Distal bilateral limb palsies presenting as foot-drop with steppage gait (with or without pes cavus), weakness of all lower leg muscles, and later wrist-drop and weakness of hands The principal cause of this syndrome is a familial polyneuropathy, mainly of the Charcot-Marie-Tooth type (see Chap. 43); the course is over decades. Also presenting in this way are paraproteinemic and inflammatory polyneuropathies, with or without motor conduction block and exceptionally, some forms of familial progressive muscular atrophy and distal types of progressive muscular dystrophy, and sarcoid myopathy. In myotonic dystrophy, there may be weakness of the leg muscles as well as the forearms, sternocleidomastoids, face, and eyes. With these exceptions, the generalization that girdle weakness without sensory changes is indicative of myopathy and that distal weakness is indicative of neuropathy is clinically useful.
Generalized or universal paralysis: Limb (but usually not cranial) muscles, involved either in attacks or as a chronic persistent, progressive deterioration (see also Chap. 46) When acute in onset and episodic, this syndrome is usually a manifestation of familial or acquired hypokalemic or hyperkalemic periodic paralysis. One variety of the hypokalemic type is associated with hyperthyroidism, another with hyperaldosteronism. Attacks of porphyric neuropathy and of Refsum disease with generalized weakness have an episodic nature. Widespread paresis (rather than paralysis) that has an acute onset and lasts many weeks is at times a feature of a severe form of idiopathic or parasitic (trichinosis) polymyositis and, rarely, of the toxic effects of certain pharmaceutical agents, particularly those used to treat hypercholesterolemia. Idiopathic polymyositis and, rarely, IBM may involve all limb and trunk muscles, but usually spare the facial and ocular muscles, whereas the weakness in trichinosis is mainly in the ocular and lingual muscles. In infants and young children, a chronic and persistent generalized weakness of all muscles, except those of the eyes, always raises the question of Werdnig-Hoffman spinal muscular atrophy or, if milder in degree and relatively nonprogressive, of one of the congenital myopathies or polyneuropathies. In these diseases of infancy, paucity of movement, hypotonia, and retardation of motor development may be more obvious than weakness, and there is arthrogryposis at birth.
Paralysis of single muscles or a group of muscles This is usually neuropathic, less often spinal or myopathic. Muscle disease does not need to be considered except in certain instances of pressure-ischemic necrosis of muscle as a result of local pressure or infarction, as in monoplegic alcoholic myopathy or in diabetic muscle infarction. The weakness of IBM has a preference for certain sites, specifically parts of the quadriceps, or of the forearm muscles, particularly the long finger flexors (flexor digitorum profundus), and also therefore enters into consideration.
From this exposition of the topographic aspects of weakness, one can appreciate that each neuromuscular disease exhibits a predilection for particular groups of muscles. Apart from these patterns that suggest certain possibilities of disease and exclude others, diagnosis depends on the age of the patient at the time of onset and tempo of progression, the coexistence of medical disorders, certain laboratory findings (serum concentrations of muscle enzymes, EMG, and biopsy findings), and genetic determinants.