Sensory nerve conduction in the spinal muscular atrophies

Ultrastructural characterization of peripheral denervation in a mouse model of Type III spinal muscular atrophy

Spinal muscular atrophy (SMA) is a heritable, autosomal recessive neuromuscular disorder characterized by a loss of the survival of motor neurons (SMN) protein, which leads to degeneration of lower motor neurons, and muscle atrophy. Despite SMA being nosographically classified as a motor neuron disease, recent advances indicate that peripheral alterations at the level of the neuromuscular junction (NMJ), involving the muscle, and axons of the sensory-motor system, occur early, and may even precede motor neuron loss. In the present study, we used a mouse model of slow progressive (type III) SMA, whereby the absence of the mouse SMN protein is compensated by the expression of two human genes (heterozygous SMN1A2G, and SMN2). This leads to late disease onset and prolonged survival, which allows for dissecting slow degenerative steps operating early in SMA pathogenesis. In this purely morphological study carried out at transmission electron microscopy, we extend the examination of motor neurons and proximal axons towards peripheral components, including distal axons, muscle fibers, and also muscle spindles. We document remarkable ultrastructural alterations being consistent with early peripheral denervation in SMA, which may shift the ultimate anatomical target in neuromuscular disease from the spinal cord towards the muscle. This concerns mostly mitochondrial alterations within distal axons and muscle, which are quantified here through ultrastructural morphometry. The present study is expected to provide a deeper knowledge of early pathogenic mechanisms in SMA.

Axonal neuropathy in female carriers of the fragile X premutation with fragile x-associated tremor ataxia syndrome

INTRODUCTION

In this study we examined whether females with the fragile X-associated tremor ataxia syndrome (FXTAS) and non-FXTAS premutation carriers have electrophysiological signs of underlying peripheral neuropathy.

METHODS

Nerve conduction studies (NCS) were performed on 19 women with FXTAS, 20 non-FXTAS carriers, and 26 age-matched controls. The results were compared with existing data on corresponding male carriers.

RESULTS

Women with FXTAS and non-FXTAS carriers had reduced sensory nerve action potential amplitudes. Also, there was a strong trend for reduced compound muscle action potential amplitudes in women with FXTAS, but not in non-FXTAS carriers. No significant slowing of nerve conduction velocities, prolongation of F-wave latencies, or associations with molecular measures was observed.

CONCLUSIONS

This study suggests an underlying axonal neuropathy in women with FXTAS. However, in comparison to men with FXTAS, the NCS abnormalities in women were less severe, possibly due to the effect of a normal X chromosome.

Peripheral nerve abnormalities in pediatric patients with spinal muscular atrophy

We examined the specific nerve conduction deficits distinguishing spinal muscular atrophy (SMA) subtypes I and II. Five SMA I patients (age, 0.2-1.1 years) and 10 SMA II patients (age, 1.0-2.8 years) were examined. Patients were compared to age-matched controls for motor and sensory conduction velocity (MCV and SCV) changes, compound muscle and sensory nerve action potential amplitudes (CMAP and SNAP), and F-wave occurrence (FO). Slower MCVs were found in three of five SMA I patients; all five exhibited markedly decreased CMAP amplitudes. Tibial nerve CMAP amplitudes significantly reduced in SMA II patients (p<0.01). Slower SCVs and decreased SNAP amplitudes were observed in three of five SMA I patients but not in SMA II patients. Although FOs were reduced in both extremities of SMA I patients, the reduction was prominent in the tibial nerve of SMA II patients (p=0.031). Loss of motor units may be widespread in the early stage of SMA I, while specific to the legs in young SMA II patients. SMA I showed sensory nerve degeneration, especially of large myelinated fibers. SMA II showed no sensory nerve abnormalities.

Diagnosis and clinical management of spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease characterized by degeneration of lower motor neurons, with resulting progressive muscle weakness. The clinical phenotype and disease severity can be varied and occupy a wide spectrum. Although many advances have been made regarding our understanding of SMA, no cure is yet available. The care of patients who have SMA can often be complex, with many medical issues to consider. When possible, a multidisciplinary team approach is effective. The current understanding of SMA, and the clinical management and rehabilitative care of patients who have SMA, are discussed in this article.

Electrodiagnostic studies in amyotrophic lateral sclerosis and other motor neuron disorders

The clinical electrodiagnostic medicine (EDX) consultant asked to assess patients with suspected amyotrophic lateral sclerosis (ALS) has a number of responsibilities. Among the most important is to provide a clinical assessment in conjunction with the EDX study. The seriousness of the diagnoses and their enormous personal and economic impact require a high-quality EDX study based on a thorough knowledge of and experience with motor neuron diseases (MNDs) and related disorders. Clinical evaluation will help determine which of the EDX tools available to the EDX consultant should be applied in individual patients. Although electromyography (EMG) and nerve conduction study are the most valuable, each of the following may be helpful in the assessment of selected patients based on their clinical findings: repetitive nerve stimulation, motor unit number estimate, single-fiber EMG, somatosensory evoked potential, autonomic function test, and polysomnography. The pertinent literature on these is reviewed in this monograph. The selection and application of these EDX tools depend on a thorough knowledge of the MNDs and related disorders.

Congenital peripheral neuropathy presenting as apnoea and respiratory insufficiency

A seven-week-old girl presented with recurrent apnoeic episodes and respiratory insufficiency. The child was hypotonic, weak, areflexic and had a paradoxical movement of the right hemidiaphragm. Cranial nerve and sphincter function was normal; there was no fasciculation. Nerve conduction studies showed a severe axonal sensory and motor peripheral neuropathy. Biopsy of the sural nerve revealed marked axonal atrophy. The infant is now over two years of age, is ventilator-dependent and has clinical and electrophysiological evidence of further progression of one of the earliest reported presentations of a congenital peripheral neuropathy.

Electromyography and biopsy correlation with suggested protocol for evaluation of the floppy infant

Eighty infants with nonarthrogrypotic floppy infant syndrome (FIS) were evaluated between 1979 and 1990. Electromyographic data were correlated with results of muscle and nerve biopsies in 41 of 80 who had concomitant biopsies (38) or other diagnostic analyses (3). A diagnosis was made of Werdnig-Hoffmann disease (WHD) in 15, a congenital infantile polyneuropathy (IPN) in 3, neuromuscular transmission defect (NMTD) in 2, myopathy in 12, and presumed "central" hypotonia in 9. A very positive correlation rate between nerve conduction studies with electromyography and biopsy results was found in 93% (14 of 15) with WHD and 100% in IPN (3 of 3). However, only 4 of 10 infants (40%) with biopsy-proven myopathy had an abnormal EMG. Only once did the results of electromyography and biopsy conflict.

Predictive value of electromyography in diagnosis and prognosis of the hypotonic infant

To investigate the diagnostic validity of electromyography in the hypotonic infant, 79 children aged 0 to 12 months, seen over a 20-year period, were studied retrospectively. The diagnoses using clinical, muscle biopsy, and laboratory characteristics were: 25 central hypotonia, 20 spinal muscular atrophy, 20 myopathy, four myotonic dystrophy, four benign congenital hypotonia, two congenital muscular dystrophy, two myasthenia gravis, one infantile inflammatory myopathy, and one arthrogryposis multiplex congenita. Using strict criteria, electromyography accurately predicted the final diagnosis in 65% of infants with spinal muscular atrophy and was consistent with the diagnosis in another 25%. In contrast, electromyography accurately predicted the final diagnosis in only 10% of infants with myopathy and was normal in 88% of infants with central hypotonia. In infants with spinal muscular atrophy, there was no difference in the predictive value of electromyography when performed in the newborn compared to older infants. Normal distal nerve conduction velocities in infants with spinal muscular atrophy may predict prognosis, since these infants had a longer survival. Electromyography thus has a high predictive value for infantile spinal muscular atrophy but not for myopathy.

Nerve conduction studies in infants and children

The electrophysiologic evaluation of peripheral nerves may provide critically important information, both with respect to diagnosis and prognosis, in the child with a suspected neuromuscular disorder. However, special attention to various technical considerations is necessary to avoid misleading results. Utilizing these techniques, both hereditary and acquired neuropathies may be identified and characterized. The latter has become especially important in view of recent advances in the treatment of acquired demyelinating neuropathies.