Incidence, severity, and time-course of motoneurone dysfunction in myotonic dystrophy: their significance for an understanding of anticipation

Association of peripheral neuropathy with sleep-related breathing disorders in myotonic dystrophies

BACKGROUND

Myotonic dystrophy (DM) type 1 and type 2 are inherited diseases characterized by myotonia and myopathy. Additional symptoms include, among others, peripheral neuropathy and sleep-related breathing disorders (SRBDs). There is growing evidence for a complex association between DM1 and DM2, which was described in patients with diabetes mellitus and in the general population. In this study, we investigated whether there is an association between peripheral neuropathy and SRBDs also in the population of patients with DM.

METHODS

The study included 16 patients with DM1 (mean age, 37.9±14.1 years; 20-69 years) and eight patients with DM2 (mean age, 47.6±14.1 years; 20-65 years), who underwent a sensory and motor nerve conduction study (NCS) and diagnostic screening for SRBDs. In both groups, the NCS parameters were correlated with respiratory parameters.

RESULTS

In both groups, the amplitude of the ulnar sensory nerve action potential (SNAP) correlated with the mean arterial oxygen saturation (SaO₂). In addition, in the DM2 group, the median SNAP correlated with the mean SaO₂. In the DM1 group, the median SNAP and the distal motor latency (DML) of the ulnar nerve correlated with the apnea-hypopnea index, while the oxygen desaturation index correlated with the DML of the tibial nerve and with conduction velocity in the sural nerve.

CONCLUSION

Our results indicate a complex association between neuropathy and SRBDs in DM1 and DM2. Axonal degeneration may contribute to nocturnal hypoxemia and vice versa. Neuropathy may contribute to muscle weakness, which in turn may cause respiratory events.

Peripheral nerve involvement in myotonic dystrophy type 2 - similar or different than in myotonic dystrophy type 1?

INTRODUCTION

Multisystem manifestations of myotonic dystrophies type 1 (DM1) and 2 (DM2) are well known. Peripheral nerve involvement has been reported in DM1 but not in genetically confirmed DM2. The aim of our study was to assess peripheral nerve involvement in DM2 using nerve conduction studies and to compare these results with findings in DM1.

METHODS

We prospectively studied patients with genetically confirmed DM2 (n=30) and DM1 (n=32). All patients underwent detailed neurological examination and nerve conduction studies.

RESULTS

Abnormalities in electrophysiological studies were found in 26.67% of patients with DM2 and in 28.13% of patients with DM1 but the criteria of polyneuropathy were fulfilled in only 13.33% of patients with DM2 and 12.5% of patients with DM1. The polyneuropathy was subclinical, and no correlation was found between its presence and patient age or disease duration.

CONCLUSIONS

Peripheral nerves are quite frequently involved in DM2, but abnormalities meeting the criteria of polyneuropathy are rarely found. The incidence of peripheral nerve involvement is similar in both types of myotonic dystrophy.

How do C9ORF72 repeat expansions cause amyotrophic lateral sclerosis and frontotemporal dementia: can we learn from other noncoding repeat expansion disorders?

PURPOSE OF REVIEW

The aim of this review is to describe disease mechanisms by which chromosome 9 open reading frame 72 (C9ORF72) repeat expansions could lead to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and to discuss these diseases in relation to other noncoding repeat expansion disorders.

RECENT FINDINGS

ALS and FTD are complex neurodegenerative disorders with a considerable clinical and pathological overlap, and this overlap is further substantiated by the recent discovery of C9ORF72 repeat expansions. These repeat expansions are currently the most important genetic cause of familial ALS and FTD, accounting for approximately 34.2 and 25.9% of the cases. Clinical phenotypes associated with these repeat expansions are highly variable, and combinations with mutations in other ALS-associated and/or FTD-associated genes may contribute to this pleiotropy. It is challenging, however, to diagnose patients with C9ORF72 expansions, not only because of large repeat sizes, but also due to somatic heterogeneity. Most other noncoding repeat expansion disorders share an RNA gain-of-function disease mechanism, a mechanism that could underlie the development of ALS and/or FTD as well.

SUMMARY

The discovery of C9ORF72 repeat expansions provides novel insights into the pathogenesis of ALS and FTD and highlights the importance of noncoding repeat expansions and RNA toxicity in neurodegenerative diseases.

Peripheral neuropathy is linked to a severe form of myotonic dystrophy in transgenic mice

Myotonic dystrophy type 1 (DM1) is a multisystem disorder with a variable phenotype. The involvement of peripheral nerves in DM1 disease is controversial. The DM1 animal model DM300 transgenic mice that carry 350 to 500 CTG repeats express a mild DM1 phenotype but do not exhibit motor or sensory pathology. Here, we investigated the presence or absence of peripheral neuropathy in transgenic mice (DMSXL) that carry more than 1,300 CTG repeats and display a severe form of DM1. Electrophysiologic, histologic, and morphometric methods were used to investigate the structure and function of peripheral nerves. We observed lower compound muscle action potentials recorded from hind limb muscles and slowing of sciatic nerve conduction velocity in DMSXL versus control mice. Morphometric analyses showed an axonopathy and neuronopathy in the DMSXL mice characterized by a decrease in numbers of myelinated motor axons in sciatic nerve and in spinal cord motor neurons. Pathologic alterations in the structure of hind limb neuromuscular junctions were also detected in the DMSXL mice. These results suggest that peripheral neuropathy can be linked to a large CTG expansion and a severe form of DM1.

Peripheral nerve axon involvement in myotonic dystrophy type 1, measured using the automated nerve excitability test

Background and Purpose

Primary involvement of the peripheral nerves in myotonic dystrophy type I (MyD1) is controversial. We investigated whether the involvement of peripheral nerves is a primary event of MyD1 or secondary to another complication such as diabetes mellitus (DM).

Methods

The subjects comprised 12 patients with MyD1, 12 with DM and no peripheral nerve involvement, and 25 healthy volunteers. We measured multiple excitability indices in the median motor axons. The strength-duration time constant was calculated from the duration-charge curve, the threshold electrotonus and current-threshold relationships were calculated from the sequential subthreshold current, and the recovery cycle was derived from double suprathreshold stimulation.

Results

The depolarizing and hyperpolarizing threshold electrotonus were significantly reduced and exhibited increased refractoriness in the MyD1 group compared with the DM and control groups. The SDTC, superexcitability, and subexcitability were not significantly altered in the MyD1 group.

Conclusions

The MyD1 group exhibited a depolarized axonal membrane potential. The significant differences in peripheral nerve excitability between the MyD1 group and the DM and normal control groups suggest that peripheral neuropathy is a primary event in MyD1 rather than a secondary complication of DM.

Motor unit estimation: anxieties and achievements

The history of motor unit number estimation (MUNE) is given, together with brief descriptions of the various methods presently available. A small muscle of the hand contains about 100 motor units and greater numbers are found in larger muscles; beyond 60 years the numbers begin to decline. In ALS approximately half the motor units cease to function within 6 months of the involvement of the motoneuron pool, while in adult spinal muscular atrophy further loss may not occur over several years. The reduction in MUNE values in myotonic dystrophy remains an enigma, but even more curious are the losses and subsequent recoveries occasionally observed in hyperthyroidism and chronic renal failure; possibly, nontransmitting ("silent") synapses are involved. MUNE may also be used to study CNS problems such as hemiplegia and congenital brachial palsy. The availability of more powerful computers for EMG should lead to advances in MUNE.

Correlation between CTG trinucleotide repeat length and frequency of severe congenital myotonic dystrophy

The myotonic dystrophy (DM) mutation has recently been identified as an unstable trinucleotide CTG repeat which is present 5-30 times in the normal population but which is amplified up to 2,000 times in DM. We have determined the status of the CTG repeat in 272 DM individuals. Infants with severe congenital DM, as well as their mothers, are shown to have on average a greater amplification of the CTG repeat than is seen in the noncongenital DM population. This fact, when viewed in conjunction with the tendency to increased CTG repeat length in our DM kindreds, provides evidence for the existence of genetic anticipation in the transmission of DM.

Invited review: motor unit estimation: methods, results, and present status

The renewed interest in motor unit estimation (counting) has coincided with the introduction of computer-based methodology and with the application of the technique to proximal as well as distal muscles. The advantages and disadvantages of the different methods are considered, together with the assumptions inherent in this type of examination. In normal subjects, the extensor digitorum brevis (EDB) muscle has approximately 200 motor units while each of the intrinsic muscles of the hand has about 100 units; larger muscles in the limbs contain greater numbers of units. Beyond the age of 60 years, there is a decline in the number of functioning motor units in both proximal and distal muscles. In denervating disorders, motor unit estimation is useful for diagnosis and assessment; abnormal values may often be observed in muscles judged clinically to be unaffected. Serial studies have enabled the rate of motor unit loss to be determined in ALS and in spinal muscular atrophy. Depletion of motor units has also been found following upper motoneuron lesions caused by injury to the spinal cord or by cerebral hemorrhage; trans-synaptic dysfunction has been presumed responsible. Rather surprisingly, reduced numbers of motor units have been observed in a variety of myopathic disorders; of these, the most consistent abnormalities have been reported in myotonic muscular dystrophy.

A review of techniques employed to estimate the number of motor units in a muscle

Being the smallest functional units under neural control, motor units play an integral role in muscle physiology. However, at the present time, there does not exist any widely accepted technique for quantifying or estimating the number of motor units in a muscle. Specifically, the existing techniques are the increment-counting technique, a technique based on spike-triggered averaging, and a macro-EMG based technique which vary in invasiveness from noninvasive to highly invasive, respectively. We discuss each of these techniques, along with their associated shortcomings, in detail.

Incomplete manifestations of myotonic dystrophy in a large kinship in Labrador

In a large kinship affected by myotonic dystrophy (MyD), 133 subjects, including 21 spouses, were examined independently by a neurologist and an ophthalmologist to determine the earliest identifying evidence of the disease. Assessment included tonometry, slit-lamp examination, facial measurements, and electromyography (EMG). Thirty-two subjects, all of whom had EMG characteristics of myotonia, were definitely affected. Low intraocular tensions were a more consistent finding than lens opacities; endocrine abnormalities were absent. Twenty-seven subjects had one to five features but lacked clinical or electrographic evidence of myotonia; they were designated as having a "partial syndrome." Signs most commonly found included weakness of the upper face, brachial hyporeflexia, low intraocular tensions, and the presence of sparse colored specks at the posterior poles of the lenses. All but 2 subjects (siblings) with the partial syndrome had a parent with either MyD or the partial syndrome. Linkage studies indicated that 7 subjects should not be carrying the gene, while others could be regarded as having a minor expression of MyD. In most persons with the partial syndrome, we could not predict whether typical MyD would develop later. Transmission of the disease may depend upon more than a single autosomal dominant gene, and nongenetic influences may also be important.

A comparative study of disopyramide and procainamide in the treatment of myotonia in myotonic dystrophy

Ten patients with myotonic dystrophy were allocated at random to treatment with disopyramide and procainamide in a cross-over trial. Disopyramide was found to be at least as effective as procainamide in the relief of myotonia; and two patients who could not tolerate procainamide both tolerated disopyramide.

Vascular adrenergic receptor responses in skeletal muscle in myotonic dystrophy

The pharmacological responses of vascular adrenergic receptors to intravenously administered epinephrine, phentolamine, and propranolol were assessed by measuring muscle blood flow (MBF) changes in the tibialis anterior muscle using the xenon 133 clearance technique and were compared in 8 normal subjects and 11 patients with myotonic dystrophy. In cases with advanced involvement of the muscle, the resting MBF was reduced and was not significantly altered by epinephrine before or after alpha- or beta-receptor blockade. In patients in whom the tibialis anterior muscle was normal or only minimally affected clinically, a paradoxical reduction in the epinephrine-induced increase in MBF was found after alpha blockade by phentolamine, and the epinephrine-induced MBF increase was not completely blocked by propranolol as in the normal subjects. These findings point to functional alteration in the properties of vascular adrenergic receptors in muscle in myotonic dystrophy. While this may be another manifestation of a widespread cell membrane defect in the disease, the possibility that the changes are secondary to the myotonic state cannot be excluded.