The diaphragm is a dome-shaped muscle positioned at the base of the chest, separating the chest cavity from the abdomen. It functions as the primary muscle for breathing, contracting and flattening during inhalation to create space for the lungs to expand, drawing air inward. As air is exhaled, the diaphragm relaxes and returns to its dome shape, assisting in the expulsion of air from the lungs. Diaphragm weakness refers to a loss of this muscle’s strength, which can impair its ability to contract and relax effectively, thereby hindering normal breathing.
Symptoms and Causes of Diaphragm Weakness
Individuals experiencing diaphragm weakness often report shortness of breath (dyspnea), particularly noticeable during physical activity. This breathlessness often worsens when lying flat (orthopnea), due to abdominal contents pushing against the weakened diaphragm. Other symptoms include paradoxical breathing, where the abdomen moves inward during inhalation instead of expanding outward, disturbed sleep patterns, morning headaches, daytime fatigue, and an increased susceptibility to recurrent pneumonia.
Diaphragm weakness can stem from conditions affecting the muscle itself or the phrenic nerves that control it. Neurological disorders like amyotrophic lateral sclerosis (ALS), multiple sclerosis, and Guillain-Barré syndrome can disrupt nerve signals to the diaphragm. Direct injury or compression of the phrenic nerve is another common cause, often resulting from surgical procedures such as cardiothoracic or cervical surgeries. Trauma, including spinal cord injuries or compression from chest tumors, can also damage these nerves. Muscular dystrophies or conditions like diabetes-related neuropathy can also directly weaken the diaphragm muscle.
Common Non-Invasive Tests
Initial assessments for diaphragm weakness often begin with pulmonary function tests (PFTs). Spirometry, a common PFT, evaluates vital capacity (VC), the maximum amount of air a person can exhale after a maximal inhalation. A significant decrease in VC when comparing measurements taken upright versus supine is a strong indicator of diaphragm weakness, as the diaphragm’s role in breathing becomes more pronounced when lying flat. While a normal individual might see a decline of less than 10-15% in VC when supine, someone with unilateral diaphragm weakness may experience a 10-20% decrease, and those with bilateral weakness could see a more substantial 30-50% reduction.
Maximal Respiratory Pressures are another non-invasive method to gauge respiratory muscle strength. Maximal Inspiratory Pressure (MIP) measures the greatest pressure a person can generate when inhaling forcefully against a blocked airway, reflecting inspiratory muscle strength, including the diaphragm. Similarly, Maximal Expiratory Pressure (MEP) assesses the strength of exhalation muscles. Reduced MIP values, particularly below -80 cmH2O, suggest inspiratory muscle weakness, with unilateral diaphragm paralysis potentially decreasing MIP by 60% and bilateral paralysis by even more, sometimes less than 30% of predicted values.
Imaging tests provide visual information about diaphragm movement. The “sniff test,” performed using diaphragm fluoroscopy, involves a continuous X-ray beam for real-time observation of the diaphragm as the patient breathes normally, deeply, and performs a quick, forceful sniff. This test helps identify paradoxical motion, where the weakened or paralyzed side of the diaphragm moves upward during inhalation instead of downward. Diaphragm ultrasound offers another non-invasive way to visualize the diaphragm, assessing its thickness, contraction, and excursion. This procedure can reveal reduced diaphragm excursion, typically less than 10 millimeters during quiet breathing, or a thickening fraction below 20% during maximal breathing, indicating compromised function.
Specialized Diagnostic Procedures
When initial non-invasive tests suggest diaphragm weakness, specialized procedures may confirm the diagnosis or identify the specific cause. Phrenic Nerve Conduction Studies (PNCS) measure the speed and strength of electrical signals along the phrenic nerves, which activate the diaphragm. During this test, electrical or magnetic stimulation is applied to the neck to assess the nerve’s response, helping determine if nerve damage contributes to the weakness. PNCS results provide insight into the integrity of the nerve pathway.
Diaphragm Electromyography (EMG) directly assesses the electrical activity of the diaphragm muscle. This procedure involves inserting a thin needle electrode into the diaphragm muscle to record its electrical signals. Diaphragm EMG helps differentiate whether the weakness originates from a nerve problem (neuropathy) or a muscle tissue problem (myopathy). While generally safe, the procedure carries a small risk, such as pneumothorax.
Understanding the Test Results
Diagnosing diaphragm weakness involves synthesizing information from a combination of these diagnostic tests. Doctors look for characteristic symptoms, such as shortness of breath that worsens when lying down or during exertion, and observations of paradoxical abdominal movement during breathing. Consistent findings across multiple tests strengthen diagnostic certainty.
A significant drop in vital capacity when moving from a sitting to a supine position, along with reduced maximal inspiratory pressures, points to compromised diaphragm function. Imaging results from a sniff test or ultrasound showing abnormal or paradoxical movement of the diaphragm further support the diagnosis. Finally, specialized tests like phrenic nerve conduction studies and diaphragm electromyography help pinpoint whether the weakness stems from nerve signal transmission or the muscle’s capacity to contract. Integrating these findings allows healthcare providers to confirm the presence and severity of diaphragm weakness and to guide the identification of its underlying cause.