Comparison of Novel Wide-Field In Vivo Corneal Confocal Microscopy With Skin Biopsy for Assessing Peripheral Neuropathy in Type 2 Diabetes

Abstract

Diabetic peripheral neuropathy (DPN) is a serious complication of diabetes, where skin biopsy assessing intraepidermal nerve fiber density (IENFD) plays an important diagnostic role. In vivo confocal microscopy (IVCM) of the corneal subbasal nerve plexus has been proposed as a noninvasive diagnostic modality for DPN. Direct comparisons of skin biopsy and IVCM in controlled cohorts are lacking, as IVCM relies on subjective selection of images depicting only 0.2% of the nerve plexus. We compared these diagnostic modalities in a fixed-age cohort of 41 participants with type 2 diabetes and 36 healthy participants using machine algorithms to create wide-field image mosaics and quantify nerves in an area 37 times the size of prior studies to avoid human bias. In the same participants, and at the same time point, no correlation between IENFD and corneal nerve density was found. Corneal nerve density did not correlate with clinical measures of DPN, including neuropathy symptom and disability scores, nerve conduction studies, or quantitative sensory tests. Our findings indicate that corneal and intraepidermal nerves likely mirror different aspects of nerve degeneration, where only intraepidermal nerves appear to reflect the clinical status of DPN, suggesting that scrutiny is warranted concerning methodologies of studies using corneal nerves to assess DPN.

Article highlights: Comparison of intraepidermal nerve fiber density with automated wide-field corneal nerve fiber density in participants with type 2 diabetes revealed no correlation between these parameters. Intraepidermal and corneal nerve fibers both detected neurodegeneration in type 2 diabetes, but only intraepidermal nerve fibers were associated with clinical measures of diabetic peripheral neuropathy. A lack of association of corneal nerves with peripheral neuropathy measures suggests that corneal nerve fibers may be a poor biomarker for diabetic peripheral neuropathy.

Figure 1

Scatterplot indicating the relationship between mCNFL and IENFD in the same participants with and without diabetes at follow-up. The linear regression is indicated by the dashed lines.

Figure 2

Scatterplot indicating the relationship between wCNFL and IENFD in the same participants with and without diabetes at follow-up. The linear regression is indicated by the dashed lines.

Figure 3

The relationship between ΔIENFD and mCNFL at follow-up, indicating subgroups with and without type 2 diabetes, and corresponding regression lines. The diabetes group lost the fewest intraepidermal nerves during follow-up and had the lowest density of corneal nerves at the final follow-up, while the nondiabetes group lost the most intraepidermal nerves during follow-up and had the highest density of corneal nerves at the final follow-up.

Figure 4

The relationship between ΔIENFD during an 8-year follow-up period and IENFD at baseline in groups with and without type 2 diabetes. Those with diabetes had the lowest IENFD at baseline and lost fewer intraepidermal nerves during follow-up, while those without diabetes had the greatest IENFD at baseline and lost relatively more intraepidermal nerves during follow-up.

Figure 5

Representative images of skin biopsy sections stained with PGP 9.5 antibody (top row, brown color) to highlight intraepidermal nerve fibers (top row, arrows) and corneal subbasal nerve plexus mosaics obtained by IVCM (bottom row), both at final follow-up. In a 69-year-old female participant with NGT (left column), intraepidermal nerve fibers were detected, while in the cornea of the same participant, a dense distribution of subbasal nerves and inferocentral circular whorl were apparent. In a 68-year-old female participant with type 2 diabetes mellitus (T2DM) diagnosed >25 years prior to examination (right column), intraepidermal nerve fibers were identified, whereas the corneal subbasal nerve plexus appeared less densely innervated. Scale bars: 50 μm (top row), 500 μm (bottom row).