Nerve damage from diabetes can take away the feeling of heat and pain in the feet, sometimes with no other sign.1 A foot that can't feel heat can burn without its owner knowing.3
What the published guidance advises, in six steps. The details and sources follow below.
Doctors group the sensory nerve fibres into small fibres and large fibres.2 Symptoms differ depending on which kind is damaged.1
Carry pain, temperature and itch.2 Clinicians check them with pinprick and with temperature.1
Carry pressure, balance, vibration and the sense of where the foot is. The vibration fork, the 10-gram monofilament and the ankle reflex all test large-fibre function.1
In diabetic nerve damage, the most common early symptoms come from the small fibres: pain and unpleasant burning sensations.1 Damage can sit mostly in the small fibres, mostly in the large fibres, or in both, and both together is the most common pattern.1 Large-fibre damage can cause numbness, tingling without pain, and loss of protective sensation.1 More advanced damage to both kinds, with loss of feeling, temperature discrimination and pain, can lead to unsteadiness, repeated minor injuries and falls.1
When the damage is only in the small fibres, vibration sense, strength and reflexes are often preserved, so a physical exam of those things often finds nothing.4 That is why the ADA's recommended exam also includes a temperature or pinprick check (below).1 A standard nerve conduction study measures only the large fibres and is normal when only the small fibres are affected.2
Up to half of diabetic peripheral neuropathy may cause no symptoms.1 With diabetic nerve damage, a person may burn their feet and not know it.3
Temperature is carried by the small fibres;2 the monofilament touch test checks a large-fibre function.1 Either kind can be damaged first, or both together,1 so a normal touch test does not by itself show that a foot can feel heat.
Many people have no symptoms, and people often do not mention symptoms unless asked.1 Things the sources say people report:
Asking about these, gently, can open the conversation with the care team.
The American Diabetes Association's 2016 position statement recommends that screening for people with diabetes include a careful history, plus either temperature or pinprick (small fibres), plus vibration with a 128‑Hz tuning fork (large fibres). It also recommends a 10‑gram monofilament test once a year for every person with diabetes, to find feet at risk of ulcers and amputation.1 Screening starts at diagnosis for type 2 diabetes and five years after diagnosis for type 1, then at least once a year.1
The monofilament tests a large-fibre function. It is mainly useful for finding more advanced nerve damage and the people at higher risk of ulcers and amputation.1 Using at least two kinds of exam together detects nerve damage more reliably than one alone.1
A question you can ask at the next visit: "Which part of the foot exam checks temperature or pinprick?"
Guidance changes. The ADA updates its Standards of Care every year, so ask the care team what they currently follow.
The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) gives the advice below on its 2017 page for people with diabetes; the heat steps are aimed at people with nerve damage.3 It is reported here as NIDDK's guidance, not as a care plan: ask the care team what suits the person you look after.
NIDDK also advises:3
A warm spot matters. A "hot spot" can be the first sign that a blister or an ulcer is starting, and for people at higher risk a doctor may suggest measuring skin temperature on different parts of the feet.3 Redness, warmth and swelling can also be how Charcot's foot starts, a rare change in which bones in the foot can later shift or break.3
NIDDK's 2017 page says to call a health care provider right away for any of these:3
Our own caution, not a source's: this list is not a waiting period. If you find a burn or anything that worries you, get it seen.
NIDDK also advises a thorough foot exam at least once a year, including a check of feeling and pulses, and a thorough foot exam at every visit for people with loss of feeling, changes in foot shape, peripheral artery disease, or a past ulcer or amputation. Ask for a referral to a podiatrist if you need one.3
The ADA lists some patterns as unusual for diabetic nerve damage, and these may prompt referral to a neurologist: weakness that is worse than the sensory loss, symptoms or signs that differ noticeably between the two sides, or nerve damage that comes on or gets worse quickly. The ADA says such referral is otherwise rarely needed.1 If you notice any of these, say so.
These tests are not part of the basic foot exam described above. Agreed diagnostic criteria and a single diagnostic protocol have not been established,4 and StatPearls notes there is not yet a gold-standard test.2 Whether any of them is right for a given person is the doctor's call.
| Test | What it does | What the sources say |
|---|---|---|
| Skin biopsy (IENFD) | A 3 mm punch of skin from the lower leg and the thigh; the lab measures the density of nerve fibres in the skin.2 | Sensitivity 88%2; another review cites sensitivity of 78–92% and specificity of 65–90%, and calls the measurement well validated and highly reproducible.4 |
| QSART | Measures the sweat the limbs produce in response to acetylcholine, to assess the nerves that run the sweat glands. Non-invasive.2 | Sensitivity up to 80%2; abnormal in more than 70% of small-fibre neuropathy.4 |
| Quantitative sensory testing (QST) | Measures the thresholds at which a person feels warm, cold, heat pain and vibration.4 | Needs the person's full cooperation, and cannot tell nerve damage in the limbs from problems in the brain or spinal cord.4 |
| Corneal confocal microscopy | Detects loss of nerve fibres in the cornea of the eye.4 | Often used for diagnostic confirmation; in painful neuropathy, corneal nerve loss correlates with the severity of the pain.4 |
Sensitivity is the share of people who have the condition that a test picks up; the rest are missed. Specificity is the share of people without it that the test correctly clears. A skin biopsy is a small procedure.
The ADA cites emerging evidence that diabetic-type nerve damage, especially its painful small-fibre form, may be present in 10–30% of people with prediabetes (impaired glucose tolerance) or metabolic syndrome.1