X-linked bulbospinal muscular atrophy (Kennedy's syndrome): a report of three cases

Decremental responses to repetitive nerve stimulation in x-linked bulbospinal muscular atrophy

BACKGROUND AND PURPOSE

X-linked bulbospinal muscular atrophy (X-BSMA) is characterized by bulbar and spinal muscular weakness and fasciculations. Although X-BSMA is a motor neuronopathy, there are several reports of myasthenic symptoms or decremental responses to repetitive nerve stimulation (RNS). We report the results of applying the RNS test to 15 patients among 41 with genetically confirmed X-BSMA; these 15 patients complained of fatigue, ease of becoming tired, or early muscular exhaustion.

METHODS

The 3-Hz RNS test was performed on the trapezius, nasalis, orbicularis oculi, flexor carpi ulnaris, and abductor digiti quinti muscles. A decrement greater than 10% was considered abnormal. Additionally, a pharmacologic response to neostigmine was identified in three patients.

RESULTS

A significant decrement was observed in 67% of patients, and was most common in the trapezius muscle (nine cases). The decrement of the trapezius muscle response ranged from 15.9% to 36.9%. The decrement was inversely correlated with the amplitude of compound muscle action potentials at rest. Neostigmine injection markedly improved the decrement in three patients, who showed noticeable decremental responses to 3-Hz RNS.

CONCLUSIONS

This study shows that myasthenic symptoms and abnormal decremental responses to low-rate RNS are common in X-BSMA.

Ocular myasthenia gravis associated with x-linked recessive spinal and bulbar muscular atrophy

We describe a 34-year-old patient who was admitted with episodic diplopia, ptosis, and swallowing difficulties of 6 months duration. He also had some muscle cramps aggravated by exercise since the age of 20. Bilateral ptosis of the eyelids, normal gaze, rare fasciculations of the tongue, easy fatigability of ocular and bilateral proximal limb muscles, atrophy of the testes, and gynecomastia were found on neurologic examination. Repetitive nerve stimulation studies and jitter measurement disclosed the defect of neuromuscular junction transmission. Acetylcholine receptor binding antibody was not detected. Acetylcholine esterase inhibitors relieved these episodic symptoms. A genetic study that showed an expansion of cytosine-adenine-guanine (CAG) repeat in the first exon of the androgen receptor (AR) confirmed the diagnosis of X-linked recessive spinal and bulbar muscular atrophy (X-SBMA). Thus, this case shows a rare association of ocular myasthenia gravis with X-SBMA.

[Multiple phenotypic manifestations of X-linked spinobulbar muscular atrophy]

Recessive X-linked amyotrophic spinobulbar muscular atrophy (SBMA) or Kennedy disease is a neuroendocrine disorder with a slowly progressive phenotype, caused by an expansion of a polymorphic tandem CAG repeat of the androgen receptor gene. Classical clinical hallmarks include onset in the third decade of life, weakness and wasting predominantly in proximal extremity muscles, variable weakness of bulbar muscles, abundant muscle fasciculations, sensory nerve action potential abnormalities and signs of androgen insensitivity such as gynecomastia and testicular atrophy. The diagnosis has been recently made easier by the availability of genetic testing but Kennedy disease is probably still underdiagnosed because of phenotypic variability. We report 11 new cases, of which seven had atypical initial manifestations presenting respectively with myasthenia, cramps and fasciculation syndrome, polyneuropathy, post-trauma monomelic neuronopathy, effort-dependent muscle intolerance and/or muscular dystrophy, with the aim to enlarge the phenotypic spectrum of the published series.

Clinical aspects of CAG repeat diseases

Seven neurodegenerative disorders are known to be caused by unstable expansions of the trinucleotide CAG within human genes, and more will be discovered in the coming years. These disorders share some clinical similarities, as well as some differences, which are summarized here. These diseases have unusual clinical genetic properties related to the dynamic nature of CAG repeat expansions, including instability of the repeat expansion in meiosis, particularly male meiosis; a strong correlation between onset age and size of the repeat expansion; anticipation (earlier disease onset in succeeding generations); new mutations arising from unstable, mutable alleles with a high-normal CAG repeat number; and reduced penetrance for alleles in the low-affected range. Much more remains to be learned about the molecular biology and clinical pathophysiology of this new class of genetic diseases.

Compound sensory action potential in normal and pathological human nerves

The compound sensory nerve action potential (SNAP) is the result of phase summation and cancellation of single fiber potentials (SFAPs) with amplitudes that depend on fiber diameter, and the amplitude and shape of the SNAP is determined by the distribution of fiber diameters. Conduction velocities at different conduction distances are determined by summation of SFAPs of varying fiber diameters, and differ in this respect, also, from the compound muscle action potential (CMAP) for which conduction velocities are determined by the very fastest fibers in the nerve. The effect and extent of temporal dispersion over increasing conduction distance is greater for the SNAP than CMAP, and demonstration of conduction block is therefore difficult. In addition, the effect of temporal dispersion on amplitude and shape is strongly dependent on the number of conducting fibers and their distribution, and, with fiber loss or increased conduction velocity variability changes of the SNAP may be smaller than expected from normal nerve. The biophysical characteristics of sensory and motor fibers differ, and this may to some extent determine divergent pathophysiological changes in sensory and motor fibers in different polyneuropathies. In this review, different factors that characterize sensory fibers and set the SNAP apart from the CMAP are discussed to emphasize the supplementary and complementary information that can be obtained from sensory conduction studies. Sensory conduction studies require particular effort and attention to theory and practical detail that may be time consuming.

Distinguishing clinical and electrodiagnostic features of X-linked bulbospinal neuronopathy

X-linked bulbospinal neuronopathy (XLBSN) or Kennedys disease is a rare inherited neuromuscular disease characterized by adult-onset muscle weakness, usually in a limb-girdle distribution. It is frequently misdiagnosed despite a distinctive clinical presentation, usually due to the absence of a clear family history, and perhaps also due to failure of recognition. Accurate diagnosis is crucial for genetic counseling purposes and because alternative diagnoses usually carry a poorer prognosis. We evaluated 4 patients with XLBSN and one symptomatic female heterozygote patient. Based on our clinical observations in these patients and a systematic review of previously reported cases, the following clinical and electrophysiologic features when present in the setting of adult-onset muscle weakness, are strongly suggestive of the disorder: 1) facial weakness, 2) facial twitching or fasciculations, 3) tongue weakness and atrophy, 4) postural hand tremor, 5) hypo- or areflexia, and 6) absent or low-amplitude sensory nerve action potentials despite clinically normal sensation. We also hypothesize regarding the possibility of partial expression of the abnormal XLBSN gene in a symptomatic heterozygote female patient.

Doença de Kennedy: Relato de dois casos

Relatamos, pela primeira vez em nosso meio, dois pacientes com atrofia muscular bulbo espinhal de início tardio (doença de Kennedy), caracterizada por fraqueza, atrofia muscular, tremores e manifestações endocrinológicas, com sinais eletromiográficos de desnervação. Esta entidade diferencia-se das demais doenças do neurônio motor pela sua peculiar herança ligada ao sexo, anormalidades endocrinas (ginecomastia, atrofia testicular e oligoespermia), tendo prognóstico relativamente favorável. Discutimos a caracterização clínica dos doentes apresentados, o aspecto fisiopatológico e a evolução.

Subclinical anterior horn cell involvement in juvenile myoclonic epilepsy

Although clinical signs of muscle wasting and weakness were not present, electromyographic (EMG) evidence of subclinical anterior horn cell involvement of spinal cord was noted in 5 patients with juvenile myoclonic epilepsy (JME). Quantitative interference pattern analysis of EMG recorded from the anterior tibial muscle showed that the ratio (amplitude:turn/turn:second, A:T/T:S) was significantly increased in 10 patients with JME and 12 patients with lower motor neuron disorders (LMND) as compared with those of 22 normal subjects and 15 patients with frequent generalized tonic-clonic seizures (GTC). Subclinical anterior horn cell involvement detected by EMG techniques can be related to a genetically determined component of JME.