Diabetic polyneuropathy (DPN) is one of the most common chronic complications of diabetes. Over 50% of diabetic patients develop DPN as their disease course progresses [
Metabolic factors including blood glucose, blood lipids, blood pressure, and body mass index (BMI) have been identified as risk factors for DPN [
The prevailing “mainstream opinions” in the medical community, therefore, reflect the belief that the neurologic impairments caused by diabetes are difficult to reverse. However, inconclusive results regarding neurological improvement of DPN may not reflect irreversibility but rather may stem from the lack of appropriately sensitive and effective evaluation methods that can be utilized to reveal the effect of glycemic control on DPN. Growing evidence supports a prominent association between corneal nerve morphology measured using corneal confocal microscopy (CCM) and DPN [
Our study sought to determine whether improved glycemic control at one-year follow-up improved corneal nerve morphology and DPN in patients with type 2 diabetes.
From September 2015 to March 2016, 60 patients (age range 30–80 years) with type 2 diabetes and HbA1c ≥ 7.0% were recruited. Type 2 diabetes was diagnosed according to World Health Organization criteria [
All patients were provided medical recommendations for antidiabetic, antihypertensive, and lipid-lowering therapy at the beginning of the study. During the next year, there were no limitations imposed on the patients regarding diabetic care. After one year had elapsed, the 32 diabetic patients who were followed up were divided into two groups (groups A and B) according to their HbA1c status relative to the control goal suggested by the Chinese Diabetes Society (CDS), which recommends an HbA1c < 7.0%. Sixteen individuals achieved an HbA1c < 7.0% and were assigned to group A, whereas group B was comprised of 16 patients who had not reached the HbA1c goal (i.e., HbA1c ≥ 7.0%) at one-year follow-up. Twelve age-matched, healthy volunteers without diabetes mellitus, prediabetes, and/or clinical or paraclinical signs or symptoms of polyneuropathy were recruited to form a control group.
Exclusion criteria were diseases affecting the central or peripheral nervous systems, malignant tumors, connective tissue diseases, acute and chronic hepatic or renal diseases, thyroid diseases, endocrinopathies, metabolic derangements, psychological conditions, diabetic foot ulcers, active oculopathy, history of ocular operation, glaucoma, acute and chronic corneal diseases, and an extended history of corneal contact lens use. Both diabetic and control participants were comprehensively examined before recruitment into the study to ascertain health status and ensure that no exclusion criteria were met.
The study was approved by the ethics committee of Beijing Hospital, and written informed consent was obtained according to the Declaration of Helsinki.
All study participants underwent medical and neurologic assessments at baseline and at one-year follow-up (15.2 ± 1.6 months). Medical assessments included the measurement of systolic (SBP) and diastolic blood pressure (DBP), HbA1c (%), and lipid fractions (concentrations of total cholesterol (TC) (mmol/L), high- (HDLC) and low-density lipoprotein cholesterol (LDLC) (mmol/L), and triglycerides (TG) (mmol/L)).
The Toronto Clinical Scoring System (TCSS) for DPN was used [
All study subjects underwent examination with the Heidelberg retina tomograph-II in vivo corneal confocal microscope. The subjects’ eyes were anesthetized using one drop of 0.4% benoxinate hydrochloride, and Viscotears were applied to the front of the eye for lubrication. One drop of viscoelastic gel was placed on the tip of the objective lens, and a sterile disposable Perspex cap was placed over the lens allowing optical coupling of the objective lens to the cornea. The patient was instructed to fixate the eye not being examined on a target. Several scans of the entire depth of the cornea were recorded by turning the fine focus of the objective lens backwards and forwards for ~2 min using the section mode, which enables manual acquisition and storage of single images of all corneal layers. This provided en face two-dimensional images with a lateral resolution of ~2 mm/pixel and final image size of 400 × 400 pixels of the subbasal nerve plexus of the cornea from each patient and control subject.
One examiner masked from patients’ HbA1c results selected and analyzed 3 to 6 high-clarity images from the central subbasal nerve plexus. Criteria for image selection were depth, focus, position, and contrast. The examiner quantified the images with semiautomated, purpose-written, proprietary software (ACCMetrics, M. A. Dabbah, Imaging Science Biomedical Engineering, University of Manchester, Manchester, UK). Three corneal nerve parameters were quantified: (1) corneal nerve fiber density (CNFD), calculated as the total number of major nerves per square millimeter of corneal tissue (n/mm2); (2) corneal nerve branch density (CNBD), calculated as the number of branches emanating from all major nerve trunks per square millimeter of corneal tissue (n/mm2); and (3) corneal nerve fiber length (CNFL), calculated as the total length of all nerve fibers and branches within the area of the corneal tissue (mm/mm2).
SPSS 23.0 for Windows was used to compute the results. Analysis included descriptive and frequency statistics. All data are expressed as means ± standard deviation (SD). Independent-sample
Table
Clinical characteristics of control subjects and patients with diabetes at baseline and one-year follow-up.
Parameter | Baseline | One-year follow-up | |||||
---|---|---|---|---|---|---|---|
Control, Ia ( |
Diabetes, IIa ( |
Control, Ib ( |
Diabetes, IIb ( |
Ia versus IIa |
Ib versus Ia |
IIb versus IIa | |
Age, y | 54.4 ± 12.7 | 56.9 ± 14.7 | 55.3 ± 12.8 | 58.1 ± 14.6 | 0.488 | ≤0.001 | ≤0.001 |
Sex (M/F) | 6/6 | 18/14 | — | — | — | ||
Duration of diabetes, y | — | 11.2 ± 9.2 | — | 12.4 ± 9.0 | ≤0.001 | ||
Weight, kg | 63.8 ± 7.4 | 71.0 ± 16.5 | 63.4 ± 8.1 | 70.1 ± 14.5 | 0.155 | 0.692 | 0.318 |
BMI, kg/m2 | 23.15 ± 2.40 | 25.80 ± 4.78 | 22.9 ± 2.20 | 25.50 ± 4.14 | 0.075 | 0.631 | 0.328 |
SBP, mmHg | 122.3 ± 11.8 | 137.8 ± 16.3 | 122.8 ± 11.4 | 133.3 ± 14.2 | 0.005 | 0.564 | 0.175 |
DBP, mmHg | 73.5 ± 5.6 | 82.6 ± 8.0 | 74.0 ± 5.5 | 81.3 ± 10.6 | 0.001 | 0.551 | 0.396 |
TC, mmol/L | 4.88 ± 0.67 | 4.52 ± 1.07 | 4.71 ± 0.59 | 4.59 ± 1.14 | 0.407 | 0.296 | 0.902 |
TG, mmol/L | 1.25 ± 0.56 | 1.94 ± 1.82 | 1.21 ± 0.51 | 1.78 ± 1.57 | 0.177 | 0.819 | 0.483 |
LDLC, mmol/L | 2.78 ± 0.64 | 2.69 ± 0.72 | 2.67 ± 0.61 | 2.56 ± 0.83 | 0.884 | 0.457 | 0.326 |
HDLC, mmol/L | 1.51 ± 0.28 | 1.22 ± 0.33 | 1.42 ± 0.28 | 1.30 ± 0.31 | 0.008 | 0.107 | 0.058 |
HbA1c, % | 5.36 ± 0.12 | 8.22 ± 1.67 | 5.38 ± 0.11 | 7.58 ± 1.47 | ≤0.001 | 0.275 | 0.058 |
TCSS | 0.6 ± 0.7 | 5.2 ± 4.5 | 0.7 ± 0.7 | 5.4 ± 4.8 | 0.001 | 0.674 | 0.338 |
PMNCV, m/s | 50.20 ± 2.84 | 44.67 ± 4.07 | 49.34 ± 2.36 | 43.43 ± 3.80 | ≤0.001 | 0.201 | 0.496 |
TMNCV, m/s | 50.56 ± 3.15 | 44.24 ± 4.60 | 50.03 ± 2.39 | 42.46 ± 4.76 | ≤0.001 | 0.341 | 0.439 |
SPSNCV, m/s | 55.44 ± 3.99 | 48.71 ± 7.36 | 54.98 ± 3.23 | 46.76 ± 7.20 | ≤0.001 | 0.491 | 0.049 |
SSNCV, m/s | 53.69 ± 3.05 | 48.34 ± 7.18 | 52.63 ± 2.00 | 44.92 ± 6.21 | 0.017 | 0.156 | 0.146 |
CNFD, n/mm2 | 29.31 ± 4.31 | 18.71 ± 4.73 | 28.29 ± 3.38 | 19.12 ± 5.99 | ≤0.001 | 0.093 | 0.643 |
CNBD, n/mm2 | 42.19 ± 13.91 | 21.80 ± 14.67 | 39.11 ± 18.11 | 20.78 ± 12.98 | 0.003 | 0.279 | 0.667 |
CNFL, mm/mm2 | 17.96 ± 2.40 | 11.81 ± 2.46 | 17.15 ± 2.44 | 11.63 ± 2.72 | ≤0.001 | 0.191 | 0.737 |
Results are expressed as mean ± SD or counts for categorical variables.
The control group showed no significant changes between examinations at baseline and follow-up. In the diabetic cohort, a significant improvement in glycemic control was demonstrated from baseline to follow-up HbA1c. Conversely, there were no significant changes in SBP and DBP, as well as levels of TC, TG, LDLC, and HDLC (Table
Table
Clinical characteristics of patients with type 2 diabetes with improved HbA1c at one-year follow-up (group A) and consistently poor glycemic control at one-year follow-up (group B).
Parameter | Baseline | One-year follow-up | |||||
---|---|---|---|---|---|---|---|
Group A0 ( |
Group B0 ( |
Group A1 ( |
Group B1 ( |
A0 versus B0 |
A1 versus A0 |
B1 versus B0 | |
Age, y | 56.1 ± 17.5 | 59.4 ± 11. | 57.3 ± 17.4 | 60.8 ± 11.1 | 0.521 | ≤0.001 | ≤0.001 |
Sex (M/F) | 9/7 | 9/7 | — | — | — | ||
Duration of diabetes, y | 11.3 ± 11.1 | 10.4 ± 7.5 | 12.6 ± 10.9 | 11.5 ± 7.3 | 0.780 | ≤0.001 | ≤0.001 |
Weight, kg | 72.0 ± 19.0 | 69.8 ± 15.7 | 70.3 ± 16.2 | 68.9 ± 14.1 | 0.233 | 0.201 | 0.940 |
BMI, kg/m2 | 27.36 ± 5.49 | 24.12 ± 7.69 | 26.73 ± 4.50 | 24.10 ± 3.25 | 0.063 | 0.207 | 0.909 |
SBP, mmHg | 135.6 ± 15.4 | 137.4 ± 17.2 | 134.9 ± 13.5 | 134.8 ± 15.4 | 0.307 | 0.867 | 0.059 |
DBP, mmHg | 82.7 ± 9.3 | 82.5 ± 8.6 | 81.4 ± 11.3 | 82.2 ± 10.8 | 0.846 | 0.552 | 0.486 |
TC, mmol/L | 4.46 ± 1.23 | 4.51 ± 1.07 | 4.44 ± 1.15 | 4.54 ± 1.11 | 0.896 | 0.954 | 0.208 |
TG, mmol/L | 1.83 ± 1.93 | 1.92 ± 1.71 | 1.31 ± 1.16 | 1.74 ± 1.47 | 0.671 | 0.190 | 0.367 |
LDLC, mmol/L | 2.55 ± 0.78 | 2.70 ± 0.73 | 2.45 ± 0.80 | 2.54 ± 0.80 | 0.424 | 0.739 | 0.677 |
HDLC, mmol/L | 1.26 ± 0.34 | 1.21 ± 0.31 | 1.39 ± 0.33 | 1.29 ± 0.30 | 0.996 | 0.073 | 0.491 |
HbA1c, % | 7.78 ± 1.62 | 8.55 ± 1.57 | 6.52 ± 0.59 | 8.79 ± 1.05 | 0.268 | 0.005 | 0.527 |
TCSS | 4.4 ± 4.4 | 6.1 ± 4.5 | 4.5 ± 4.8 | 6.3 ± 4.7 | 0.293 | 0.544 | 0.468 |
PMNCV, m/s | 44.47 ± 4.10 | 44.62 ± 4.15 | 44.46 ± 4.33 | 43.39 ± 3.84 | 0.895 | 0.984 | 0.124 |
TMNCV, m/s | 44.15 ± 4.86 | 44.12 ± 4.35 | 43.56 ± 4.86 | 42.54 ± 4.66 | 0.793 | 0.269 | 0.951 |
SPSNCV, m/s | 50.42 ± 6.81 | 48.66 ± 7.48 | 50.14 ± 7.19 | 46.64 ± 7.21 | 0.261 | 0.287 | 0.056 |
SSNCV, m/s | 48.87 ± 7.89 | 47.93 ± 7.20 | 47.28 ± 6.05 | 44.67 ± 6.43 | 0.982 | 0.293 | 0.024 |
CNFD, n/mm2 | 18.55 ± 5.25 | 17.19 ± 5.31 | 21.78 ± 6.13 | 15.67 ± 4.16 | 0.070 | 0.005 | 0.001 |
CNBD, n/mm2 | 21.76 ± 16.10 | 19.33 ± 12.82 | 26.19 ± 13.87 | 14.23 ± 6.56 | 0.349 | 0.122 | 0.033 |
CNFL, mm/mm2 | 11.62 ± 2.89 | 11.16 ± 2.57 | 13.04 ± 2.44 | 9.90 ± 1.75 | 0.137 | 0.029 | 0.011 |
Results are expressed as mean ± SD or counts for categorical variables.
At one-year follow-up, group A showed a significant decrease in HbA1c compared to baseline (from 7.78 ± 1.62% to 6.52 ± 0.59%,
Changes of CCM (from top to bottom are CNFD, CNBD, and CNFL) from baseline to follow-up in group A (a) and group B (b).
CCM images from a diabetic patient of group A at baseline (a) and follow-up (b).
Group B showed no significant change at one-year follow-up and remained with an average HbA1c ≥ 7.0% (from 8.55 ± 1.57% to 8.79 ± 1.05%,
CCM images from a diabetic patient of group B at baseline (a) and follow-up (b).
We have retrospect all the patient’s outpatient records; patients in group A had 152 times of outpatient records for diabetes in the past year in all, while patients in group B had 78 times of outpatient records for diabetes in all. This suggested that patients in group A have higher compliance (Table
Correlation coefficients between corneal nerve parameters and other indexes.
Correlation coefficients ( |
Change in CNFD | Change in CNBD | Change in CNFL |
---|---|---|---|
Age | 0.085 | −0.220 | −0.056 |
Duration of diabetes | 0.036 | −0.047 | 0.206 |
Change of Weight | −0.203 | −0.199 | |
Change of SBP | −0.006 | −0.126 | −0.006 |
Change of DBP | −0.241 | 0.121 | 0.064 |
Change of TC | −0.146 | 0.000 | −0.061 |
Change of TG | −0.031 | 0.01 | 0.034 |
Change of LDLC | −0.138 | −0.084 | −0.119 |
Change of HDLC | −0.042 | 0.180 | 0.110 |
Change of HbA1c | −0.127 | 0.200 | 0.077 |
Multiple linear regression and correlative analysis showed that the changes of corneal nerve parameters (CNFD, CNBD, and CNFL) had not shown significant correlations with the duration of diabetes, changes of weight, blood pressure, blood lipids, and HbA1c (
Longitudinal data from the Rochester cohort support the contention that the duration and severity of exposure to hyperglycemia are related to the progression and hence severity of neuropathy [
Studies of DPN in people with type 2 diabetes often have confusing results [
Diabetic neuropathy is a complication of diabetic microangiopathy that is specifically related to glucose control but also related to multiple metabolic factors especially in type 2 diabetes. We observed improvement of corneal nerve parameters but not TCSS or NCV in patients with type 2 diabetes with good glucose control, indicating that the corneal nerve, which is reflective of small-fiber neuropathy, can highly sensitively detect the improvement of neuropathy. In addition to glucose control, positive trends towards the improvement in blood pressure and blood lipids especially in group A also contribute to the improvement of corneal nerve morphology. In patients with type 1 diabetes who underwent combined pancreas and kidney transplantation, one year following transplantation, among all diagnostic parameters used to evaluate DPN, only CNFL, CNFD, and CNBD showed improvements [
We did not find any correlations between changes in corneal nerve parameters and the disease course of type 2 diabetes, weight, SBP, DBP, blood lipids, or HbA1c. In a previous 24-month observational study, the decrease in HbA1c value was significantly associated with an increase in CNFD [
Limitations of the current study include the small size of the study sample and the lack of randomization. Larger randomized studies with active intervention are required to confirm our findings. Another limitation is that HbA1c only reflects the glycemic status of 3 months; in this observational study, we could not get HbA1c values of every person every 3 months, so we do not know how long were the patients in good glycemic control before there were changes in the corneal nerve morphology. Nevertheless, the present data suggest that CCM may be a convenient, noninvasive technique to assess the progression of nerve damage and potentially to assess the effects of therapeutic intervention in future clinical trials of human diabetic neuropathy.
The results of this study suggest that morphological repair of corneal nerve fibers can be detected when glycemic control improves. In vivo CCM could be a sensitive method that can be applied in future longitudinal or interventional studies on diabetic neuropathy.
All data generated or analyzed during this study are included in this article and available from the corresponding author upon request.
The authors declare that there is no conflict of interest regarding the publication of this article.