Electromyography (EMG), electroneurography (ENG)

Understanding EMG and Nerve Conduction Studies (NCS/ENG)

Electromyography (EMG) and Nerve Conduction Studies (NCS), also sometimes referred to as Electroneurography (ENG), are essential electrodiagnostic tests used to evaluate the health and function of peripheral nerves and muscles. These tests measure the electrical activity generated by muscles and the speed and efficiency with which electrical signals travel along nerve pathways. They are often performed together during the same appointment as they provide complementary information about the neuromuscular system.

Nerve Conduction Studies (NCS / ENG)

NCS primarily assess the function of peripheral nerves. During this part of the test, small electrodes are taped to the skin over specific nerves. A stimulating electrode delivers brief, mild electrical impulses to the nerve, while recording electrodes detect the nerve's response further along its pathway. By measuring the time it takes for the signal to travel between points and the strength (amplitude) of the response, the physician can determine the speed and integrity of nerve signal conduction. This helps identify nerve damage (neuropathy), locate the site of injury (e.g., nerve compression like in carpal tunnel syndrome), and differentiate between types of nerve damage (e.g., affecting the myelin sheath vs. the nerve axon).

Electromyography (EMG)

EMG directly assesses the electrical activity within muscles. This part of the test involves inserting a very fine, sterile needle electrode (similar in size to an acupuncture needle) through the skin into specific muscles. The electrode detects the electrical signals generated by muscle fibers both when the muscle is at rest and during voluntary contraction. Abnormal electrical activity at rest or characteristic changes in the signals during contraction can indicate nerve damage supplying the muscle (denervation), muscle disease (myopathy), or problems with the neuromuscular junction (where nerve signals are transmitted to muscle).

While NCS involves mild electrical sensations often described as tapping or tingling, and EMG involves needle insertion which can cause temporary discomfort similar to an injection, the procedures are generally well-tolerated by most patients. They provide invaluable diagnostic information that often cannot be obtained through other means.

What Information Do These Tests Provide?

EMG and NCS are highly informative and help clinicians to:

• Detect nerve damage or disease: Confirm the presence of neuropathy or nerve injury.
• Detect muscle disease: Identify primary muscle disorders (myopathies).
• Localize the problem: Pinpoint the specific nerves or muscles affected (e.g., distinguishing carpal tunnel syndrome from a neck root problem).
• Determine the type of nerve injury: Differentiate between problems affecting the nerve axon (the core fiber) versus the myelin sheath (the insulating cover), which impacts prognosis and potential treatment.
• Assess severity: Quantify the degree of nerve or muscle damage.
• Evaluate chronicity: Distinguish between acute (recent) and chronic (long-standing) processes.
• Monitor progression or recovery: Track changes in nerve or muscle function over time or after treatment.

These tests are often performed following a thorough neurological examination and may be used in conjunction with imaging studies like MRI to provide a complete picture of the patient's condition and guide further management, including decisions about surgery.

Common Indications for EMG/NCS

EMG and NCS are frequently used to diagnose or evaluate symptoms such as numbness, tingling, pain, weakness, or cramping. Common conditions investigated include:

• Mononeuropathies (Damage to single nerves):
  • Carpal tunnel syndrome (Median nerve at the wrist)
  • Cubital tunnel syndrome (Ulnar nerve at the elbow)
  • Peroneal (common fibular) nerve neuropathy (Often causing foot drop)
  • Bell's palsy (facial nerve neuropathy)
  • Radial nerve neuropathy
  • Median nerve neuropathy (other locations)
  • Ulnar nerve neuropathy (other locations)
  • Sciatic nerve neuropathy
  • Tibial nerve neuropathy
  • Trigeminal nerve neuropathy (less common use for EMG/NCS, often clinical/MRI based)
• Polyneuropathies (Widespread nerve damage):
  • Diabetic neuropathy
  • Alcoholic neuropathy
  • Toxic neuropathies
  • Guillain-Barré syndrome (acute inflammatory neuropathy)
  • Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)
• Radiculopathies (Pinched nerves in the spine):
  • Cervical radiculopathy (neck) - causing Cervicalgia and brachialgia
  • Lumbar radiculopathy (lower back) - often causing Sciatica
• Traumatic nerve injuries
• Myopathies (Primary muscle diseases): e.g., muscular dystrophy, polymyositis
• Neuromuscular junction disorders: e.g., Myasthenia Gravis, Lambert-Eaton syndrome
• Motor Neuron Diseases: e.g., Amyotrophic Lateral Sclerosis (ALS)
• Neuralgia, neuralgic pain (To identify underlying nerve damage, though often clinical diagnosis, e.g., Trigeminal neuralgia)

References

1. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic Correlations. 4th ed. Elsevier; 2021. (Standard textbook for EMG/NCS)
2. Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 4th ed. Oxford University Press; 2013. (Classic comprehensive reference)
3. Stålberg E, Nandedkar SD, Sanders DB, Falck B. Quantitative motor unit potential analysis. J Clin Neurophysiol. 1996 Jul;13(4):401-22. DOI: 10.1097/00004691-199607000-00001. PMID: 8814801. (Focuses on quantitative EMG analysis)
4. American Association of Neuromuscular & Electrodiagnostic Medicine (AANEM). Recommended policy for electrodiagnostic medicine. Muscle Nerve. 2015 Feb;51(2):304-5. DOI: 10.1002/mus.24528. (Guidelines on practice)
5. Mallik A, Weir AI. Nerve conduction studies: essentials and pitfalls in practice. J Neurol Neurosurg Psychiatry. 2005 Jun;76 Suppl 2(Suppl 2):ii23-31. DOI: 10.1136/jnnp.2005.069138. PMID: 15961865. (Practical guide to NCS)