Diabetes is now recognized as a chronic, debilitating and costly disease and the global population predicted to double to 366 million by 2030. A major complication of diabetes is diabetic peripheral neuropathy (DPN) according to the International Neuropathy Guidelines this can be defined as the presence of symptoms and/or signs of peripheral nerve dysfunction in people with diabetes after exclusion of other causes. Neuropathy can present with a loss of protective sensation, defined as a level of sensory loss sufficient for a patient to injure himself or herself without recognizing the injury. About 47% of diabetics suffer some peripheral neuropathy and at a conservative estimate, 7.5% present with symptoms at the time of their initial diabetes. Research confirms early detection of the insensate foot is critical, but made all the more difficult because diabetic peripheral neuropathy is often asymptomatic. Despite clear and authoritative clinical guidelines research supports first-line providers do not screen enough and their subsequent care quality suffers as a result.
What causes Diabetic Peripheral Neuropathy?
The aetiology (cause) of diabetic neuropathy is still poorly understood although glycation is probably a major factor. (The importance of glycaemia has been confirmed by studies showing incidence is greatly reduced by strict glycaemic control.) Changes to vasculature (micro circulation) also play an important part in causation of nerve damage. Resultant disruption to neuronal integrity and failure to regenerate results in progressive neuropathy, characteristically presenting in a distal–proximal direction (or stocking distribution). Loss of protective sensation means people unwittingly can damage themselves. Delayed healing and the diabetics’ propensity to infection mean small cuts and other abrasions have the potential to ulcerate. It is well recognized regular foot screening helps monitor and prevent these serious complications in people coping with peripheral neuropathy and peripheral vascular disease.
Signs and Symptoms: The clinical conundrum
Some people present with acute sensory neuropathy i.e. severe polyneuropathic pain i.e. burning, hyperesthesiae, paresthesia, and dysesthesia [an unpleasant, abnormal sense of touch], but with minimal deficit. All are prone to nocturnal exacerbation. Clinical examination is usually relatively normal, sometimes with allodynia on sensory testing; a normal motor exam, and occasionally reduced ankle reflexes. Others, more commonly, present with chronic sensorimotor neuropathy and have a complete insensate foot with no symptoms. This neurological deficit may only ever be discovered during a routine neurological foot examination.
Testing for Peripheral Neuropathy
Currently there is no gold standard for chair side assessment of Diabetic Peripheral Neuropathy (DPN). Conventional wisdom supports symptoms alone have relatively poor diagnostic accuracy hence a combination of signs and symptoms approach is considered more appropriate. Symptoms may be primarily sensory, motor or both. Again research supports simple composite examination scores are as accurate as more complex examinations. Best evidence indicates the combined of a 10g monofilament test for pressure sensation combined with a modified Neuropathy Disability Score (NDS) is appropriate to chart protective sensation and identify those most likely to progress to ulcerative stage. NDS is a score based on vibration perception, pin-prick sensation, temperature perception, and ankle (Achilles) reflexes.
From the 80s, Semmes-Weinstein monofilament examination (SWME) has been used in testing protective sensation. Initially they were developed to assess neuropathy in Hansen’s disease. At a given pressure to the skin (10 grams) the nylon filament bends. The 5.07 (10g) filament is used to ascertain the level of protective sensation. With eyes closed three sites are tested i.e. the plantar aspects of the great toe, the third metatarsal, and fifth metatarsal (The Australian Diabetes Society). Research suggests people unable feel the monofilament have a 7.7-fold increase in ulceration risk. Monofilaments have high inter and intra examiner reliability and are generally considered effective, inexpensive and simple screening for ‘at risk’ feet. The half -life of a Semmes-Weinstein monofilament is approximately 100 patients. Filaments fatigue and bend too easily, giving false positives after testing about 10 patients. They must be rested for 24 hours before regaining their firmness.
Neuropathy Disability Score (NDS)
A modified Neuropathy Disability Score (NDS) has been shown to be the most reliable test for sensitivity and grading diabetic peripheral neuropathy. Combination of different examination scores gives better sensitivity and specificity.
Diabetic peripheral neuropathy manifests with a wide variety of sensory, motor, and autonomic symptoms. Smaller fibres are often the first to be affected and with continued hyperglycemia, larger fibres follow. The smallest sensory nerves are responsible respectively for thermal/burning pain (0.2-1.5 mu); and sharp pain (1-5 mu). Pin Prick sensation is tested with neurological pins.
Altered thermal thresholds such as lowered heat-pain thresholds are associated with early changes in distal nerve segments and can be tested with simple heat/cold tests.
Bigger A-alpha (I) and A- beta (II) responsible for propioception (13 -20 mu), vibration and pressure (6 -12 mu). A loss of propioception may result in a positive Romberg’s sign. Vibration Perception threshold is tested using a 128Hz tuning fork or Rydell Seiffer tuning fork.
Vibration Perception thresholds have a strong co-relation with nerve conduction velocities and are a reliable indicator of “risk” for future ulceration across a wide range of ages and durations of diabetes. (Neurothesiometer or Biothesiometer).
Deep tendon reflexes are tested with a tendon hammer and in neuropathy these are commonly hypoactive or absent.
Motor weakness is unusual, although small muscle wasting in the feet and also the hands may also be seen in more advanced cases. Motor problems include distal, proximal, or more focal weakness. Symptoms include weakness or atrophy of intrinsic foot muscles and associated foot deformities. Foot slapping and toe scuffing or frequent tripping may be reported. Proximal limb weakness include difficulty climbing up and down stairs, difficulty getting up from a seated or supine position, as well as falls due to the knees giving way. As standard examination muscle strength is tested in the lower and foot.
Autonomic neuropathy may involve the cardiovascular, gastrointestinal, and genitourinary systems and the sweat glands (sudomotor). People with generalized autonomic neuropathies may report ataxia, gait instability, or near syncope/syncope. Sudomotor neuropathy may produce heat intolerance, heavy head, neck, and trunk sweating with anhidrosis of lower trunk and extremities. Signs might include warm dry skin (in the absence of peripheral vascular disease) and the presence of plantar callus under pressure-bearing areas. The “at-risk” foot for neuropathic ulceration might also have foot arch complications combined with metatarsals phalangeal subluxation resulting in lesser toe deformities. However, it must be emphasized that all patients with DPN with or without obvious foot deformities must be considered as being at risk of neuropathic complications, such as Charcot’s neuroarthopathy or foot ulceration.
Peripheral vascular circulation
Claudication can be a useful symptom, but peripheral arterial disease (PAD) is commonly asymptomatic. Palpation of foot pulses is however a good simple test to determine the presence of peripheral arterial disease.
The ankle-brachial pressure index (ABPI or ABI), using Doppler ultrasound is a useful adjunct to assess foot perfusion. Results can however be falsely elevated in the presence of arterial calcification and in this event clinicians are using photoplethysmography to measure toe-brachial pressure index or toe pressures.
In the ‘toe’ examination, if the flow is deemed adequate patients are managed medically, surgically, and mechanically with the foci to heal and prevent severe recurrence. If the blood supply is determined to be inadequate, the patient is prioritised for revascularisation or ‘flow’. Once an appropriate blood supply is established the patient is returned to ‘toe’ for preventive management.
Foot Deformity Score
A Foot Deformity Score helps identify high risk areas where repetitive trauma can cause breakdown, progressive ulceration, and infection. Screening includes signs such as: abnormal bony prominences, subluxation of the metatso-phalangeal joints, lesser toe deformities due to small muscle wasting, Pes Planus (or Pes Valgus), previous amputation, and Charcot’s neuroarthropathy. About half of all amputees experience a subsequent amputation as the disease progresses. Morbidity rates for limb amputees are high with a life expectance of approximate five-years. Other foot symptoms include dryness, tinea pedis, cracks, onychomycoses, acute erythema and tenderness, and fluctuance under calluses.
Inadequate footwear makes a significant contributory factor in causation of diabetic ulcers i.e. 35% -50% of cases. Shoes are made in standard sizes and feet are not symmetrical like shoes. Poorly fitting shoes cause sheer which may lead to secondary skin changes. When shoes fail to support feet, high intermittent pressure result which puts the insensate foot at risk. A significant number of people wear shoes that do not fit their feet and research continues to shown poorly fitting shoes are more prevalent in the demographic with diabetic foot wounds than in those without wounds with or without peripheral neuropathy. A routine foot assessment includes size, volume, suitability and wear marks of presenting footwear are reviewed and discussed with the patient.
There are several systems to categorize risk with the best known the University of Texas Diabetic Foot Risk System (with 7 categories). For simplicity however the demographic can be divided into four main divisions:
Low risk i.e. normal sensation and palpable pulses. Recommend an annual foot inspection with education.
Intermediate risk i.e. neuropathy or absent pulses or foot deformity. Recommend 3 to 6 monthly reviews.
High risk i.e. where two or more risk factors are present (neuropathy, peripheral arterial disease or foot deformity) and/or a previous history of foot ulcer/amputation have been recorded. Recommend 3 to 6 monthly reviews.
Critical category Recommend GP/Hospital referral within 24 hours.
Increased assessment frequency helps monitor rapidly-developing problems, such as ulcers, infections, gangrene, and Charcot’s neuroarthropathy. All of which need immediate intervention.
In Australia the Enhanced Primary Care Plan gives greater access to clients who are potentially at risk. Foot Health Education Programs promote daily self foot inspections with informed foot hygiene that minimize inadvertent self harm. Advice on appropriate foot gear and where to get it is combined with and open access in the case of emergency. To achieve improved Quality Adjusted Life Years the early detection of the insensate limb is critical and achieved through regular foot screening and foot care; in conjunction with intensive glycaemic control. To this end effective care involves clear communications between patients and other health care professionals.
Australian Podiatry Council
American Diabetes Association