Diaphragm electromyography (EMG) is a specialized diagnostic test that measures the electrical activity produced by the diaphragm, the primary muscle responsible for breathing. The procedure assesses the function of the muscle and the phrenic nerve that controls it. By recording electrical signals during respiration, the test provides an objective measure of muscle strength and nerve signal transmission speed, helping physicians localize the source of breathing difficulties.
Understanding the Diaphragm’s Role in Respiration
The diaphragm is a dome-shaped sheet of muscle situated at the base of the chest cavity, separating the chest from the abdomen. It receives its motor command exclusively from the phrenic nerve, which originates from the third, fourth, and fifth cervical spinal nerves (C3-C5) in the neck.
When the phrenic nerve signals, the muscle contracts and moves downward, flattening toward the abdomen. This action increases the volume of the chest cavity, creating negative pressure inside the lungs. The resulting pressure gradient forces air to rush in, completing inhalation. When the signal stops, the muscle relaxes and passively returns to its dome shape, pushing air out during quiet exhalation.
Clinical Reasons for Testing
A physician typically orders a Diaphragm EMG when a patient experiences unexplained shortness of breath (dyspnea), particularly when lying down (orthopnea). The evaluation is helpful in cases of suspected phrenic nerve injury or palsy, where nerve damage prevents correct muscle stimulation.
The procedure helps pinpoint the problem’s location, distinguishing between a neuropathic cause (a disorder affecting the phrenic nerve) or a myopathic cause (a primary disease of the muscle tissue). Generalized neuromuscular disorders, such as Amyotrophic Lateral Sclerosis (ALS) or Myasthenia Gravis, often involve the diaphragm; the EMG provides specific data on the extent of this respiratory muscle involvement. This objective assessment of functional status is important for managing respiratory failure or when weaning a patient from ventilator support.
How the Diaphragm EMG Procedure is Performed
The Diaphragm EMG is an outpatient procedure that generally takes about 30 minutes. Patients are often asked to fast for a few hours before the test to ensure the stomach is empty, as the most accurate method involves placing a sensor near the muscle. Patients must also discuss all medications with their physician beforehand, as certain drugs, particularly blood thinners, may need temporary adjustment.
The electrical activity is measured using two primary methods:
Esophageal catheter: This is a thin, flexible tube containing electrodes inserted through the nose or mouth. The catheter is advanced until the electrodes are positioned adjacent to the diaphragm, providing a high-quality signal less prone to interference. The nasal passage may be numbed with a topical spray to increase comfort during insertion.
Surface electrodes: These are small adhesive patches placed on the skin over the lower rib cage and upper abdomen, typically on the right side to minimize heart interference. While less invasive, these electrodes may pick up signals from other nearby respiratory muscles, making the esophageal catheter the preferred method for precise readings.
During the test, the patient performs various breathing maneuvers, such as quiet breathing and deep maximal breaths, while the electrodes record the electrical signals.
Analyzing the Results
The recorded electrical signals provide three primary parameters used to analyze the diaphragm’s function:
Amplitude: This measures the strength of the electrical signal, often called the Compound Muscle Action Potential (CMAP). Low amplitude suggests muscle weakness or a reduction in contracting muscle fibers, pointing toward a myopathic disorder.
Latency: This measures the time required for the signal to travel from the phrenic nerve stimulation point to the diaphragm muscle. Prolonged latency indicates nerve damage and slow signal conduction, a hallmark of phrenic nerve injury. If the nerve is non-functional, no signal may be recorded.
Recruitment Pattern: This refers to the rate and orderly fashion in which individual motor units (a nerve fiber and the muscle fibers it controls) are activated. An abnormal pattern, such as reduced active motor units or excessive spontaneous activity at rest, can signify chronic nerve damage or denervation.