Persistent and transient “conduction block” in motor neuron diseases

Abstract

Although conduction block indicates dysfunction of peripheral nerve, it may occur in patients with clinically typical motor neuron disease. There are no universally accepted criteria to identify conduction block, so diagnosis may be difficult. In some peripheral neuropathies, conduction block persists over long periods of time. If conduction block persists in motor neuron disease, then a more reproducible means for identification would be available. We repeatedly studied 9 patients with different forms of motor neuron diseases; conduction block was suspected because of excessive loss of the amplitude of motor evoked responses between distal and proximal stimulation sites. Five showed persistent amplitude loss at intervals between 12 and 36 months. All had focal loss of amplitude and area across a specific segment; all were men; none had definite upper motor neuron signs, 2 had probable and 3 had no upper motor neuron signs; 1 had IgM paraproteinemia, one elevated anti-GM1 titers; the duration of symptoms spanned 4-13 years. Four patients had transient loss of amplitude that was not reproduced in intervals between 3 and 13 months. None had focal loss of both amplitude and area; 2 were men; all had definite upper motor neuron signs and none had symptoms for more than 3-13 months; and none had immunological abnormalities. Thus, patients with persistent amplitude loss fulfill other criteria for conduction block, have prolonged survival but otherwise have clinical syndromes indistinguishable from ALS, except that definite upper motor neuron signs seem to be exceptional.

Criteria for early detection of conduction block in multifocal motor neuropathy (MMN): a study based on control populations and follow-up of MMN patients

“…When an abnormal amplitude reduction in the proximal CMAP owing to temporal dispersion is present in conditions that cause chronic partial denervation, such as motor-neuron disease [13], conduction block cannot be reliably assessed [9]. In our ALS cases, as well as in a larger study performed on 133 patients [1], the motornerve conduction abnormalities never approached levels suggestive of demyelination and MCBs were not evident.…”

Central motor conduction time in patients with multifocal motor conduction block

“…In these patients, the abnormal amplitude reduction of the CMAP elicited with proximal nerve stimulation may be due, at least in part, to temporal dispersion and phase cancellation in a hypoexcitable nerve, with a reduced number of axons and zones of demyelination. 20,24,26 According to several investigators, true CB can be distinguished from CMAP amplitude reduction due to other factors by requiring a decrease of at least 50% in amplitude of the negative phase, with less than 30% increase in duration, in a segment that is less than 25 cm long. 24,26 These criteria were met by all our patients, who also exhibited CBs in at least two nerves, and most of them in more than one nerve segment.…”

Nerve conduction studies in amyotrophic lateral sclerosis

“…31,32 Certain motor conduction abnormalities, nevertheless, are well recognized in ALS. The distal motor latency may be increased, 3,36 a feature that has been correlated with the degree of atrophy, 17,23,37,49 but which may also occur in muscles that are only mildly affected.…”