154 (148-164) | 161 (158-163) | 158 (149-164) | ||
| Range | 126-176 | 153-166 | 126-176 | ||
| Nasal | Mean (SD) | 80 (7) | 85 (3) | 82 (6) | 0.008 |
| Median (IQR) | 81 (78-83) | 87 (83-88) | 83 (79-86) | ||
| Range | 65-95 | 80-89 | 65-99 | ||
| Temporal | Mean (SD) | 81 (6) | 80 (2) | 81 (5) | 0.221 |
| Median (IQR) | 81 (78-85) | 80 (77-81) | 81 (78-84) | ||
| Range | 66-92 | 76-83 | 66-92 | ||
| Bilateral average | |||||
| Mean (SD) | 112.3 (6.7) | 119.4 (1.1) | 114.1 (6.6) | <0.001 | |
| Median (IQR) | 111.3 (108-118) | 119.6 (118.8-120) | 116.6 (109.1-119.6) | ||
| Range | 99.5-125.6 | 117.5-121.3 | 99.5-125.6 |
IQR=interquartile range, RNFL=retinal nerve fiber layer, SD=standard deviation, Bold numbers are statistically significant (p value < 0.05)
Following the initial hypothesis, this study found statistically significant differences in RNFL thicknesses between ALS patients and healthy controls. The results of the study demonstrated that the RNFL thickness in the left eye (mean 111.2 ± 7.3 μm) was lower in ALS patients in comparison to age- and sex-matched healthy controls (mean 119.3 ± 1.3 μm). Furthermore, the degree of RNFL thinning was different in the right and left eyes of ALS patients (113.5 ± 6.3 vs. 111.2 ± 7.3 μm), suggesting significant asymmetricity (P < 0.001) [Table 3].
Table 3.
Comparison of bilateral average RNFL thickness (μm) among the ALS subgroups
| ALS subcategories | Mean (SD) | Median | Range | P |
|---|---|---|---|---|
| Probable ALS (n=16) | 115.9 (6.5) | 117.9 | 99.5-125.6 | <0.001 |
| Definite ALS (n=14) | 108.2 (4.2) | 108.9 | 100.1-113.5 | |
| Spinal-onset ALS (n=24) | 112.9 (6.9) | 113.6 | 99.5-125.6 | 0.251 |
| Bulbar-onset ALS (n=6) | 110.2 (5.5) | 110.9 | 102.5-117.5 | |
| Short duration (≤24 months) (n=14) | 115.5 (4.9) | 117.7 | 106.8-122.1 | 0.01 |
| Long duration (>24 months) (n=16) | 109.5 (6.9) | 109.5 | 99.5-125.6 | |
| Mild ALS (a) | 118.8 (3.3) | 118.5 | 113.6-125.6 | 0.031 (a vs. b) |
| Moderate ALS (b) | 111.2 (5.6) | 111.9 | 100.1-118.4 | <0.001 (a vs. c) |
| Severe ALS (c) | 106.7 (4) | 106.8 | 99.5-113 | 0.416 (b vs. c) |
| Right eye (cases) (n=30) | 113.5 (6.3) | 114.6 | 100.2-127.8 | <0.001 |
| Left eye (cases) (n=30) | 111.2 (7.3) | 111.3 | 97-123.4 | |
| Right eye (controls) (n=10) | 119.8 (1.5) | 119.8 | 117.8-121.8 | 0.227 |
| Left eye (controls) (n=10) | 119.2 (1.3) | 119.3 | 116.5-121.5 |
ALS=amyotrophic lateral sclerosis, SD=standard deviation, Bold fonts are statistically significant, means P < 0.05
In this study, we divided our cases into two groups based on the duration of illness, that is, short duration (<24 months) and long duration (>24 months). More than half of them (53.3%) had a long duration; the rest, 46.7%, had a short duration (no patient had duration of less than 6 months). The mean duration was 29.6 ± 14.5 months, ranging from 10 to 59 months with a median of 26 months. The mean bilateral average RNFL thickness among short-duration ALS patients (115.5 ± 4.9 μm, ranging from 106.8 to 122.1 μm with a median of 117.7 μm) was significantly higher than the long-duration ALS patients (109.5 ± 6.9 μm, ranging from 99.5 to 125.6 μm with a median of 109.5 μm) [Table 3].
In our study, the mean bilateral average RNFL thickness among probable ALS patients (115.9 ± 6.5 μm, ranging from 99.5 to 125.6 μm with a median of 117.9 μm) was much higher than that of definite ALS patients (108.2 ± 4.2 μm, ranging from 100.1 to 113.5 μm with a median of 108.9), with a P value <0.001. Bilateral average RNFL among spinal-onset ALS patients was 112.9 ± 6.9 μm (ranging from 99.5 to 125.6 μm with a median of 113.6 μm), and the mean average bilateral RNFL among bulbar-onset ALS patients was 110.2 ± 5.5 μm (ranging from 102.5 to 117.5 μm with a median of 110.9 μm). No significant difference in the mean bilateral average RNFL thickness was noticed between spinal- or bulbar-onset disease [Table 3].
Correlation of disease duration and severity (ALSFRS-R score) with RNFL parameters
For statistical analysis, we grouped our cases into three groups based on the ALSFRS-R scores (mild >40, moderate 31–40, and severe ≤30). On comparative analysis, we found the mean bilateral average RNFL in the mild group (>40 ALFRS-R scores) was 118.8 ± 3.3 μm, ranging from 113.6 to 125.6 μm with a median of 118.5 μm. The mean bilateral average RNFL in the moderate group (31–40 ALSFRS-R scores) was 111.2 ± 5.6 μm (ranging from 100.1 to 118.4 μm with a median of 111.9 μm). The mean average bilateral RNFL in the severe group (≤30 ALSFRS-R scores) was 106.7 ± 4.0 μm (ranging from 99.5 to 113 μm with a median of 106.8 μm). In addition, a significant difference in bilateral average RNFL thickness was noticed across different severity groups (P < 0.001). On pairwise comparison, a significant difference in mean bilateral average RNFL was seen between mild and moderate (P = 0.031) and mild and severe (P < 0.001) groups, but no such significant difference was observed between moderate and severe groups (P = 0.416) [Table 3].
On correlation analysis, the ALSFRS-R score demonstrated a positive correlation with bilateral average RNFL thickness and RNFL thickness in the superior and inferior quadrants. In addition, the ALSFRS-R score also had a positive correlation with RNFL thickness in the nasal and temporal quadrants on both sides. All of these correlations were statistically significant. Thus, bilateral average RNFL thickness was positively correlated with the ALSFRS-R score. There was a negative correlation between duration of illness and bilateral average RNFL thickness and left eye RNFL thickness, mostly in the average and superior quadrants. On the other hand, disease duration negatively correlated with the right eye RNFL thickness in the superior and inferior quadrants and with the left eye RNFL thickness in the inferior and temporal quadrants, which was statistically significant. Thus, increased disease duration was associated with a substantial decrease in RNFL thickness in bilateral average and right superior and inferior and left superior, inferior, and temporal quadrants [Fig. 1, Table 4].
Figure 1.
(a) Correlation of RNFL thickness parameters of right eye with disease duration and ALSFRS-R score (n = 30). (b) Correlation of RNFL parameters of left eye with disease duration and ALSFRS-R score (n = 30). (c) Correlation of bilateral average RNFL with disease duration and ALSFRS-R score (n = 30). ALSFRS-R = revised amyotrophic lateral sclerosis functional rating scale, RNFL = retinal nerve fiber layer
Table 4.
Correlation of RNFL thickness (μm) parameters with disease duration and ALSFRS-R score (n=30)
| RNFL thickness | Disease duration | ALSFRS-R score | ||
|---|---|---|---|---|
|
|
|
|||
| Correlation coefficient | P | Correlation coefficient¥ | P | |
| Bilateral average | −0.687 | <0.001 | 0.893 | <0.001 |
| Right average | −0.652 | <0.001 | 0.882 | <0.001 |
| Right superior | −0.634 | <0.001 | 0.761 | <0.001 |
| Right inferior | −0.556 | 0.001 | 0.737 | <0.001 |
| Right nasal | −0.211 | 0.262 | 0.383 | 0.037 |
| Right temporal | −0.284 | 0.129 | 0.589 | 0.001 |
| Left average | −0.723 | <0.001 | 0.879 | <0.001 |
| Left superior | −0.811 | <0.001 | 0.774 | <0.001 |
| Left inferior | −0.651 | <0.001 | 0.771 | <0.001 |
| Left nasal | −0.196 | 0.299 | 0.408 | 0.025 |
| Left temporal | −0.4 | 0.029 | 0.602 | <0.001 |
ALSFRS-R=revised amyotrophic lateral sclerosis functional rating scale, RNFL=retinal nerve fiber layer, Bold numbers are statistically significant (p value < 0.05)
Visual evoked potentials showed no statistically significant difference between the cases and controls in either eye [Table 5]. Only two patients had increased P100 latency. Table 6 shows all the parameters of cases and controls of our study.
Table 5.
Comparison of visual evoked potential parameters between cases and controls
| Visual evoked potential | Parameters | Cases (n=30) | Controls (n=10) | Total (n=40) | P |
|---|---|---|---|---|---|
| P100 latency right | Mean (SD) | 95 (8) | 93 (4) | 94 (7) | 0.89 |
| Median (IQR) | 93 (88-101) | 92 (90-94) | 92 (89-101) | ||
| Range | 83-116 | 87-101 | 83-116 | ||
| P100 amplitude right | Mean (SD) | 6.4 (2.4) | 5.9 (1.2) | 6.3 (2.1) | 0.233 |
| Median (IQR) | 6.7 (5.5-7.9) | 6.3 (5.3-6.8) | 6.6 (5.4-7.8) | ||
| Range | 1.4-9.8 | 3.5-7.2 | 1.4-9.8 | ||
| P100 latency left | Mean (SD) | 95 (9) | 96 (5) | 95 (8) | 0.77 |
| Median (IQR) | 94 (88-102) | 98 (93-99) | 95 (88-101) | ||
| Range | 81-116 | 86-101 | 81-116 | ||
| P100 amplitude left | Mean (SD) | 6.9 (1.9) | 6.2 (1.3) | 6.8 (1.8) | 0.331 |
| Median (IQR) | 6.9 (5.4-8.7) | 6.6 (5.4-6.9) | 6.9 (5.4-8.4) | ||
| Range | 3.5-9.7 | 3.7-7.8 | 3.5-9.7 |
IQR=interquartile range, SD=standard deviation
Table 6.
All parameters of cases and controls
| Age (years) | Gender | ALS type | Onset Bulbar | Onset Spinal | Duration (months) | ALSFRS-R score | RNFL Right (AVG) | RNFL Right (SUP) | RNFL Right (INF) | RNFL Right (NAS) |
|---|---|---|---|---|---|---|---|---|---|---|
| 25 | M | P | Y | 18 | 46 | 120.2 | 153 | 160 | 85 | |
| 38 | F | P | Y | 10 | 43 | 117 | 150 | 154 | 87 | |
| 46 | M | P | Y | 12 | 48 | 122.7 | 154 | 167 | 84 | |
| 55 | M | P | Y | 26 | 47 | 127.75 | 148 | 176 | 92 | |
| 54 | M | D | Y | 47 | 33 | 103.2 | 129 | 149 | 66 | |
| 59 | M | D | Y | 48 | 35 | 114 | 138 | 141 | 87 | |
| 64 | M | D | Y | 23 | 32 | 107.5 | 121 | 148 | 81 | |
| 53 | M | D | Y | 59 | 25 | 104.25 | 129 | 149 | 71 | |
| 43 | F | D | Y | 48 | 30 | 108.7 | 122 | 147 | 82 | |
| 37 | M | P | Y | 26 | 42 | 115.7 | 138 | 158 | 84 | |
| 71 | M | P | Y | 26 | 40 | 118.25 | 149 | 167 | 79 | |
| 49 | M | P | Y | 54 | 30 | 106.75 | 120 | 142 | 85 | |
| 48 | M | P | Y | 28 | 42 | 114.75 | 145 | 159 | 74 | |
| 51 | M | P | Y | 18 | 44 | 119.2 | 154 | 164 | 80 | |
| 55 | M | D | Y | 32 | 29 | 112.25 | 139 | 147 | 81 | |
| 39 | M | D | Y | 58 | 26 | 105.75 | 127 | 148 | 78 | |
| 36 | M | P | Y | 20 | 45 | 120.2 | 148 | 165 | 85 | |
| 57 | F | D | Y | 16 | 33 | 114.5 | 134 | 164 | 81 | |
| 70 | M | P | Y | 26 | 23 | 100.2 | 116 | 126 | 78 | |
| 54 | M | D | Y | 24 | 29 | 108.5 | 127 | 147 | 77 | |
| 67 | M | D | Y | 28 | 33 | 115.75 | 146 | 157 | 83 | |
| 47 | F | D | Y | 20 | 37 | 111.2 | 147 | 166 | 67 | |
| 34 | M | P | Y | 24 | 44 | 118.5 | 139 | 158 | 87 | |
| 49 | M | D | 46 | 29 | 107.5 | 127 | 145 | 80 | ||
| 58 | M | P | Y | 48 | 30 | 110.75 | 129 | 154 | 82 | |
| 27 | F | P | Y | 20 | 40 | 116.2 | 148 | 150 | 79 | |
| 34 | M | P | 12 | 48 | 118.75 | 145 | 165 | 85 | ||
| 57 | M | D | Y | 36 | 30 | 114.75 | 139 | 159 | 80 | |
| 69 | M | D | Y | 21 | 25 | 111.75 | 139 | 151 | 76 | |
| 51 | M | P | Y | 15 | 44 | 118.25 | 145 | 154 | 89 | |
|
| ||||||||||
| Healthy controls | ||||||||||
|
| ||||||||||
| 45 | M | 121 | 152 | 166 | 77 | |||||
| 51 | M | 120.25 | 151 | 163 | 81 | |||||
| 34 | F | 119.25 | 150 | 161 | 78 | |||||
| 49 | F | 117.75 | 148 | 160 | 83 | |||||
| 55 | M | 118.5 | 150 | 154 | 81 | |||||
| 48 | M | 118.5 | 149 | 158 | 79 | |||||
| 47 | F | 121.25 | 159 | 166 | 77 | |||||
| 45 | M | 118.25 | 151 | 153 | 82 | |||||
| 52 | M | 121.75 | 168 | 160 | 76 | |||||
| 56 | M | 121 | 159 | 161 | 81 | |||||
|
| ||||||||||
| RNFL Right (TEM) | RNFL Left (AVG) | RNFL Left (SUP) | RNFL Left (INF) | RNFL Left (NAS) | RNFL Left (TEM) | VEP | MRI findings | |||
|
| ||||||||||
| P100 (right) | P100 (left) | Amplitude (right) | Amplitude (left) | |||||||
|
| ||||||||||
| 83 | 119.25 | 152 | 158 | 79 | 88 | 89 | 88 | 5.6 | 7.9 | 1 |
| 77 | 118 | 159 | 154 | 72 | 87 | 92 | 96 | 6.4 | 9.2 | 1 |
| 86 | 121.5 | 152 | 167 | 80 | 87 | 93 | 94 | 3.8 | 6.9 | 1 |
| 95 | 123.4 | 131 | 175 | 93 | 95 | 93 | 92 | 2.2 | 3.5 | 5 |
| 69 | 97 | 112 | 143 | 61 | 72 | 88 | 86 | 1.6 | 7.2 | 2 |
| 90 | 104 | 119 | 137 | 76 | 84 | 84 | 85 | 6.6 | 8.8 | 4 |
| 80 | 110 | 137 | 143 | 81 | 79 | 103 | 106 | 6.5 | 9.2 | 4 |
| 68 | 101.75 | 119 | 147 | 66 | 75 | 105 | 107 | 9.5 | 9.5 | 3 |
| 84 | 107.25 | 122 | 146 | 80 | 81 | 100 | 102 | 6.2 | 8.4 | 6 |
| 83 | 112.75 | 136 | 152 | 81 | 82 | 89 | 88 | 2.7 | 9.5 | 1 |
| 78 | 118.5 | 149 | 166 | 79 | 80 | 101 | 103 | 7.8 | 8.6 | 1 |
| 80 | 103.5 | 117 | 140 | 78 | 79 | 110 | 112 | 8.4 | 9.7 | 4 |
| 81 | 112.5 | 141 | 156 | 70 | 83 | 101 | 102 | 9 | 7.5 | 1 |
| 79 | 117.7 | 153 | 164 | 79 | 75 | 87 | 86 | 9.1 | 5.7 | 1 |
| 82 | 109.5 | 129 | 145 | 80 | 84 | 92 | 94 | 6.8 | 6.9 | 1 |
| 70 | 99.25 | 116 | 140 | 74 | 67 | 116 | 116 | 3.2 | 5.2 | 3 |
| 83 | 120 | 149 | 167 | 84 | 80 | 105 | 106 | 1.4 | 4.6 | 1 |
| 79 | 112.5 | 133 | 163 | 76 | 78 | 87 | 88 | 7.6 | 5.7 | 5 |
| 81 | 98.75 | 119 | 123 | 77 | 82 | 89 | 87 | 7.9 | 4.7 | 4, 5 |
| 83 | 105 | 129 | 143 | 73 | 75 | 86 | 83 | 7.7 | 8.7 | 1 |
| 77 | 111.25 | 145 | 150 | 71 | 79 | 90 | 91 | 5.5 | 4.5 | 1 |
| 65 | 109.5 | 142 | 167 | 60 | 69 | 83 | 81 | 6.8 | 9.2 | 1 |
| 90 | 117.25 | 149 | 157 | 74 | 89 | 88 | 87 | 6.4 | 6.5 | 1 |
| 78 | 104.75 | 126 | 141 | 74 | 78 | 89 | 91 | 4.5 | 5.4 | 1 |
| 78 | 109.25 | 124 | 147 | 88 | 78 | 98 | 97 | 6.6 | 6.7 | 1 |
| 88 | 115 | 145 | 149 | 79 | 87 | 101 | 100 | 7.2 | 6.2 | 1 |
| 80 | 120.75 | 149 | 157 | 86 | 91 | 103 | 102 | 7.8 | 5.7 | 1 |
| 81 | 111.25 | 134 | 151 | 78 | 82 | 99 | 98 | 9.8 | 4.1 | 2 |
| 81 | 106.5 | 128 | 149 | 70 | 79 | 96 | 94 | 9.3 | 4.5 | 3 |
| 85 | 117.75 | 147 | 158 | 80 | 86 | 87 | 88 | 8.1 | 8.3 | 1 |
|
| ||||||||||
| Healthy controls | ||||||||||
|
| ||||||||||
| 87 | 121.5 | 153 | 168 | 78 | 87 | 92 | 99 | 7.2 | 7.7 | |
| 86 | 118.75 | 145 | 163 | 79 | 88 | 94 | 87 | 3.5 | 3.7 | |
| 88 | 120 | 151 | 158 | 82 | 89 | 90 | 86 | 6.2 | 6.9 | |
| 80 | 119.25 | 154 | 159 | 79 | 85 | 100 | 99 | 7.1 | 7.8 | |
| 89 | 119.75 | 151 | 164 | 81 | 83 | 101 | 100 | 6.3 | 6.9 | |
| 88 | 116.5 | 149 | 159 | 76 | 82 | 92 | 93 | 5.3 | 5.4 | |
| 83 | 118.25 | 158 | 163 | 75 | 77 | 89 | 101 | 4.7 | 4.8 | |
| 87 | 119.25 | 149 | 161 | 80 | 87 | 87 | 99 | 6.8 | 6.2 | |
| 83 | 119.25 | 159 | 161 | 81 | 76 | 91 | 96 | 6.6 | 6.9 | |
| 83 | 119 | 155 | 157 | 81 | 83 | 94 | 95 | 5.6 | 5.9 | |
ALS=amyotrophic lateral sclerosis, ALSFRS-R=revised amyotrophic lateral sclerosis functional rating scale, AVG=average, D=definite ALS, INF=inferior quadrant, MRI=magnetic resonance imaging, NAS=nasal quadrant, P=probable ALS, RNFL=retinal nerve fiber layer, SUP=superior quadrant, TEM= temporal quadrant, Y=yes. MRI findings (1=normal, 2=frontal lobe atrophy, 3=hyperintensity in the posterior limb of the internal capsule, 4=generalized cerebral atrophy, 5=periventricular white matter hyperintensity, 6=mild temporal lobe atrophy)
In our study, though we found some changes in conventional MRI brain, like periventricular hyperintensity, generalized or focal atrophy, and hyperintensity in the posterior limb of the internal capsule, they are not specific to the disease. Most of the patients had normal brain MRI.
Discussion
The dominant feature of ALS, as to our present understanding, is still motor neuron degeneration, but there is more and more evidence that hints at a more widely spread pathological process, which includes decaying of nonmotor systems.[2,4,5] Given the common embryological evolution of cells of the retina and neurons of the central nervous system (CNS), it is not far-fetched to hypothesize that neurodegenerative conditions that affect the brain and spinal cord in many different ways may, at least in some situations, also affect the neurons in the retina. No clear pathophysiological mechanism between neurodegeneration in the CNS and retinal degeneration has been confirmed as of now, but several theories are currently considered to be plausible. Among them, anterograde degeneration caused by ganglion cell death in the retina and retrograde degeneration caused by neurodegeneration in the cerebral cortex seem to be two major credible processes that occur in this disease group.[2] Although visual acuity seems not to be affected in ALS, there is a link between ophthalmic involvement and neuronal degeneration in the motor cortex.[6,7]
Current biomarkers of motor neuron disease (MND) each have limitations. Markers such as forced vital capacity (FVC%) are challenging in patients with bulbar and cognitive dysfunction.[8] Functional rating scales depend upon assessors’ interpretation,[9] cerebrospinal fluid (CSF) and serum biomarkers’ utility in longitudinal assessment remains uncertain, and neuroradiological techniques like conventional MRI are used mostly to exclude ALS mimics, with limited sensitivity in diagnosing ALS.[10–12]
Multiple studies have shown that MNDs are misdiagnosed in 26%–42% cases. Even if an MND is suspected, it is difficult for any clinician to tell a patient with progressive weakness or disability or both that their diagnosis is only probable or possible. Furthermore, with recent US Food and Drug Administration (US FDA) approval of intravenous (IV) medication edaravone for early treatment of ALS, the validation of diagnostic criteria has become more important.[13,14]
Identification of patients at early disease stages is a critical area of research and an unmet need not only in ALS, but also in other neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease. These pathologies are characterized by slow development with an asymptomatic or pauci-symptomatic period, followed by gradual emergence of typical symptoms. As a consequence, these conditions are diagnosed when marked neuronal death has already occurred and therapeutic interventions are less likely to be effective. Importantly, patients diagnosed in later stages may have missed the window to enroll in clinical trials, many of which require an early ALS diagnosis as an entry criterion. Diagnostic delays also have a negative impact on research efforts revolving around the identification of biomarkers and mechanisms of early disease.
Few studies, especially those using the OCT to determine visual anomalies, relate ALS with the visual system.[7,15–17] Our primary goal is to provide evidence regarding retinal changes in ALS patients and identify a possible correlation between RNFL thickness in different spectra of ALS patients and its association with disease duration and severity. The unequal degree of damage in right and left eyes indicates that asymmetric CNS damage in ALS patients is not limited to the motor system. An important finding of this study was asymmetry of the contralateral retinal lesion. The unequal degree of eye involvement indicates that asymmetry is not limited to motor system only. However, Rohani et al.[12] found some evidence of asymmetric RNFL involvement. The asymmetric CNS involvement was indicated by the asymmetricity in the thickness of the RNFL in the right and left eyes in ALS patients. In other words, our study provides proof that this laterality is not confined to the motor system. Thinning of the RNFL is either due to death of the ganglion cells or Wallerian degeneration secondary to death of the cortical neurons.[13,18–20]
The study found statistically significant differences in RNFL thicknesses between ALS patients and healthy controls and a considerable asymmetricity between the right and left sides. Several other studies like Ringelstein et al.[15] and Hübers et al.[16] did not consider laterality. However, Rohani et al.[12] found some evidence of asymmetric RNFL involvement. Although several studies[18] have shown asymmetric CNS involvement in ALS, our study is the first in India to assess ALS’s asymmetricity by evaluating the thickness of the RNFL in the right and left eyes. In other words, our study proves that this laterality is not confined to the motor system.
Roth et al.[17] reported that OCT-based measures of RNFL do not differ between patients and controls, probably due to a lower number of definite ALS patients in their research, unlike ours. The result of our study is supported by Ringelstein et al.[15] and Rohani et al.[12] Volpe et al.[7] and Hübers et al.[16] also published their results, agreeing with our findings. Though Ringelstein et al.[15] described RNFL thinning in ALS compared to healthy controls,[21] in their study, no detailed ophthalmic evaluation or exclusion for retinal diseases by history was reported. Later, in 2016, Hübers et al.[16] reported statistically significant RNFL thinning in ALS patients compared to healthy controls.
Previous studies by Hübers et al.[16] and Volpe et al.[7] did not analyze segmental RNFL thickness. Roth et al.[17] examined a larger group of 76 MND patients and concluded that OCT does not support the involvement of optic nerve or retina in MND. However, the participants reviewed by Roth et al.[17] did not undergo a complete ophthalmic examination; patients were excluded based on clinical records and interviews with the examiner. This might be a reason for the lack of positive findings. But Ringelstein et al.[15] found a small but statistically significant reduction in RNFL thickness in a smaller (24 patients) MND group, suggesting optic nerve axonal degeneration. Rohani et al.[12] found RNFL thickness to be substantially involved in the superior and nasal quadrants of the left eyes; there was nonsignificant involvement in the superior nasal quadrant. However, Mukherjee et al.[21] analyzed RNFL thickness in six sectors and found significant thinning in bilateral mean total RNFL, including temporal, right superonasal quadrants, and left superior and temporal quadrants.
Pairwise comparison showed a significant difference in mean bilateral average RNFL between mild versus moderate and mild versus severe ALS according to the ALSFRS-R score. The ALSFRS-R score demonstrated a positive correlation with bilateral average RNFL thickness and RNFL thickness in the superior and inferior quadrants and a moderately positive correlation with RNFL thickness in the nasal and temporal quadrants on both sides. Although several studies have demonstrated a decline in RNFL thickness of ALS patients without ocular pathology,[12,21,22] there is controversy about its correlation with the ALSFRS-R values.[12,21] For example, a study by Mukherjee et al.[21] in 2017 did not demonstrate any correlation between RNFL thickness and the ALSFRS-R score due to the inclusion of a maximum number of mild cases. However, our study included patients of varying severity, with the ALSFRS-R scores ranging from 23 to 48, and found a significant association between RNFL thickness and disease severity. Hence, RNFL thickness can be utilized as an imaging biomarker for disease progression. We also found a positive correlation with different segments of both eyes in superior, inferior, and nasal quadrants. But Rojas et al.[22] did not find any correlation in the inferior quadrant. In addition, we noted a significant reduction in average RNFL thickness in the long-duration group. Several studies correlated RNFL thickness with disease duration, but only a few studies did subgroup analysis between RNFL and time. For example, Zhang et al.[18] in 2022 did not find statistical differences in RNFL thickness with disease duration-grouped patients.
In our study, almost half of the cases (53.3%) had an onset more than 24 months back. None had onset below 6 months and the rest, 46.7%, had an onset of 6–24 months. Mean duration was 29.6 ± 14.5 months, ranging from 10 to 59 months with a median of 26 months.
Explanation for this wide variation in duration from onset is being a chronic neurodegenerative disease,[23] it follows a slow course with a median survival time of 24–48 months, albeit with a wide variation,[24] and it depends upon several factors like body mass index (BMI), symptom onset site, FVC%, age, sex, levels of neurofilaments (NFs) in blood, presence of frontotemporal dementia, and the level of daily functions at the time of diagnosis.[11,25] Also, delay in diagnosis is another explanation for such a wide variation in duration and its cause is multifactorial. First, the time from symptom onset to the first doctor visit is long, which is either due to delay in seeking medical attention because of slow and insidious course of the disease or a possibility that patients may wait until symptoms become more noticeable. Secondly, ALS presenting symptoms can mimic a variety of other conditions.[26]
Few studies have analyzed the VEPs in ALS patients. In our study, 30 patients were analyzed of whom only two had VEP abnormalities (increased latency time); however, these alterations were mild. In other studies, wave latency and amplitudes were within normal limits in all ALS patients.[27] On comparing VEP between cases and controls, we did not find any statistically significant difference.
In our study, though we found some changes in conventional MRI brain, like periventricular hyperintensity, generalized or focal atrophy, and hyperintensity in the posterior limb of the internal capsule, they are not specific to the disease. Most of the patients had normal brain MRI. In patients with ALS, conventional MRI is used principally to exclude other pathologies, such as cerebral lesions, skull base lesions, cervical spondylotic myelopathy, other myelopathies, conus lesions, and thoracolumbar/sacral radiculopathy.[28–30] Once diseases that mimic ALS are ruled out, subtle neuroimaging signs that are supportive of ALS are looked into, like hyperintensity in the corticospinal tract (CST), hypointensity in the motor cortex, and brain atrophy, which are indicative but nonspecific to disease. Though we found some MRI changes in patients with old age, long duration of illness, and low ALSFRS score, they are not specific to ALS. How ever, several studies which had been done previously showed varied results and did not report correlation with clinical score.[29,31]
Conclusion
The study provides an in-depth analysis of the retinal involvement in ALS patients, like other neurodegenerative diseases. The OCT showed a significant decrease in RNFL thickness in ALS patients compared to controls, which decreased significantly according to disease severity and duration of illness. So, the retinal changes can serve as a marker for diagnosing and monitoring patients with ALS and checking the treatment efficacy through a simple, noninvasive, and cheaper technology.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
- 1.Soldatov V, Kukharsky M, Belykh A, Sobolev AM, Deykin A. Retinal damage in amyotrophic lateral sclerosis:Underlying mechanisms. Eye Brain. 2021;13:131–46. doi: 10.2147/EB.S299423. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Mancino R, Cesareo M, Martucci A, Di Carlo E, Ciuffoletti E, Giannini C, et al. Neurodegenerative process linking the eye and the brain. Curr Med Chem. 2019;26:3754–63. doi: 10.2174/0929867325666180307114332. [DOI] [PubMed] [Google Scholar]
- 3.Ludolph A, Drory V, Hardiman O, Nakano I, Ravits J, Robberecht W, et al. A revision of the El Escorial criteria- 2015. Amyotroph Lateral Scler Front Degener. 2015;16:291–2. doi: 10.3109/21678421.2015.1049183. [DOI] [PubMed] [Google Scholar]
- 4.London A, Benhar I, Schwartz M. The retina as a window to the brain—from eye research to CNS disorders. Nat Rev Neurol. 2013;9:44–53. doi: 10.1038/nrneurol.2012.227. [DOI] [PubMed] [Google Scholar]
- 5.Braak H, Brettschneider J, Ludolph AC, Lee VM, Trojanowski JQ, Del Tredici K. Amyotrophic lateral sclerosis--A model of corticofugal axonal spread. Nat Rev Neurol. 2013;9:708–14. doi: 10.1038/nrneurol.2013.221. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Moss HE, Samelson M, Mohan G, Jiang QL. High and low contrast visual acuity are not affected in amyotrophic lateral sclerosis. PLoS One. 2016;11:e0168714. doi: 10.1371/journal.pone.0168714. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Volpe NJ, Simonett J, Fawzi AA, Siddique T. Ophthalmic manifestations of amyotrophic lateral sclerosis (An American Ophthalmological Society Thesis) Trans Am Ophthalmol Soc. 2015;113:T12. [PMC free article] [PubMed] [Google Scholar]
- 8.Czaplinski A, Yen AA, Appel SH. Forced vital capacity (FVC) as an indicator of survival and disease progression in an ALS clinic population. J Neurol Neurosurg Psychiatry. 2006;77:390–2. doi: 10.1136/jnnp.2005.072660. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Cedarbaum JM, Stambler N, Malta E, Fuller C, Hilt D, Thurmond B, et al. The ALSFRS-R:A revised ALS functional rating scale that incorporates assessments of respiratory function. BDNF ALS Study Group (Phase III) J Neurol Sci. 1999;169:13–21. doi: 10.1016/s0022-510x(99)00210-5. [DOI] [PubMed] [Google Scholar]
- 10.Zucchi E, Bonetto V, Sorarù G, Martinelli I, Parchi P, Liguori R, et al. Neurofilaments in motor neuron disorders:Towards promising diagnostic and prognostic biomarkers. Mol Neurodegener. 2020;15:58. doi: 10.1186/s13024-020-00406-3. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Poesen K, Van Damme P. Diagnostic and prognostic performance of neurofilaments in ALS. Front Neurol. 2018;9:1167. doi: 10.3389/fneur.2018.01167. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Rohani M, Meysamie A, Zamani B, Sowlat MM, Akhoundi FH. Reduced retinal nerve fiber layer (RNFL) thickness in ALS patients:A window to disease progression. J Neurol. 2018;265:1557–62. doi: 10.1007/s00415-018-8863-2. [DOI] [PubMed] [Google Scholar]
- 13.Devine MS, Kiernan MC, Heggie S, McCombe PA, Henderson RD. Study of motor asymmetry in ALS indicates an effect of limb dominance on onset and spread of weakness, and an important role for upper motor neurons. Amyotroph Lateral Scler Front Degener. 2014;15:481–7. doi: 10.3109/21678421.2014.906617. [DOI] [PubMed] [Google Scholar]
- 14.Nzwalo H, de Abreu D, Swash M, Pinto S, de Carvalho M. Delayed diagnosis in ALS:The problem continues. J Neurol Sci. 2014;343:173–5. doi: 10.1016/j.jns.2014.06.003. [DOI] [PubMed] [Google Scholar]
- 15.Ringelstein M, Albrecht P, Südmeyer M, Harmel J, Müller A-K, Keser N, et al. Subtle retinal pathology in amyotrophic lateral sclerosis. Ann Clin Transl Neurol. 2014;1:290–7. doi: 10.1002/acn3.46. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Hübers A, Müller HP, Dreyhaupt J, Böhm K, Lauda F, Tumani H, et al. Retinal involvement in amyotrophic lateral sclerosis:A study with optical coherence tomography and diffusion tensor imaging. J Neural Transm Vienna Austria 1996. 2016;123:281–7. doi: 10.1007/s00702-015-1483-4. [DOI] [PubMed] [Google Scholar]
- 17.Roth NM, Saidha S, Zimmermann H, Brandt AU, Oberwahrenbrock T, Maragakis NJ, et al. Optical coherence tomography does not support optic nerve involvement in amyotrophic lateral sclerosis. Eur J Neurol. 2013;20:1170–6. doi: 10.1111/ene.12146. [DOI] [PubMed] [Google Scholar]
- 18.Zhang Y, Liu X, Fu J, Zhang Y, Yang X, Zhang S, et al. Selective and Inverse U-shaped curve alteration of the retinal nerve in amyotrophic lateral sclerosis:A potential mirror of the disease. Front Aging Neurosci. 2021;13:783431. doi: 10.3389/fnagi.2021.783431. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Fiori F, Sedda A, Ferrè ER, Toraldo A, Querzola M, Pasotti F, et al. Exploring motor and visual imagery in amyotrophic lateral sclerosis. Exp Brain Res. 2013;226:537–47. doi: 10.1007/s00221-013-3465-9. [DOI] [PubMed] [Google Scholar]
- 20.d'Ambrosio A, Gallo A, Trojsi F, Corbo D, Esposito F, Cirillo M, et al. Frontotemporal cortical thinning in amyotrophic lateral sclerosis. AJNR Am J Neuroradiol. 2014;35:304–10. doi: 10.3174/ajnr.A3753. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Mukherjee N, McBurney-Lin S, Kuo A, Bedlack R, Tseng H. Retinal thinning in amyotrophic lateral sclerosis patients without ophthalmic disease. PLoS One. 2017;12:e0185242. doi: 10.1371/journal.pone.0185242. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Rojas P, de Hoz R, Ramírez AI, Ferreras A, Salobrar-Garcia E, Muñoz-Blanco JL, et al. Changes in retinal OCT and their correlations with neurological disability in early ALS patients, a follow-up study. Brain Sci. 2019;9:337. doi: 10.3390/brainsci9120337. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Phukan J, Pender NP, Hardiman O. Cognitive impairment in amyotrophic lateral sclerosis. Lancet Neurol. 2007;6:994–1003. doi: 10.1016/S1474-4422(07)70265-X. [DOI] [PubMed] [Google Scholar]
- 24.Chiò A, Logroscino G, Traynor BJ, Collins J, Simeone JC, Goldstein LA, et al. Global epidemiology of amyotrophic lateral sclerosis:A systematic review of the published literature. Neuroepidemiology. 2013;41:118–30. doi: 10.1159/000351153. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25.Chiò A, Logroscino G, Hardiman O, Swingler R, Mitchell D, Beghi E, et al. Prognostic factors in ALS:A critical review. Amyotroph Lateral Scler. 2009;10:310–23. doi: 10.3109/17482960802566824. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26.Brooks BR. El Escorial World Federation of Neurology criteria for the diagnosis of amyotrophic lateral sclerosis. Subcommittee on Motor Neuron Diseases/Amyotrophic Lateral Sclerosis of the World Federation of Neurology Research Group on Neuromuscular Diseases and the El Escorial “Clinical limits of amyotrophic lateral sclerosis”workshop contributors. J Neurol Sci. 1994;124((Suppl)):96–107. doi: 10.1016/0022-510x(94)90191-0. [DOI] [PubMed] [Google Scholar]
- 27.Ghezzi A, Mazzalovo E, Locatelli C, Zibetti A, Zaffaroni M, Montanini R. Multimodality evoked potentials in amyotrophic lateral sclerosis. Acta Neurol Scand. 1989;79:353–6. doi: 10.1111/j.1600-0404.1989.tb03799.x. [DOI] [PubMed] [Google Scholar]
- 28.Chan S, Kaufmann P, Shungu DC, Mitsumoto H. Amyotrophic lateral sclerosis and primary lateral sclerosis:Evidence-based diagnostic evaluation of the upper motor neuron. Neuroimaging Clin N Am. 2003;13:307–26. doi: 10.1016/s1052-5149(03)00018-2. [DOI] [PubMed] [Google Scholar]
- 29.Agosta F, Chiò A, Cosottini M, Stefano ND, Falini A, Mascalchi M, et al. The present and the future of neuroimaging in amyotrophic lateral sclerosis. Am J Neuroradiol. 2010;31:1769–77. doi: 10.3174/ajnr.A2043. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30.Wang S, Melhem ER. Amyotrophic lateral sclerosis and primary lateral sclerosis:The role of diffusion tensor imaging and other advanced MR-based techniques as objective upper motor neuron markers. Ann N Y Acad Sci. 2005;1064:61–77. doi: 10.1196/annals.1340.013. [DOI] [PubMed] [Google Scholar]
- 31.Hecht MJ, Fellner F, Fellner C, Hilz MJ, Neundörfer B, Heuss D. Hyperintense and hypointense MRI signals of the precentral gyrus and corticospinal tract in ALS:A follow-up examination including FLAIR images. J Neurol Sci. 2002;199:59–65. doi: 10.1016/s0022-510x(02)00104-1. [DOI] [PubMed] [Google Scholar]