Ultrasonographic Assessement Of Diaphragm In Neuromuscular Diseases In Pediatric Patients

The diaphragm is the main muscle of respiration during resting breathing (1), and is formed by two muscles with dual innervation, joined by a central tendon. When it is contracted, the caudal movement increases the volume of the rib cage, generating the negative pressure necessary for inspiratory flow (2). When respiratory demands are increased or diaphragm function is impaired, rib cage muscles and expiratory muscles are progressively recruited. In some patients with diaphragm dysfunction, this compensation is associated with minimal or no respiratory symptoms. In other patients, this compensation is associated with significant respiratory symptoms. Early diagnosis of diaphragmatic dysfunction is essential, because it may be responsive to therapeutic intervention (3). The ultimate causes of diaphragmatic dysfunction can be broadly grouped into three major categories: disorders of central nervous system or peripheral neurons, disorders of the neuromuscular junction and disorders of the contractile machinery of the diaphragm itself (4). So In summary, motion and contractile force of the diaphragm may be affected by pathological alterations of the following anatomical structures:

• - Central nervous system
• - Phrenic nerve
• - Neuromuscular junction
• - Diaphragm muscle
• - Thoracic cage
• - Upper abdomen In patients on mechanical ventilation, the positive end expiratory pressure (PEEP) level also decrease diaphragmatic motion by increasing the end expiratory lung volume and thereby lowering the diaphragmatic dome at the end of expiration (3).

Diaphragm muscle dysfunction is increasingly recognized as an important element of several diseases including neuromuscular diseases leading to a restrictive respiratory pattern (1). The assessment of respiratory muscle function is of paramount interest in patients with neuromuscular disorders. In patients with neuromuscular diseases, respiratory symptoms are subtle and usually appear late in the clinical course of the disease, partly because of the limited mobility of patients due to peripheral muscle weakness, except in the case of acute respiratory failure due to infection. Clinical presentation is quite variable in cases of diaphragmatic failure. Orthopnea may be present and paradoxical abdominal motion may be observed during inspiration, with the abdomen moving inward while the rib cage expands (3). Different structural and functional techniques are available for evaluating the diaphragm. Each technique has its strengths and weaknesses (5). Imaging of respiratory muscles was divided into static and dynamic techniques. Static techniques comprise chest radiography, B-mode (brightness mode) ultrasound, CT and MRI, and are used to assess the position and thickness of the diaphragm and the other respiratory muscles. Dynamic techniques include fluoroscopy, M-mode (motion mode) ultrasound and MRI, used to assess diaphragm motion in one or more directions (6). The recent development of diaphragmatic ultrasound has revolutionized diaphragm evaluation (2). Diaphragm ultrasonography was first described in the late 1960s as a means to determine position and size of supra- and subphrenic mass lesions, and to assess the motion and contour of the diaphragm (1). Two decades later, Wait et al, developed a technique to measure diaphragm thickness based on ultrasonography. Later on the investigators reported a close correlation between diaphragm thickness measured in cadavers using ultrasound imaging and thickness measured with a ruler (7). it has been shown to be similar in accuracy to most other imaging modalities for diaphragm assessment (5), as it can be used to assess bilateral diaphragmatic morphology and function in real time, permitting follow-up without exposure to radiation. It is, moreover, affordable and ubiquitous. (2). First developed in intensive care, mainly for weaning from mechanical ventilation, its use is now extending to pulmonology. Different measurements are described such as diaphragmatic excursion, diaphragmatic thickness and diaphragmatic thickening fraction (8). US measurements of diaphragm muscle thickness and thickening with inspiration have been shown to be superior to phrenic nerve conduction studies (NCS), chest radiographs, and fluoroscopy for detection of neuromuscular disease affecting the diaphragm. The main use in pulmonology is for the respiratory evaluation of patients with neuromuscular diseases, for the search of isolated diaphragmatic impairment and for patients with chronic obstructive lung diseases. Numerous studies are in progress to better determine the role of diaphragmatic ultrasound (5).