NCS and EMG Tests Explained: What Happens, Does It Hurt, and What It Diagnoses

Nerve conduction studies (NCS) and electromyography (EMG) are the neurologist's most powerful tools for diagnosing peripheral nerve and muscle diseases — yet they remain mysterious and anxiety-provoking for most patients. Here is exactly what happens and what these tests reveal.

What Is NCS (Nerve Conduction Study)?

NCS measures how fast electrical signals travel through peripheral nerves, and how strong those signals are. Surface electrodes (sticky pads) are placed on the skin over specific nerves and muscles. A small electrical pulse is applied to the nerve at one point, and the response is recorded at another point. The machine measures: conduction velocity (how fast the signal travels — slow velocity suggests demyelination, i.e., damage to the nerve's myelin sheath); amplitude (the size of the electrical response — reduced amplitude suggests axonal loss, i.e., the nerve fibre itself is damaged).

The electrical stimulus feels like a brief mild electric shock — described by most patients as "a flick" or "a slight sting." It is not pleasant but not severely painful. The discomfort is brief and stops immediately when the stimulus is off.

What Is EMG (Electromyography)?

EMG measures the electrical activity of muscles — at rest and during voluntary contraction. A fine needle electrode is inserted into specific muscles. The needle is thin (much finer than an injection needle) and records the electrical activity of individual muscle fibres. This can feel like a dull ache when the needle is in the muscle — similar to an intramuscular injection.

At rest, healthy muscle is electrically silent. Denervated muscle (muscle whose nerve supply is damaged) produces abnormal spontaneous activity — "fibrillation potentials" and "positive sharp waves" — detectable on EMG. During voluntary contraction, the pattern of motor unit firing reveals the health of the nerve-muscle connection.

What Conditions Are Diagnosed by NCS/EMG?

• Peripheral neuropathy (diabetic, B12-related, hereditary)
• Carpal tunnel syndrome — compression of the median nerve at the wrist
• Cervical or lumbar radiculopathy — nerve root compression from disc disease
• Guillain-Barré syndrome
• Motor neurone disease (MND)
• Myopathies (muscle diseases) — distinguishes nerve from muscle origin of weakness
• Myasthenia gravis (repetitive nerve stimulation test)
• Ulnar neuropathy, peroneal neuropathy (nerve entrapments)

How to Prepare

Warm hands and feet (cold reduces nerve conduction velocity and can produce false abnormal results — wear warm socks to the appointment). No lotions or creams on the limbs on the day. Continue all regular medications. Tell the doctor if you are on anticoagulants (warfarin, newer anticoagulants) before the EMG needle component.

NCS and EMG at Sri Anand CNC's neuro lab, Chanda Nagar, Hyderabad. Call +91 90633 66983.