The diaphragm is a dome-shaped structure located beneath the lungs that serves as the primary muscle responsible for respiration. When a person inhales, this muscle contracts and flattens, moving downward to increase the volume of the chest cavity. This action creates a negative pressure gradient, pulling air into the lungs. Conversely, when the muscle relaxes and returns to its dome-like position, it pushes air out during passive exhalation. Diaphragm weakness or paralysis occurs when this mechanical function is compromised, often due to damage to the phrenic nerve or underlying neuromuscular diseases. Specific diagnostic tests are required to accurately measure the degree of muscle dysfunction and determine the appropriate course of treatment.
Clinical Indicators for Testing
The most common symptom prompting diaphragm function testing is unexplained shortness of breath (dyspnea), particularly during physical activity. Another specific indicator is orthopnea, which is difficulty breathing that worsens when the patient lies flat. In the supine position, abdominal contents push against the weakened diaphragm, impeding its movement and decreasing lung capacity.
A physical examination may reveal paradoxical abdominal movement, a distinct sign of weakness. During inhalation, a functioning diaphragm pushes the abdomen outward, but a paralyzed diaphragm is sucked upward into the chest, causing the abdomen to move inward. Patients may also report chronic fatigue, morning headaches, or recurrent respiratory infections. Testing is often initiated in patients with conditions known to affect the phrenic nerve, such as Amyotrophic Lateral Sclerosis (ALS), Guillain-Barré syndrome, or those recovering from cervical or cardiothoracic surgery.
Standard Screening and Imaging Methods
The initial step in assessing diaphragm function often involves simple imaging, starting with a standard Chest X-ray. A static X-ray may show an elevated hemidiaphragm, suggesting weakness or paralysis, especially if one side is two centimeters higher than the other. However, this static image cannot confirm whether the muscle is actually moving.
Dynamic assessment frequently uses the fluoroscopic sniff test, a real-time X-ray examination. During this procedure, the patient is asked to inhale forcefully and quickly, a maneuver that normally causes both sides of the diaphragm to descend. A positive result for paralysis occurs when the weakened hemidiaphragm moves paradoxically upward into the chest cavity, pulled by the negative pressure generated by the healthy side. This test is considered the imaging standard for unilateral paralysis, but it relies on the patient’s cooperation and effort.
Diaphragm Ultrasound provides a non-invasive, radiation-free alternative for functional assessment. This technique measures two parameters: diaphragm excursion and Diaphragm Thickening Fraction (DTF). Excursion measures the distance the muscle travels downward during inspiration. The DTF is calculated as the percentage change in thickness between end-expiration and end-inspiration. A paralyzed or chronically weak diaphragm will often fail to thicken during inhalation, providing numerical evidence of dysfunction.
Quantitative Functional Assessment
To obtain objective, numerical data on the force-generating capacity of the respiratory muscles, physicians employ specific pressure measurements.
Maximal Inspiratory Pressure (MIP)
The Maximal Inspiratory Pressure (MIP) test is a common tool, performed by having the patient inhale with maximum effort against a closed mouthpiece connected to a pressure manometer. MIP measures the strength of all inspiratory muscles collectively. A value greater than -80 cmH₂O in adults is generally used to rule out clinically significant inspiratory muscle weakness. The primary limitation of this test is its reliance on the patient’s maximal effort and ability to cooperate, which can introduce variability.
Transdiaphragmatic Pressure (Pdi)
A more specific and invasive method is the measurement of Transdiaphragmatic Pressure (Pdi), which isolates the mechanical force generated by the diaphragm itself. This technique involves inserting two pressure catheters with balloons: one into the esophagus to approximate chest cavity pressure, and one into the stomach to measure abdominal pressure. Pdi is calculated by subtracting the esophageal pressure from the gastric pressure, quantifying the pressure gradient created across the diaphragm during contraction. Measuring Pdi during a maximal sniff maneuver (Pdi,sniff) is a reliable method for assessing diaphragmatic force, though it still requires patient effort.
Phrenic Nerve Stimulation
Phrenic Nerve Stimulation assesses the diaphragm’s contractility independent of patient effort. This procedure involves applying electrical or magnetic stimulation to the phrenic nerve in the neck, triggering an involuntary muscle contraction. The resulting force, measured as the twitch transdiaphragmatic pressure (Pdi,tw), is considered the definitive standard for quantifying maximal diaphragm strength. By comparing the force generated during voluntary effort (MIP or Pdi,sniff) with the involuntary force (Pdi,tw), clinicians can distinguish between true muscle weakness and an effort-related limitation.
Interpretation of Results and Next Steps
The confirmed diagnosis of diaphragm weakness relies on integrating dynamic imaging findings with objective pressure measurements. A Maximal Inspiratory Pressure (MIP) less than 30% of the predicted value is a strong indicator of severe bilateral diaphragm weakness. A definitive sign of severe dysfunction is a low twitch transdiaphragmatic pressure (Pdi,tw), with values typically falling below 8 cmH₂O. While unilateral weakness may result in a mild reduction in total lung capacity, bilateral weakness can reduce it to 50% or less of predicted capacity.
Following diagnosis, management focuses on alleviating symptoms and addressing the underlying cause. Treatment often includes the use of non-invasive positive pressure ventilation, such as CPAP, to support nocturnal breathing and improve gas exchange. For patients experiencing persistent symptoms from a paralyzed hemidiaphragm, surgical plication may be considered to tighten the weakened side and prevent its paradoxical motion.