Types of neuropathy

Large fiber neuropathy: numb feet, poor balance and lost reflexes, how it’s tested, and how it differs from small fiber

Large nerve fibers carry touch, vibration and the sense of where your feet are, and they drive your muscles and reflexes. When they’re damaged, the result is usually numbness and unsteadiness rather than burning pain. Here is what large fiber neuropathy feels like, what causes it, how a nerve test shows it, and what helps.

Written by the Managing Neuropathy editorial team under the direction of Dr. Tom Biernacki, DPM, FACFAS · Clinical review pending · Last updated 13 November 2026

The shadow of an older person walking with a cane on a sunlit sidewalk

The short answer. Large fiber neuropathy is damage to the thick, insulated nerve fibers that carry touch, vibration and position sense, and that drive muscles and reflexes. It typically causes numbness, poor balance that is worse in the dark or with your eyes closed, falls, lost ankle reflexes and, in some people, weakness. Unlike small fiber neuropathy, it usually shows up on a standard nerve conduction study. Causes include diabetes (the most common), low vitamin B12, too much vitamin B6, some chemotherapy drugs, immune conditions such as CIDP and Guillain-Barré syndrome, and inherited forms such as Charcot-Marie-Tooth disease. Treatment targets the cause, while balance training and fall-proofing protect you in the meantime.

What large nerve fibers do

A peripheral nerve is a bundle of fibers of different sizes. The large ones are thickly coated in myelin, which makes them fast. They carry light touch, vibration and position sense, the ability to know where your feet are without looking, they carry the signals behind your reflexes, and the motor fibers among them make your muscles contract. Small fibers carry pain and temperature and run automatic functions such as sweating. Most neuropathies damage both to some degree, and “large fiber” describes which kind dominates.

Symptoms

Numbness, usually starting in the toes and creeping up the feet and legs, sometimes with tingling.
Poor balance, worse in the dark or with your eyes closed, because your brain leans on your eyes once position sense fades. In our experience, people often notice it first in the shower or on the way to the bathroom at night.
Stumbles and falls, especially on uneven ground.
Clumsy hands, such as trouble with buttons, when the hands are involved.
Weakness, cramps and thinning muscles when motor fibers are involved, such as trouble lifting the front of the foot and tripping over your toes. Foot drop.
Lost reflexes, which you won’t notice yourself but an examiner will.
Loss of protective sensation: stepping on something sharp, or wearing a blister, without feeling it.

Burning, stabbing and electric pains come mainly from small fibers, so a mostly large fiber neuropathy is often more numb than painful. That is part of what makes it dangerous for the feet.

Video: what neuropathy is, what causes it, and how it is treated, from Dr. Tom Biernacki’s Michigan Foot Doctors channel.

Large fiber vs small fiber neuropathy

	Large fiber	Small fiber
What the fibers carry	Touch, vibration and position sense; muscle power and reflexes	Pain and temperature; automatic functions such as sweating
Typical symptoms	Numbness, poor balance, falls, sometimes weakness	Burning, sharp or electric pain, tingling, sweating and temperature changes
What the exam finds	Reduced vibration, position sense and monofilament touch; weak or absent reflexes; swaying with the eyes closed; sometimes weakness	Reduced pinprick and temperature sense, often with normal vibration, reflexes and strength
The test that shows it	Nerve conduction study and EMG	Skin biopsy for nerve fiber density, or sweat testing; nerve conduction studies are typically normal

Many people, especially with diabetes, have both. Small fiber neuropathy, in full.

What causes it

Diabetes, the leading cause of neuropathy. Its earliest symptoms usually come from small fibers, and large fiber involvement adds numbness, balance problems and falls. Diabetic neuropathy.
Low vitamin B12, which can damage nerves even without anemia and cause numbness, tingling and balance problems. Long-term metformin use raises the risk. Vitamin B12 neuropathy.
Too much vitamin B6, usually from supplements, which injures sensory nerves and causes numbness and unsteady walking. Some reports describe nerve damage at doses below 500 mg a day. In the classic reports, people improved after stopping. Vitamin B6 toxicity.
Chemotherapy, especially the platinum drugs cisplatin and oxaliplatin (carboplatin less often), which damage sensory nerve cells and can keep getting worse for months after treatment ends. Chemotherapy-induced neuropathy.
Immune attacks on the nerves: Guillain-Barré syndrome, which comes on over hours to weeks, and CIDP, in which the immune system attacks the nerves’ myelin coating and weakness and numbness progress more slowly. CIDP.
Inherited neuropathies, most often Charcot-Marie-Tooth disease, which typically shows up as high arches, curled toes and weakness from the feet up. Charcot-Marie-Tooth disease.
Abnormal blood proteins, called paraproteins, which a blood test called protein electrophoresis looks for.
Alcohol, kidney disease, an underactive thyroid, copper deficiency (for example after weight-loss surgery), HIV or hepatitis C, cancers such as myeloma, and some medicines, among other causes.

Sometimes no cause turns up despite a full workup. That is called idiopathic neuropathy.

How it’s diagnosed

The bedside exam does most of the work. A clinician checks:

Vibration, with a 128 Hz tuning fork on the big toe.
Position sense, by moving your big toe up or down while your eyes are closed.
Touch, with a 10-gram monofilament.
Ankle reflexes and strength, especially lifting the foot and toes.
Balance, including the Romberg test: standing with your feet together, first with your eyes open, then closed. Swaying or stepping when the eyes close points to lost position sense.

A nerve conduction study then measures the large fibers directly, and EMG checks the muscles they supply. Together they can show whether sensory or motor fibers are affected, whether the damage is to the nerve fiber itself or to its myelin coating, and whether it is widespread or patchy, which narrows down the causes. Axonal neuropathy.

Blood tests come next. The American Academy of Neurology lists blood sugar, vitamin B12 with methylmalonic acid, and serum protein electrophoresis with immunofixation as the highest-yield tests for the common pattern of neuropathy that starts in both feet, and says a glucose tolerance test may be considered if routine tests don’t show diabetes. Others depend on your history.

When to get help urgently

Go to an emergency department or call 911 for weakness or tingling that spreads up from your feet over hours or days, or any trouble breathing: Guillain-Barré syndrome can progress quickly and weaken the breathing muscles. Sudden numbness or weakness on one side of the body can be a stroke: call 911. So can sudden loss of balance or sudden trouble walking. Numbness in the groin or buttocks, or new trouble controlling your bladder or bowels, is an emergency too.

See a clinician soon for:

Numbness or unsteadiness that is new or getting worse over weeks, or any fall.
Symptoms that are worse on one side, start in the hands, or include weakness.
Numbness after starting chemotherapy, or any supplement that contains vitamin B6, including B-complex, multivitamin and magnesium products.

See a foot specialist urgently for a sore that isn’t healing, or a foot that turns red, warm or swollen, even if it doesn’t hurt. Charcot foot.

Treatment: the cause first, then your balance

No medicine directly regrows damaged nerve fibers. Treatment targets the cause: correcting low B12, stopping excess B6, managing blood sugar, adjusting chemotherapy where possible, or immune treatment: IVIG or plasma exchange for Guillain-Barré syndrome (steroids don’t help it), and IVIG, steroids or plasma exchange for CIDP. Once the cause is fixed, nerves can recover and regrow, but slowly and sometimes only partly, so the sooner that happens, the more there is to save. Can neuropathy be reversed?

Meanwhile, protect your balance and your feet:

Train your balance.
Programs that combine balance work with endurance exercise improve balance and mobility in diabetic neuropathy, although the American Diabetes Association rates the strength of that evidence as low. Balance work helps chemotherapy-related neuropathy too. Neuropathy exercises.

Light the way at night.
Put night lights between the bed and the bathroom, because your eyes are now doing part of the job your feet used to.

Clear the floors.
Remove loose rugs and clutter, and keep a hand free for the railing on stairs.

Wear real shoes.
Nonskid, rubber-soled, low-heeled shoes, indoors and out. Never walk barefoot, and don’t walk on floors or stairs in socks or in smooth-soled shoes or slippers. Choosing shoes.

Ask about a cane, walker or brace.
A physical or occupational therapist can help you choose the right aid and use it safely, and an ankle-foot brace helps if you have foot drop. Foot drop braces.

Check your feet every day.
Numb feet don’t warn you about blisters, cuts or burns. The daily foot check.

Our take. Large fiber neuropathy is the kind that knocks people over. Numbness gets the attention, but in our clinic the bigger dangers are the fall on the way to the bathroom at night and the blister nobody felt. If your balance is worse in the dark, say so, and ask for the simple bedside tests: tuning fork, position sense, reflexes and the Romberg test. Then make sure the basic blood work is done, because low B12 and too much B6 are cheap to find and among the causes that can improve once fixed.

Common questions

What is large fiber neuropathy?

Damage to the large, myelin-coated nerve fibers that carry touch, vibration and position sense and that control muscles and reflexes. It causes numbness, poor balance, lost reflexes and sometimes weakness.

What is the difference between large and small fiber neuropathy?

Large fibers carry touch, vibration, position sense and muscle signals; small fibers carry pain and temperature and run automatic functions. Large fiber damage causes numbness and imbalance and shows up on nerve conduction studies. Small fiber damage causes burning pain and is usually confirmed with a skin biopsy.

Is large fiber neuropathy painful?

It is more often numb than painful. Burning and electric pains come mainly from small fibers, but many people have both kinds of damage.

Why is my balance worse in the dark?

When position sense in your feet fades, your brain relies on your eyes to keep you upright. Take away the light, or close your eyes, and you sway. That is the basis of the Romberg test.

Does a nerve conduction study show large fiber neuropathy?

Usually. Nerve conduction studies measure the large motor and sensory fibers, and can show whether the damage is to the nerve fiber or its myelin coating. They are typically normal in pure small fiber neuropathy.

What causes large fiber neuropathy?

Diabetes, low vitamin B12, too much vitamin B6, platinum chemotherapy, immune conditions such as CIDP and Guillain-Barré syndrome, inherited forms such as Charcot-Marie-Tooth disease, and abnormal blood proteins, among others.

Can large fiber neuropathy be reversed?

Sometimes, depending on the cause. In the classic reports, neuropathy from too much B6 improved after stopping, and treating low B12 or an immune cause early gives the best chance of improvement. Recovery is slow, and long-standing damage may be permanent.

Does large fiber neuropathy shorten life expectancy?

Not directly, in its common forms. The risk comes from what it takes away: foot injuries you don’t feel, which can turn into infections, and falls. A few less common causes are serious diseases in their own right, which is another reason to find the cause. What risk neuropathy actually carries.

Can exercise help large fiber neuropathy?

Balance training, combined with endurance exercise such as walking or cycling, can improve balance and mobility, based mainly on studies in diabetic and chemotherapy-related neuropathy. It isn’t a cure for the nerve damage, but it trains the balance you still have.

Numb feet and
shaky balance?

A foot and ankle exam can check vibration, position sense, reflexes and balance in a few minutes, look for causes worth treating, and protect feet that can’t feel properly. Dr. Biernacki’s Michigan practice can start there.

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Dr. Tom Biernacki, DPM, FACFAS

Dr. Tom Biernacki, DPM, FACFAS is a double board-certified foot and ankle surgeon (ABFAS and ABPM) and a Fellow of the American College of Foot & Ankle Surgeons. He is the founder of Balance Foot & Ankle in Howell and Bloomfield Township, Michigan, where peripheral neuropathy, diabetic foot risk and loss of protective sensation are part of his weekly clinic. His patient education videos have drawn more than 950,000 subscribers on YouTube. Managing Neuropathy is written by its editorial team under his direction, to put the explanation he gives in the exam room in front of the people who cannot get to one — including when a product is worth buying and when it is not.