Testing for nerve damage typically starts with a hands-on neurological exam and then moves to electrodiagnostic tests like nerve conduction studies and electromyography. The specific combination depends on the type of nerve damage your doctor suspects, since different tests target different nerve fibers. Some detect problems with large motor nerves, others catch damage to the tiny sensory fibers in your skin, and still others measure how well your autonomic nerves control functions like sweating.
The Physical Exam Comes First
Before ordering any lab work or electrical tests, a doctor will run through a series of simple bedside checks. These take just minutes but can reveal a surprising amount about where and how severely your nerves are affected.
For sensation, the most common screening tools are a safety pin and a cotton wisp. The pin is lightly pressed against your face, torso, and each limb while you report whether it feels sharp or dull, and whether it feels the same on both sides. A cotton wisp dragged across the skin tests light touch. Vibration sense is checked with a 128-cycle tuning fork placed on a finger or toe joint. You tell the examiner the moment you stop feeling the buzzing, and that timing is compared against when the examiner stops feeling it through your joint. If you lose the vibration sensation noticeably earlier, it points to damage in the nerves that carry deep sensory information.
Reflexes are tested with a standard reflex hammer at the knee, ankle, elbow, and wrist. Absent or diminished reflexes suggest the nerve pathway between your spinal cord and that muscle is disrupted. The doctor will also check your muscle strength by asking you to push or pull against resistance, and may watch you walk to look for foot drop or balance problems. All of this helps narrow down which nerves are involved before more formal testing begins.
Nerve Conduction Studies
A nerve conduction study (NCS) measures how fast and how strongly electrical signals travel through your nerves. Small electrode patches are placed on your skin, and a brief electrical pulse is delivered to stimulate the nerve at one point. Sensors downstream record how quickly the signal arrives and how large it is.
Two numbers matter most: conduction velocity (how fast the signal moves, measured in meters per second) and amplitude (how strong the signal is when it arrives). For a healthy median nerve in the forearm, normal motor conduction velocity is roughly 49 meters per second, while the ulnar nerve runs around 52 meters per second. When the insulating sheath around a nerve is damaged, velocity drops. When the nerve fibers themselves are dying off, amplitude drops instead. That distinction helps doctors tell the difference between conditions that strip the nerve’s insulation (like Guillain-Barré syndrome) and those that kill the nerve fibers outright (like diabetic neuropathy).
The electrical pulses feel like a quick, sharp tap or a static shock. Most people find it uncomfortable but tolerable. The test typically takes 30 to 60 minutes. One important prep detail: skip lotions, creams, and perfumes on test day, because residue on the skin interferes with the electrodes and can throw off results.
Electromyography (EMG)
EMG is often performed in the same session as a nerve conduction study, and together they give a more complete picture. While NCS tests the nerve itself, EMG tests the muscle that the nerve controls. A thin needle electrode is inserted into the muscle, and the electrical activity is recorded both at rest and while you contract the muscle.
Healthy muscle is electrically quiet at rest. When a nerve supplying a muscle is damaged or severed, the muscle fibers become hypersensitive. Extra receptors sprout across the muscle membrane, making the fibers prone to firing on their own. This produces characteristic signals called fibrillation potentials and positive sharp waves, both small, brief electrical discharges that show up while the muscle should be silent. These are the hallmarks of denervation on EMG.
One important limitation is timing. After a nerve injury, these abnormal signals don’t appear immediately. It takes roughly one to five weeks for fibrillation potentials to develop, depending on how far the injury site is from the muscle being tested. An EMG performed too soon after an acute injury may come back normal even when real damage has occurred. Doctors sometimes schedule the test three to four weeks after the suspected injury for this reason.
The needle insertion feels like a brief pinch, and you may feel a dull ache when the muscle contracts around the needle. The discomfort is real but short-lived for each muscle tested.
Skin Biopsy for Small Fiber Damage
Standard nerve conduction studies are excellent at detecting damage to large nerve fibers, but they can completely miss small fiber neuropathy. This is the type of nerve damage responsible for burning pain, prickling sensations, and temperature sensitivity, and it’s common in early diabetic neuropathy, fibromyalgia, and autoimmune conditions.
The gold standard for diagnosing it is a skin biopsy. A tiny punch of skin, usually about 3 millimeters, is taken from the lower leg near the ankle. In the lab, pathologists count the number of nerve fibers that penetrate the outer layer of skin per millimeter of tissue. In healthy adults, the distal leg normally has around 13.8 nerve fibers per millimeter. A count that falls below the fifth percentile of the normal range (roughly 3.8 fibers per millimeter at the distal leg) confirms small fiber neuropathy.
The biopsy itself is done under local anesthetic and leaves a wound smaller than a pencil eraser. Results typically take about 14 days. The test has a diagnostic efficiency of 88% and a negative predictive value of 90%, meaning a normal result is quite reliable at ruling out the condition.
Autonomic Nerve Testing
Your autonomic nerves control involuntary functions: heart rate, blood pressure regulation, digestion, and sweating. When these nerves are damaged, you might experience dizziness when standing, abnormal sweating, or digestive problems. Standard NCS and EMG won’t catch this type of damage.
The most common test is the quantitative sudomotor axon reflex test, or QSART. It measures your sweat glands’ response to stimulation. Electrodes are placed on the skin at several sites (typically the forearm, proximal leg, distal leg, and foot). A chemical that naturally triggers sweating is applied to the skin, and a small electrical current drives it into the sweat glands. The machine then measures the volume of sweat produced and how long it takes your body to start sweating.
Abnormal results include a delayed onset of sweating, reduced sweat volume, or an exaggerated sweat response. The pattern of which sites are abnormal helps localize the damage. For instance, reduced sweating only at the foot and lower leg suggests a length-dependent neuropathy, the same pattern seen in diabetes, where the longest nerves are affected first.
MRI and MR Neurography
When doctors need to see the nerve itself, rather than just measure its function, they turn to MR neurography. This specialized MRI technique produces detailed images of individual nerves and can reveal structural damage that electrical tests can’t pinpoint.
On MR neurography, damaged nerves appear enlarged and abnormally bright on certain image sequences. The degree of brightness correlates with injury severity. In cases of nerve compression or partial injury, the nerve looks swollen with increased signal intensity caused by blocked flow within the nerve fibers on one side and degeneration on the other. As the nerve heals, it returns to normal size and signal, so MR neurography can also track recovery over time.
For more severe injuries, MR neurography can distinguish between a nerve that’s damaged but still in one piece and a nerve that’s been completely severed. A severed nerve forms a characteristic mass at its end that resembles a balloon on a string. A nerve that’s injured but intact forms a spindle-shaped swelling with nerve continuity visible on either side. This distinction is critical for surgical planning, since a severed nerve often requires surgical repair while an intact but damaged nerve may recover on its own.
Which Tests You’ll Actually Get
Most people suspected of nerve damage will start with a nerve conduction study and EMG, often done together in a single appointment lasting about an hour. If those come back normal but symptoms persist, particularly burning pain, numbness in the feet, or temperature sensitivity, a skin biopsy may follow to check for small fiber involvement. Autonomic testing is added when symptoms suggest problems with blood pressure regulation, sweating, or digestion. MR neurography is typically reserved for traumatic injuries, suspected nerve tumors, or cases where surgical intervention is being considered.
No single test catches every type of nerve damage. Large fiber neuropathies show up clearly on NCS and EMG but are invisible on skin biopsy. Small fiber neuropathies are the reverse. Autonomic damage requires its own dedicated testing. A thorough workup often combines two or three of these approaches to build a complete picture of which nerve fibers are affected and how severely.