Myopathy and Motor Neuron Disorders

The first part of this chapter deals with the main varieties of muscular dystrophy and their differential diagnosis, many of which are instantly recognizable. Handles are given for the main neuromuscular junction disorders including, myasthenia gravis and Lambert-Eaton myasthenic syndrome. Inflammatory and dysthyroid myopathies are evaluated. Also appraised are periodic paralysis, McArdle’s syndrome, and mitochondrial, distal, and medication-induced myopathy. In the second section, motor neuron disorders are discussed, in particular the characteristic features of amyotrophic lateral sclerosis and its mimics as well as various forms of bulbar palsy. Hereditary spastic paraplegia and its variants are discussed and how they can simulate other diseases.

Myopathy and Motor Neuron Disorders

This chapter deals with the main varieties of muscular dystrophy and their differential diagnosis, including dystrophia myotonica, Duchenne muscular dystrophy, Becker muscular dystrophy, facioscapulohumeral muscular dystrophy, limb girdle muscular dystrophy, Emery-Dreifuss muscular dystrophy and oculopharyngeal muscular dystrophy. Diagnostic clues are given for the main neuromuscular junction disorders, including myasthenia gravis and Lambert-Eaton myasthenic syndrome. Inflammatory and dysthyroid myopathies are evaluated. Also appraised are periodic paralysis, McArdle’s syndrome, mitochondrial, distal, and medication-induced myopathy. The characteristic features of amyotrophic lateral sclerosis and its mimics are debated as well as various forms of bulbar palsy. Hereditary spastic paraplegia and its variants are also included in the chapter.

Potassium and ventilation in exercise

The drive to breathe in exercise is thought to result from a combination of neural and humoral factors, but the exact nature of the controlling signals is controversial. This review examines evidence suggesting that potassium could be a signal that drives ventilation (VE) in exercise. Potassium is lost from working muscle, which results in a marked hyperkalemia in the arterial plasma. The rise in potassium is directly proportional to the increase in carbon dioxide production during exercise and is also well correlated with VE in normal subjects and in subjects who do not produce acid (McArdle's syndrome). In the anesthetized and decerebrate cat, physiological levels of hyperkalemia stimulate VE by direct excitation of the peripheral arterial chemoreceptors, because surgical and chemical denervation of the chemoreceptors abolishes the increase in VE caused by potassium. The effect of hyperkalemia on chemoreceptor activity is further enhanced by hypoxia, but an abrupt switch to hyperoxia removes this effect. From these studies, it is suggested that potassium fulfills many of the criteria of being the special substance or “work factor” that was postulated over a century ago to stimulate VE in exercise. Although there is no direct proof that potassium causes an increase in breathing during exercise, circumstantial evidence strongly supports the idea that it should be considered as a stimulus to exercise hyperpnea.