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Author: Ausma Ahmed, University of Ottawa,
Class of 2013
Introduction: The morbidity of type 1 diabetes
includes abnormality of small nerve fiber morphology occurring
early in the course of diabetic sensorimotor polyneuropathy (DSP).
This can be detected by examination of intra-epidermal nerve fibers
by invasive skin biopsy. Alternatively, small nerve fibers can be
examined non-invasively by a novel in-vivo Corneal Confocal
Microscopy (CCM) method. We aimed to determine the CCM parameter
and threshold value with optimal operating characteristics for DSP
Methods: At the Toronto General Hospital 89
individuals with type 1 diabetes and a broad spectrum of neuropathy
severity were selected. Concurrent with clinical and
electrophysiological examination for the determination of DSP (the
gold standard), CCM was performed to determine corneal nerve fiber
length (CNFL), corneal nerve fiber density (CNFD), corneal nerve
branch density (CNBD), and fiber tortuosity (TC). Receiver
operating characteristic (ROC) curve analysis was used to determine
the parameter with the largest area under the curve and the
threshold values with optimal sensitivity and specificity.
Results: Of the 89 participants with type 1
diabetes, 33 (37%) were classified as DSP cases and 56 (63%) as
non-neuropathic controls. DSP was associated with older age, longer
diabetes duration, greater body mass index (kg/m2), higher blood
pressure and higher glycosylated hemoglobin A1c (%). CNFL was
significantly lower in those with DSP as compared to those without
(1278 ± 411 vs. 1928 ± 491 µm/mm2, P <
0.001) as was CNFD (28 ± 9 vs. 39 ± 10 fibers/mm2, P
< 0.001) and CNBD (18 ± 12 vs. 29 ± 16
branches/mm2, P < 0.001). Differences in TC were not
significant. Area under the ROC curve was greatest for CNFL (0.88,
p=0.0001) compared to all other parameters. The optimal CNFL
threshold value was 1612 µm/mm2, with 85% sensitivity and 84%
specificity, and positive and negative likelihood ratios of 5.3 and
0.18, respectively. An alternate approach in which a threshold
value that maximizes sensitivity (=1823 µm/mm2, sensitivity
91%, negative likelihood ratio 0.16) and a separate threshold that
maximizes specificity (=1320 µm/mm2, specificity 93%,
positive likelihood ratio 8.5) permitted correct classification of
60 (67%) of subjects.
Conclusion: Among all of the corneal
parameters, corneal nerve fiber length had the best capacity to
discriminate DSP. A single, optimal threshold offers clinically
acceptable operating characteristics, however, an alternative
strategy that uses separate thresholds to respectively rule in and
rule out DSP has excellent performance while maximizing classified
subjects. We hypothesize that values between these thresholds
indicate incipient nerve injury that represents those individuals
at future risk of neuropathy. Together, this data suggests that CCM
is a potential diagnostic test for DSP in clinical practice.
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