Introduction
Diabetic retinopathy is a chronic progressive disorder predominantly affecting the microscopic vessels of the retina.1 The neuroretinal ganglion cell involvement occur before microvascular changes starts, which leads to thinning of GC-IPL and detected on OCT. Thinning of GC-IPL is an early sign of diabetic neurodegeneration and requires careful clinical evaluation.2 Retinal neurodegeneration is described as a consequence of neural apoptosis, reactive gliosis, glutamate excitotoxicity, decrease in neuroprotective factors and impairment of the neurovascular coupling.3
Diabetic peripheral neuropathy (DPN) cannot be diagnosed alone by clinical tests and it is difficult to detect mild neuropathy in early DPN patients. For diagnosis, a combination of clinical symptoms, physical examination and electrodiagnostic findings are used. Electrodiagnostic tests include a NCS, which is very informative and specific. For assessing peripheral neuropathy, nerve conduction study test of peroneal motor nerve, posterior tibial motor nerve (conduction velocity, terminal latency) are used.2
DR is one of the three major microvascular complications of diabetes along with neuropathy and nephropathy. These are caused by microvascular disturbances and arise sequentially: First neuropathy occurs, followed by retinopathy and nephropathy respectively. Diabetic peripheral neuropathy has been observed in patients with impaired glucose tolerance test.4
Moreover, neuronal damage in the distal segment of the peripheral nerves is associated with objective measurements of neuropathy, such as nerve conduction velocity (NCV). The normal range of terminal latency is 3.2 to 4.5 ms in both the peroneal motor and posterior tibial motor nerve.2 The normal ranges of conduction velocity in the peroneal motor, the posterior tibial motor, and the sural sensory nerve are 43–62 m/s, 41–61 m/s and 34–49 m/s, respectively. The normal range of F-wave latency is 44.7 ± 4.7 ms in the peroneal motor and 44.9 ± 6.2 ms in the posterior tibial motor nerve. In diabetics, retinal neuroglial dysfunction occurs before ophthalmoscopically visible signs of retinopathy (i.e., vascular changes) are seen. This raises the possibility that NCV may be a biomarker of diabetes progression, including ocular complications, in the earlier stages.4
Aim of this study was to show the association between retinal neuro-degeneration with peripheral nerve conduction in diabetic (type-II) patients. In this study, we study the association between various parameters, including NCV, ophthalmoscope findings and measurement of retinal structure, in type 2 diabetics with early DR, in order to search for novel biomarkers of DR.
Material and Methods
This study was carried out during December 2018 to November 2019. Patients were enrolled at M.D. eye Hospital, Regional Institute of Ophthalmology Prayagraj. The study was conducted in accordance with the ethical standards which were approved by institutional ethics review board with informed consent obtained from all participants.
It is a cross sectional study. A total number of 30 diabetic patients and 30 non diabetic controls were enrolled consecutively. The patients were evaluated of for mean retinal thickness (μm) of the parafoveal area within diameters 1-3 mm, GC- IPL thickness and nerve conduction study of DPN, CPN was analysed.
The exclusion criteria
Duration of diabetes >10 years.
Diagnosis of any peripheral neurologic disease except diabetes- related neuropathy, medication for peripheral neuropathy prior to the nerve conduction test.
Cardiovascular disease.
Clinically significant macular edema.
Previous diagnosis of glaucoma.
Ocular hypertension, uveitis other retinal disease, any history of retinal treatment.
Diagnosed case of moderate/severe or PDR.
Additionally, 35 -75 age matched patients were taken as control group without diabetes for comparison of ganglion cell –inner plexiform layer thickness.
Investigations like routine blood Investigation, urine examination, Fasting plasma glucose and 2hr post prandial glucose, lipid profile and HbA1cwas advised and undertaken. Visual acuity was done by Snellen’s chart. A detailed fundus examination was done by indirect ophthalmoscopy. NPDR and PDR were diagnosed by the presence of fundus findings.
Spectral-Domain -OCT imaging was performed with the cirrus HD-OCT (Software version 6.5.0.772; Carl Zeiss Meditec; Inc.) We analyzed the mean retinal thickness (µm) of the parafoveal area within diameters 1–3 mm and the GC-IPL thickness in 6 macular regions was analyzed using Cirrus HD-OCT (Carl Zeiss Meditec). A color-coded macular thickness map, i.e., the macular cube (512 × 128 scan) was obtained with Cirrus HDOCT. The diameters of the circular areas were 1, 3 and 6 mm. The built-in algorithms of the Cirrus HD-OCT software (version 6.5.0.772) are capable of automatically identifying the outer boundary of the macular RNFL and the outer boundary of the IPL. The difference between the RNFL and the IPL outer boundary segmentation yields the GC-IPL thickness.
The average, minimum, and 6-sectoral (superior temporal, superior, superior nasal, inferior nasal, inferior, and inferior temporal) GC-IPL thicknesses are measured in an elliptical annulus with a vertical outer diameter of 4.0 mm and horizontal diameter of 4.8 mm. In this study, the average CC-IPL thickness of 6 sectors was used for statistical analysis. Nerve conduction studies are used mainly for evaluation of paraesthesia (numbness, tingling, burning) and/or weakness of the arms and legs. The nerve conduction study consists of the following components
Motor NCS are performed by electrical stimulation of a peripheral nerve and recording from a muscle supplied by this nerve. The time it takes for the electrical impulse to travel from the stimulation to the recording site is measured. This value is called the latency and is measured in milliseconds (ms). The size of the response – called the amplitude – is also measured. Motor amplitudes are measured in millivolts. By stimulating in two or more different locations along the same nerve, the NCV across different segments can be determined. Calculations are performed using the distance between the different stimulating electrodes and the difference in latencies. In general, different pathological processes result in changes in latencies, motor, and/or sensory amplitudes, or slowing of the conduction velocities to differing degrees.
Results
A cross sectional control study with 30 cases (60 eyes) and 30 controls (60 eyes) was done to study the GC-IPL thickness imaging by Cirrus HD OCT in diabetic patients, neuropathy by nerve conduction study test.
Relationships are considered significant if p value ˂0.05. The intergroup differences are analysed by the chi-square test statistics. The differences between groups are defined by z-test.
Table 1
Distribution of patients according to sex group
| Gender | Number | Percentage |
| Male | 17 | 56.66% |
| Female | 13 | 43.33% |
Table 2
Distribution of patients in relation to retinopathy and neuropathy
| Diabetic patient | Number | Percentage |
| Without retinopathy | 17 | 56.66% |
| With retinopathy | 13 | 43.33% |
| With no neuropathy | 9 | 30% |
| With neuropathy | 21 | 70% |
Table 3
Association of abnormal Ganglion cell-inner plexiform layer thickness and diabetes with neuropathy among cases
Table 4
Comparison of cases and controls with abnormal GC-IPL thickness
Table 5
Association of abnormal G.C. thickness and with neuropathy among controls
Table 6
Association of average Ganglion cell -inner plexiform thickness to peripheral neuropathy in diabetic patients
6 shows difference of mean “ganglion cell –inner Plexiform thickness” in diabetic patients with neuropathy (Group-A) and without neuropathy (Group-B) in both, left and right eyes. In right eye, mean Ganglion cell thickness was 67.44±8.64SD in Group A which was comparatively lower i.e.82.2±8.64SD than Group B patients. This difference in mean was found to be statistically significant at 5% level of significance and 95% confidence interval (t=3.28, p=0.0027).
Similarly, in left eye mean “ganglion cell inner plexiform thickness” was 69.71±11.56 SD in Group A and 81.78±SD in Group B patients. The difference between means thickness of these two groups was found two be significant at same level of significance and confidence interval (t=4.61, p=0.014).
Both the significant difference indicate that diabetic patients with neuropathy (Group A), were associated with lower mean of “ganglion cell–inner Plexiform thickness” in both the eyes ascompared to without neuropathy patients (Group B).
Discussion
50% diabetic patients develop diabetic polyneuropathy (DPN) but only about 20% develop clinical features of neuropathy at the time of diabetes diagnosis, since it is often under diagnosed owing to adverse and vague symptoms. OCT finding of thinner ganglion cell-inner plexiform layer can be an early signs of diabetic neurodegeneration and requires careful clinical evaluation regarding diabetic neuropathy.5
Total 30 diabetic patients were studied between 35-75 years age group. Maximum 33% patients came in 35-45 years age group. The mean age in our study is 51.59±11 years, which is slightly less that quoted by Jin A Choi et al.6 to be 55.7 years. Eduardo Buchele Rodrigues et al.7, 3 studied that diabetic induced changes in neuroretina before retina vessels on 102 patients and mean age was found to be 58.67 (±10.7) which is slightly higherthanour study. Jay Chhablani et al.8 found mean age 57.5 years. P. Carpineto et al.9 60.9±8.3 years. Kiyoungkim et al.5 63.61±12.52 years.
In present study we found that mean average GC-IPL thickness of diabeticin right eye is72.46±10.43, in left eye 73.36±12.86 thinner than healthy controls mean average GC-IPL thickness in right eye is82.73±5.6, in left eye 82.66±5.9. Higher proportion (56%) of cases with decreased GC-IPL thickness if found in right eyes as well as left eye (56.66%) in comparison to controls of both eyes (13.33% for both eyes) and this is found to be statistically significant (p=0.00044 and p=0.00044 respectively) at 5% level of significance and 95% confidence interval.
Jay Chhablani et al.8 conducted retrospective study on 62 subjects with diabetes. In this studyaverage GC-IPL thickness and minimum GC-IPL thickness were significantly lower in diabetics than non-diabetic healthy control group (p value<0.05). P Carpineto et al.9 in their study found that mean average GC-IPL thickness was 80±8.1µm and 85±9.9µm in diabetic patients and controls, respectively (p=0.001). Dorothy et al.10 conducted observational case-control study. In this study mean average GC-IPL thickness was 4.49 thinner (95% CI: 2.92 to 6.06) in diabetic then healthy controls.
In our study we found that in right eye mean GC-IPL (Ganglion cell-inner plexiform layer) thickness was 67.44±8.64SD with neuropathy which was comparatively lower i.e.82.2±8.64SD without neuropathy patients. This difference in mean was found to be statistically highly significant at 5% level of significance and 95% confidence interval (t=3.28, p=0.0027). Similarly, in left eye mean GC-IPL (ganglion cell inner plexiform layer) thickness was 69.71±11.56 SD in neuropathy patients and 81.78±SD in without neuropathy patients. The difference between means thickness of these two groups was found two be significant at same level of significance and confidence interval (t=4.61, p=0.014).
Kiyoung Kim, Seung-Young Yu et al.2 evaluated GC-IPL (ganglion cell inner plexiform layer) in diabetic patients with definite neuropathy 75.2±4.2 μm and diabetic patients without neuropathy 82.0±5.8 μm. There were significant differences between the no neuropathy vs neuropathy (82.0±5.8 and 75.2±4.2, p value =0.005).
Kiyoung Kim et al.5 conducted retrospective cohort study. They selected 87 patients with type II diabetes. Change in mGCIPL thickness was related to baseline mGCIPL thickness (r =-0.30, p = 0.005). Study shows those patients who exhibit diabetic retinopathy progression had lower mGC-IPL (p<0.001) thickness higher rate of change of mGC-IPL thickness (p 0.041), and lower conduction velocity of peripheral nerve (p<0.001) compared with those who exhibit no diabetic retinopathy progression
Sangeetha Srinivasan et al.11 Studied association between full retinal thickness of macula with diabetic peripheral neuropathy in 2016, and found that in subjects with diabetic peripheral neuropathy, overall (p=0.02) retinal macular thickness significantly reduced. In the macula parafoveal (p value =0.004), superior (p value =0.01), Inferior (p value=0.002) was found significantly thinner in diabetic retinopathy.
A M Shahidi et al.12 conducted a study on 105 individuals with type 2 diabetes with or without neuropathy. In this study a significant association was found between nerve fibre layer thickness and diabetic neuropathy cases (p value =0.03). In this study diabetic without neuropathy and no diabetic without neuropathy had no significant differences in nerve fibre layer thickness (p value = 0.78) also.
Conclusion
Majority of patients (33%) were in the age group of 35-45 years, 30% were of 46-55 years age group, 26.66% were in between 56-65 years and only 10% were more than 66 years. In this study population out of 30 patients, majority 56.66% were male while females were 43.33%. Among 30 patients, 56.66% were without diabetic retinopathy and 43.33% were with diabetic retinopathy. Majority of patients, 70% had neuropathy while only 30% patients were without neuropathy. In this study we found a positive correlation between decrease in Ganglion cell-inner plexiform layer (GC-IPL) thickness and decrease peripheral nerve conduction in diabetic patients. Decrease in Ganglion cell-inner plexiform layer (GC -IPL) is predictive of early retinal neurodegeneration in diabetic patients, according to our study.
Once Diabetic Retinopathy has started, its progression is predictable, leading to visual loss and blindness if undetected and/ or untreated. Thinning of GC-IPL is early sign of diabetic retinal neurodegeneration. Hence by careful clinical evaluation regarding retinopathy and peripheral nerve symptoms, it can be detected earlier in type-2 diabetic patients and more strict diabetic control is recommended in hope of delaying future retinopathy.
