Weakness of respiratory and trunk Muscles Usually the diaphragm, chest, and trunk muscles are affected in association with shoulder and proximal limb muscles, but occasionally, isolated weakness of the respiratory muscles is the initial or the dominant manifestation of a muscle disease. Dyspnea and diminished vital capacity first bring the patient to the pulmonary clinic. The main causes are motor neuron disease, myasthenia gravis and less often because of their rarity, glycogen storage disease (acid maltase deficiency—Pompe disease), mitochondrial myopathies, and nemaline myopathy. Polymyositis may cause respiratory weakness, but pulmonary difficulty is more often the result of interstitial lung disease. Unilateral paralysis of the diaphragm may result from compression of the phrenic nerve in the thorax by tumor or aortic aneurysm; an idiopathic or postinfectious variety may be related to brachial plexitis (see Chap. 43). The diaphragm and accessory muscles may be severely affected in some types of muscular dystrophies, but usually in association with pelvocrural and shoulder muscle weakness. Nocturnal dyspnea, sleep apnea, and respiratory arrest may occur, particularly in myasthenics and in patients with glycogen storage myopathies, and respiratory failure may threaten life in severe myasthenia gravis, Guillain-Barré syndrome, and poliomyelitis.
As a general observation, in the acute neuromuscular paralyses, the cervical and shoulder muscles and the diaphragm, all of which share a common innervation, show a similar degree of weakness. Asking the patient to count aloud on 1 maximal breath can help detect diaphragmatic weakness (counting to 20 equates with a vital capacity of approximately 2 L). Paradoxical inward movement of the abdomen with inspiration is another sign of diaphragm weakness. Disorders of breathing and ventilation are discussed in Chap. 25 that discusses respiratory control and Chap. 43 in relation to its most dramatic presentation in the Guillain-Barré syndrome.
Bibrachial palsy and the dangling-arm (flail-arm) syndrome Weakness, atrophy, and fasciculations of the hands, arms, and shoulders characterize the common form of motor neuron disease, ALS. Primary diseases of muscle hardly ever weaken these parts disproportionately. A diffuse weakness of both arms and the shoulder muscles may occur in the early stages of Guillain-Barré syndrome, paraneoplastic neuropathy, and amyloid polyneuropathy, in special forms of immunoglobulin (Ig) M-related paraproteinemic, or in inflammatory polyneuropathy (e.g., brachial neuritis) and porphyric polyneuropathy. A lesion affecting the central portion of the spinal cord in the cervical region produces this same pattern, but in that case there is an associated loss of pain and thermal sensation in the upper limbs and shoulders, signs that exclude disease of muscle.