Because it may produce a motor-predominant radiculopathy, Lyme infection is sometimes considered in the differential diagnosis of ALS. Some clinics screen for Lyme antibodies using both an enzyme-linked immunosorbent assay (ELISA) and the more sensitive and specific Western immunoblot, but we have never detected such a case and doubt there is much similarity. Infrequently, we have seen myelopathic motor findings and motor radiculopathy with vitamin B12 deficiency (or copper deficiency), and there are exceptional reports of myeloradiculopathy with lead poisoning; we sometimes include tests for these conditions. Another entity that may simulate ALS is inclusion body myositis (IBM), an atypical myopathy that begins asymmetrically and involves distal muscles, usually without much elevation of serum CPK levels. In a recent series of 70 patients with this condition, 13 percent were initially diagnosed as having ALS (Dabby et al). Features distinguishing the IBM cases included normal corticospinal function, preservation of deep tendon reflexes in weak muscles, and finger flexor weakness. One concludes from this series that a detailed, quantitative EMG and possibly a muscle biopsy are indicated in cases that display predominantly lower motor features. Fully developed ALS is difficult to confuse with these conditions. Acid maltase deficiency may also simulate ALS in causing fatigability and early respiratory failure.
Over the years, the authors have encountered young men with localized and asymmetrical amyotrophy of the forearm that became arrested and did not advance over a decade or two, typical of Hirayama disease. Reports of such a partial cervical spinal amyotrophy have appeared (Hirayama et al; Moreno Martinez et al). In the type described by Hirayama and associates, young men are affected with progressive and asymmetrical amyotrophy of the forearm and hand that has been traced to ligamentous hypertrophy and buckling in the ventral spinal canal. This causes a compression of the cervical spinal cord gray matter, presumably by a chronic ischemic effect as discussed in detail in Chap. 42. In a familial variety of pure restricted amyotrophy, only the vocal cords became paralyzed over a period of years in adult life; only later were the hands affected.
Some patients who have recovered from paralytic poliomyelitis may develop progressive muscular weakness 30 or 40 years later; the nature of this relationship is obscure. We favor the explanation that atrophy of anterior horn cells with aging brings to light a critically depleted motor neuron population (see further on). It appears to progress little if at all.
An observation of interest is the finding of a form of progressive spinal muscular atrophy in patients with GM2 gangliosidosis, the storage disease that presents in infancy as Tay-Sachs disease (Kolodny and Raghavan). The onset is in late adolescence and early adult life and the atrophic paralysis is progressive, so that this condition is often mistaken for Wohlfart-Kugelberg-Welander disease or ALS. A number of cases of this type were discovered in
Ashkenazi Jews by the use of lysosomal enzyme analysis and currently, by genetic testing. The rare and incompletely characterized entity of polyglucosan disease, discussed in other sections of the book, has simulated ALS.
The differential diagnosis of the purely spastic state of primary lateral sclerosis is broad and has been listed earlier. An estimate of the frequency of all the aforementioned alternative diagnoses may be appreciated from a study of cases by Visser and colleagues that were initially presumed to be PMA but turned out to represent another process. In 17 of 89 patients the diagnosis proved to be anti-GM1 motor conduction block, chronic inflammatory demyelinating polyneuropathy, and various myopathies. This notwithstanding, ALS or the more discrete forms of motor system disease rarely offer any difficulty in diagnosis.