Diabetic Neuropathy

Diabetic neuropathy is defined — according to the International Consensus Group on Neuropathy — as 'the presence of symptoms and/or signs of peripheral nerve dysfunction in people with diabetes, after exclusion of other causes'. The prevalence of peripheral neuropathy in diabetes is 23-42% and is higher (50-60%) among older type 2 diabetic patients. It should be mentioned that the prevalence of symptomatic peripheral neuropathy (burning sensation, pins and needles or allodynia in the feet, shooting, sharp and stabbing pain or muscle cramps at the legs) is only 15-20% and the majority of the patients with neuropathy are free of symptoms. Often, the first sign of peripheral neuropathy is a neuropathic ulcer. Other patients have neuropathic pain and on examination are found to have severe loss of sensation. This combination is described as 'painful-painless legs' and these patients are at increased risk for foot ulceration.

All patients with diabetes should be examined annually for peripheral neuropathy,

Pathway Diabetic Ulcer
Figure 1.1 Pathways to foot ulceration in diabetic patients. (From Boulton AJM. The pathway to ulceration: Aetiopathogenesis. In Boulton AJM, Connor H, Cavanagh PR (Eds), The Foot in Diabetes (3rd edn). Chichester: Wiley, 2000; 61-72, with permission)

so that those at risk for ulceration can be identified. The tests for peripheral neuropathy are many and some of them are quite sophisticated, and are undertaken only in specialist centers. However, the tests that are used to characterize the patient with loss of protective sensation are simple, fast and easily carried out at the outpatient clinic. These tests are as follows.

1. Questioning the patient to ascertain whether symptoms of peripheral neuropathy, as described above, are present. Typically neuropathic symptoms are worse during the night and may wake the patient, who finds relief on walking.

2. Loss of sensation of (a) pain (using a disposable pin; this test is carried out only when the skin is intact), (b) light touch (using a cotton wisp), and (c) temperature (using two metal rods, one at a temperature of 4 °C and the other at 40 °C) on the dorsum of the feet. Typically, in diabetic peripheral neuropathy the sensory deficit is pronounced at the periphery of the extremities (in a 'glove and stocking distribution'). A zone of hypoesthesia is found between the area of loss of sensation and a more central area of normal sensation. Achilles tendon reflexes may be reduced or absent. Wasting of small muscles of the feet results in toe deformities (claw, hammer, curly toes) and prominent metatarsal heads. Vibration perception is tested using a 128-Hz tuning fork on the dorsal side of the distal phalanx of the great toes (Figure 1.2). A tuning fork should be placed perpendicular to the foot at a constant pressure. During examination the patient is prevented from seeing where the examiner has placed the tuning fork. Examination is repeated twice and there is at least one 'sham' application in which the tuning fork is not vibrating. The patient has

128 Tuning Fork Foot
Figure 1.2 Examination of vibration perception by the use of tuning fork

normal sensation when his reactions are correct in two out of three tests, but is at risk for ulceration when they are incorrect in two out of the three tests. 3. Pressure perception is tested with Sem-mes-Weinstein monofilaments. Many studies have shown that inability to perceive pressure is related to a several-fold increase in the risk for foot ulceration. The filaments are available in large sets with varying levels of force required to bend them. Diabetic neuropathy can be detected using the 5.07 monofilament (this filament bends with the application

Diabetic Neuropathy
Figure 1.3 Semmes-Weinstein (5.07) monofilament examination

of a 10-g force). Monofilament should be applied perpendicular to the skin surface and with sufficient force so that it bends or buckles (Figure 1.3). Total duration of skin contact of the filament should be approximately 2 s. During examination the patient is prevented from seeing if and where the examiner applies the filament. The patient is asked to say whether he can feel the pressure applied (yes/no) and in which foot (right/left foot). Examination is repeated twice at the same site and there is at least one 'sham' application, in which no filament is applied (a total of three questions per site). The patient has normal protective sensation when the correct answer is given for two out of the three tests and is at risk for ulceration when they are not. The International Consensus on the Diabetic Foot suggested three sites to be tested on both feet: the plantar aspect of the great toe, the first and the fifth metatarsal heads. The filament must be applied at the perimeter and not at an ulcer site, callus, scar or site of necrotic tissue.

4. Determination of vibration perception thresholds using a biothesiometer or a neurothesiometer. Vibration perception threshold is measured at the tip of the great toes with the vibrating head of the device balanced under its own weight (Figure 1.4). The vibrating stimulus is increased until the patient feels it, the stimulus is then withdrawn and the test repeated. This test is usually carried out three times at each site and the mean value is calculated. Several studies have shown that a vibration perception threshold over 25 V is associated with a 4- to 7-fold increase in risk for foot ulceration.

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