Chronic Inflammatory Demyelinating Polyradiculoneuropathy
This disorder has many features in common with AIDP. These patients typically show a subacute or chronic onset lasting more than 4 weeks, and the disorder continues with either a chronic or relapsing course. Electrophysiologic findings generally show more marked slowing of conduction velocity (often below 10 m/s) and elevated stimulation thresholds. As in AIDP, there is evidence of focal conduction block, temporal dispersion, prolongation of DMLs, and prolonged or absent H-wave and F-wave responses. These late responses may be absent due to proximal conduction block. Needle EMG may show a paucity of abnormal spontaneous rest activity and normal or slightly enlarged MUAPs, which exhibit a neuropathic firing pattern.
Axonal GBS/Acute Motor Axonal Neuropathy
In this disorder, children often present with rapid onset, quadriparesis, bulbar dysfunction, and respiratory insufficiency (92). The patients may have inexcitable motor nerves or very low-amplitude CMAPs. The
FIGURE 6.10 Median motor nerve conduction in a 4-year-old child with Guillain-Barre syndrome. Distal latency is prolonged at 16.9 milliseconds, and conduction velocity is slowed at 9 m/s. Note the conduction block (amplitude drop from 2.734 to 0.260 mV) and temporal dispersion.
author has observed such a case with clinical findings mimicking cerebral death (93). The child had combined demyelinating and axonal findings and eventually had near complete recovery over 18 months. In general, children with the axonal form of GBS are more likely to require assisted ventilation, develop severe quadriparesis, and require a much longer period of time to become ambulatory. Campylobacter jejuni has been implicated as a precipitating agent in many cases.
One recent study (94) evaluated an electrophysiologic classification of a GBS population into demyelinating and axonal subtypes, to investigate how serial recordings changed the classification and to underline the pitfalls in electrodiagnosis of GBS subtypes. In the first test, the electrodiagnostic findings resulted in 65% to 67% of patients classifiable as demyelinating AIDP, 18% classifiable as axonal GBS, and 14% to 16% equivocal. At follow-up, 24% of patients changed classification: AIDP decreased to 58%, axonal GBS increased to 38%, and equivocal patients decreased to 4%. The majority of shifts were from AIDP and equivocal groups to axonal GBS, and the main reason was the recognition by serial recordings of the reversible conduction failure and of the length-dependent CMAP amplitude reduction patterns as expression of axonal pathology. Axonal GBS is pathophysiologically characterized not only by axonal degeneration but also by reversible conduction failure at the axolemma of the Ranvier node. The lack of distinction among demyelinating conduction block, reversible conduction failure, and length-dependent CMAP amplitude reduction may fallaciously classify patients with axonal GBS as having AIDP. Serial electrophysiologic studies are mandatory for proper diagnosis of GBS subtypes and the identification of pathophysiologic mechanisms of muscle weakness. More reliable electrodiagnostic criteria taking into consideration the reversible conduction failure pattern should be devised.
Neuropathies Associated With Central Disorders
A variety of metabolic disorders produce abnormalities of both the central and peripheral nervous system. Abnormalities of lipid metabolism such as metachromatic leukodystrophy may produce a severe demyelinating peripheral neuropathy with electrophysiologic findings of high stimulation threshold and low CVs. Somatosensory evoked potentials may show both central and peripheral delay, and visual evoked potentials show central delay. Other disorders showing both central and peripheral nervous system involvement include Krabbe disease, Refsum's disease (phytanic acid storage disease), Tangier disease (hereditary high-density lipoprotein deficiency), A-beta lipoproteinemia (a vitamin E deficiency syndrome), Fabry's disease (alpha galactosidase A deficiency), Niemann-Pick disease (a variant of sphingomyelin lipidoses), peroxisomal disorders such as adre-noleukodystrophy porphyria, which produces axonal degeneration of predominantly motor fibers, and tyrosinemia, which produces primary axonal degeneration with secondary segmental demyelination.
Krabbe disease is associated with marked central and peripheral demyelination and NCSs typically show a mixed sensorimotor demyelinating peripheral neuropathy. The peripheral neuropathy occurs very early in the neonatal period in Krabbe disease and affects the nerves uniformly. NCSs may provide a highly sensitive tool to screen this patient population (95).
In ataxia telangiectasia, there is a loss of large, predominantly sensory, myelinated fibers due to a primary axonal degeneration. In Friedreich's ataxia, an autosomal recessive condition, there is a primary axonal degeneration of peripheral nerve fibers producing reduced or absent sensory compound action potential amplitudes.