A Week’s Worth of Reasons to Reconsider Monofilament Testing for DPN: Sunday
Sunday: Monofilament testing under ideal conditions assesses light touch by applying pressure to the skin of the foot. Light touch is mediated mostly by large fiber encapsulated nerve receptors and some small nerve fiber mechanoreceptors (see Figure one). For decades, providers have been advised by researchers and expert consensus to accept this as a valid “proxy” test for loss of protective sensation (LOPS)(1). LOPS, more accurately interpreted as a loss of pain sensation, is a small nerve fiber function mediated by specialized free nerve endings known as nociceptors. The obvious question is whether or not a test assessing mostly large fiber nerve function can imply a loss of small fiber nerve function? Although there is thought to be some correlation between the two it is difficult to make a direct comparison. For example, nerve conduction velocity (NCV) testing, acknowledged as the “Gold Standard” for diagnosing neuropathy, tells us nothing about small fiber nerve function. This lack of direct correlation is heightened in light of recent research demonstrating that small fiber neuropathy (SFN) precedes large fiber neuropathy in the feet of diabetic patients (2).
Given these facts, the argument can be made that there is an intrinsic flaw in the test itself. Although not commonly discussed in the literature, providers have no doubt seen its impact over the years in the false negatives elicited by the test. The most glaring examples are those patients who will “pass” the monofilament exam while failing to react to a sharp stimulus or complain of pain when presenting with a full-thickness foot ulcer. In these cases, patients most likely are passing the exam with their intact large fiber nerve receptors. Although these may be exceptions, periodic false negatives combined with the errors induced by confounding variables noted in days Monday-Saturday lead one to reconsider the validity of the test as medicine moves into the 21st century. Ultimately providers should consider whether LOPS as diagnosed by the monofilament is effective at further reducing diabetic foot complications.
Fig. 1. Large and small nerve fiber functions in the skin of the foot. Note that pain perception resides on the small fiber side and light touch is the only shared function.
1.Boulton, AJ, Armstrong, DG, et al. Comprehensive Foot Examination and Risk Assessment. Diabetes Care 31(8):1679-1685, 2008.
2. Breiner A, Lovblom LE, Perkins BA, Bril V. Does the prevailing hypothesis that small-fiber dysfunction precedes large-fiber dysfunction apply to type 1diabetic patients? Diabetes Care. 2014 May;37(5):1418-24.