Positive sensory symptoms can arise spontaneously or as a response to stimulation, and they are often classified into painful and non-painful descriptors (Box 17.3). Numbness and prickling are the most common symptoms, and they usually occur earlier. Allodynia is the perception of pain from a non-noxious stimulus. The prevalence of painful symptoms varies from 3% to 20%. The natural history of painful neuropathy is unclear, but there is some suggestion that the intensity of symptoms may subside with worsening quantitative measures of nerve function. Similarly, the risk factors for painful neuropathy are ill defined. Hypoesthetic neuropathy is associated with minimal or negative sensory symptoms, and therefore is best detected by quantitative sensory testing.
In identifying feet at risk of ulceration, the 10 g monofilament has a sensitivity of 86 – 100%. These are patients who are unable to feel the monofilament when applied with sufficient pressure at the handle to buckle the filament (Figure 17.4). The monofilament should be applied to the sole of each foot in four places (over the hallux and metatarsal heads 1, 3, 5). More sophisticated instruments for measuring vibration detection threshold (VDT) and quantitative sensory testing (three modalities – vibration, thermal and pain thresholds) are also useful for predicting patients with neuropathy who are at high risk of ulceration and amputation.
Positive symptoms of neuropathy are distressing, often occur at night, are disabling and difficult to treat. Patients with painful peripheral neuropathy often have warm, dry feet because of autonomic involvement, which results in dilated arteriovenous shunts and absent sweating. The most important complications are:
- foot ulceration
- neuropathic oedema, caused by increased blood flow in the foot, which has reduced sympathetic innervation
- Charcot arthropathy, with chronic destruction, deformity and inflammation of the joints and bones of the mid-foot. There is reduced bone density, possibly because of increased blood flow (Figure 17.5) (see Chapter 21).