IgM-containing fraction suppressed voltage-gated potassium channels in acquired neuromyotonia

OBJECTIVES

Acquired neuromyotonia (ANM) is an autoimmune disorder caused by antibodies to voltage-gated potassium channels (VGKC). Previously, we reported a patient with immunoglobulin M (IgM), instead of immunoglobulin G (IgG), anti-VGKC antibody. The purpose of this study was to determine the function of IgM-containing fraction in ANM patients.

MATERIALS AND METHODS

We determined whether anti-VGKC antibodies in the IgG or IgM-containing fractions suppressed outward potassium current (OKC) using the patch clamp method in three patients with ANM. Whole sera from all patients suppressed OKCs.

RESULT

Only the purified IgG, not the IgM-containing fractions from two patients suppressed VGKCs, whereas in a patient with IgM anti-VGKC antibody, only the IgM-containing fractions, not the IgG-containing fractions suppressed VGKCs.

CONCLUSION

Anti-VGKC antibodies belonging to the IgM subclass should be determined in seronegative ANM patients.

Sequential antibodies to potassium channels and glutamic acid decarboxylase in neuromyotonia

A patient with thymoma-associated neuromyotonia and voltage-gated potassium channel (Kv1.2 and Kv1.6) antibodies by immunoprecipitation and rat brain immunolabeling was treated successfully with immunoadsorption and cyclophosphamide. Curiously, glutamic acid decarboxylase antibodies, absent at onset, appeared later. Stiff-person syndrome was absent, but fast blink reflex recovery suggested enhanced brainstem excitability. The range of antibodies produced in thymoma-associated neuromyotonia is richer, and the timing of antibody appearance more complex, than previously suspected.

Isaacs' syndrome: Clinical and electrophysiological response to gabapentin

A patient with Isaacs' syndrome had generalized muscle spasms, twitching, and progressive muscle stiffness for 21 years. Electrodiagnostic study showed continuous spontaneous motor-unit activity and the presence of M-wave afterdischarges. He responded dramatically to treatment with carbamazepine but developed a drug rash; his treatment was changed to gabapentin with continued improvement. Subsequent nerve conduction studies showed decreased amplitude and later an absence of M-wave afterdischarges. Gabapentin thus appears to be an effective treatment for Isaacs' syndrome.

Finger flexion resembling focal dystonia in Isaacs' syndrome

We describe a patient with a 5-month history of gradually progressive painless flexion of the left ring finger associated with cramps in both thighs. She has severe chronic obstructive pulmonary disease and was on salbutamol. Serum anti-voltage-gated potassium channel antibodies was positive. Electromyography showed generalized neuromyotonia and myokymic discharges. The cramps were partially relieved by phenytoin. We would like to highlight that finger flexion resembling dystonia can be a presenting sign of Isaacs' syndrome.

Neuromyotonia with neuropathy and muscle hypertrophy: association or cause?

Continuous muscle fibre activity in a patient with demyelinating neuropathy is rare. We report an 18 year old boy who presented with walking difficulty and continuous rippling in muscles of upper and lower limbs. He had dysarthric speech, hypertrophied arm and calf muscles with normal power, tone, reflexes and sensations. Myokymic discharges were seen in deltoid, biceps, quadriceps and calf muscles. His blood counts, chemistry, thyroid profile, DNA, Rh factor were normal and CPK was raised. CSF showed protein 50 mg/dl and 4 lymphocyte/mm3. Nerve conduction study revealed conduction block and absence of peroneal F wave. EMG showed neuromyotonic discharges which disappeared on regional neuromuscular blocker but not on nerve block or general anaesthesia. He responded partially to prednisolone. Acquired demyelinating neuropathy may result in neuromyotonia and muscle hypertrophy which may partially respond to prednisolone.

Neuromyotonia: Clinical profile of twenty cases from northwest India

OBJECTIVES

We are presenting 20 cases of the intriguing clinico-electromyographic entity, now considered a potassium channel disorder, Neuromyotonia. Our experience with the clinical manifestations, underlying abnormalities and response to various therapies is documented.

MATERIALS AND METHODS

Patients with diffuse pain or undulating muscle movements, with or without stiffness were sent for electromyographic and further studies. Patients with "neuromyotonic discharges" were included after exclusion of hypocalcaemia.

RESULTS

Our cases included 19 males and one female of age group 15 to 52 years, the majority being between 30 to 45 years. Undulating movements were seen in 19, of which two had focal twitching. Muscle stiffness was a complaint in five; pain was the chief presenting complaint of 19, which started in the calf in all. Irritability, insomnia and a peculiar worried pinched face were present in 12 patients. CSF was abnormal with mildly raised protein in eight. Curiously, 11 of these patients had taken ayurvedic treatment for various complaints in the preceding one month. Bell's palsy was associated in four, peripheral neuropathy in two and residual poliomyelitis in two. Electromyographic evidence of spontaneous activity in the form of "neuromyotonic discharges" was seen in all. Antibodies to voltage gated potassium channels was tested in one patient and was positive (titer was 1028 pM). Membrane stabilizers (e.g, phenytoin sodium) in our experience did not provide adequate rapid relief; we tried high-dose intravenous Methylprednisolone in 19 with significant amelioration of complaints. One patient was offered intravenous immunoglobulin, to which he responded.

CONCLUSIONS

Neuromyotonia is a heterogeneous condition and can present in varied ways including diffuse nonspecific pain. This uncommon condition is potentially treatable and can be picked up with high index of suspicion.

Clinical electrophysiological characterization of the acquired neuromyotonia phenotype of autoimmune peripheral nerve hyperexcitability

Acquired autoimmune neuromyotonia is regarded as part of the spectrum of peripheral nerve hyperexcitability disorders. We aimed to use clinical neurophysiological measurements to study the extent, distribution, and characteristics of spontaneous motor unit potentials in 11 patients with acquired neuromyotonia. Investigations revealed that most spontaneous discharges recorded were motor unit, or partial motor unit potentials of normal size. Bursts of motor unit potentials arose more commonly from distal portions of the peripheral nerve and had abnormal absolute and relative refractory periods. Spontaneous discharges in some patients occurred in semirhythmic bursts in certain muscles. No patient had neurophysiological abnormalities detectable in first-order neurons of the central nervous system when using transcranial magnetic stimulation to estimate the threshold for corticomotor excitation and determine central motor conduction time. Only patients with coexistent myasthenia gravis had neurophysiologically detectable defects in neuromuscular transmission. The pathogenic region of abnormality in peripheral nerve hyperexcitability disorders therefore seems to lie within the terminal branches of peripheral motor nerves.

Continuous motor unit activity syndromes: a video-polysomnographic study

OBJECTIVE

To ascertain the presence of abnormalities of sleep in patients with continuous motor unit activity with and without symptoms of central nervous system involvement.

METHODS

Five patients with isolated neuromyotonia (Isaacs' syndrome) and 1 patient with Morvan syndrome underwent 24-h videopolysomnographic recording to investigate sleep structure, motor activities and autonomic variables during sleep.

RESULTS

Macro- and microstructural organization of sleep and of the attending autonomic variables were substantially normal in patients with Isaacs' syndrome. On the contrary, sleep structure was severely disrupted with subcontinuous dream enactment and hallucinations in the patient with Morvan syndrome. The pattern of the neuromyotonic discharges, however, was not different between the patients with Isaacs' syndrome compared to Morvan syndrome, the EMG discharges persisting throughout the 24 h of recording and affecting wakefulness and sleep equally.

CONCLUSIONS

Neuromyotonia is compatible with normal organization of sleep. The severe sleep abnormalities observed in Morvan syndrome cannot be simply attributed to the effects of neuromyotonia of peripheral origin.

SIGNIFICANCE

Even though neuromyotonia is common to both Isaacs' and Morvan syndromes, the two conditions differ significantly in regard to CNS involvement with sleep abnormalities and lumping the two conditions together is not justified on clinical and neurophysiological grounds.

The origin of spontaneous discharges in acquired neuromyotonia. A Macro EMG study

OBJECTIVE

To study the generator sites of spontaneous discharges in patients with immune-mediated neuromyotonia.

METHODS

Macro EMGs triggered by both spontaneously and voluntarily activated single action potentials were recorded and the mean peak-to-peak amplitude and area of the macro motor unit potentials were compared in two patients with typical acquired neuromyotonia having positive antibodies against voltage-gated potassium channels.

RESULTS

Mean peak-to-peak amplitude and area of Macro EMG motor unit potentials (macro MUPs) triggered by spontaneous discharges were significantly smaller than those triggered by voluntary activation in both patients. However, a few macro MUPs triggered by spontaneous discharges resembled those triggered by voluntary activation.

CONCLUSIONS

Spontaneous discharges in two patients with immune-mediated neuromyotonia seem to be mostly generated at sites distal to the terminal axon branching points.

SIGNIFICANCE

This finding may provide a new insight in the understanding of spontaneous discharges in immune-mediated neuromyotonia.

Thymoma-associated neuromyotonia with antibodies against voltage-gated potassium channels presenting as chronic intestinal pseudo-obstruction

Chronic intestinal pseudo-obstruction can occur as a paraneoplastic disorder, and several cases have been reported in association with thymoma or small-cell lung cancer. Autoantibodies against voltage-gated potassium channels (VGKCs) are found in acquired neuromyotonia (Isaac's syndrome), and have been reported in one case of slow transit constipation without apparent neurological disease. We describe a patient with VGKC antibodies, acquired neuromyotonia and thymoma, who first presented with a severe slow-transit constipation and in whom the gastrointestinal symptoms responded well to plasmapheresis. We suggest that VGKC antibodies might be helpful in patients with possible paraneoplastic chronic intestinal pseudo-obstruction, and a positive result should stimulate the search for a thymoma or other tumour and raise the possibility of immunotherapy.

Expanding the phenotype of potassium channelopathy: severe neuromyotonia and skeletal deformities without prominent Episodic Ataxia

We report an unusual family in which the same point mutation in the voltage-gated potassium channel gene KCNA1 resulted in markedly different clinical phenotypes. The propositus presented in infancy with marked muscle stiffness, motor developmental delay, short stature, skeletal deformities, muscle hypertrophy and muscle rippling on percussion. He did not experience episodic ataxia. His mother presented some years later with typical features of Episodic Ataxia type 1 (EA1), with episodes of ataxia lasting a few minutes provoked by exercise. On examination she had myokymia, joint contractures and mild skeletal deformities. A heterozygous point mutation in the voltage-gated K(+) channel (KCNA1) gene (ACG-AGG, Thr226Arg) was found in both. We conclude that mutations in the potassium channel gene (KCNA1) can cause severe neuromyotonia resulting in marked skeletal deformities even if episodic ataxia is not prominent.

Immunological features of neurological paraneoplastic syndromes

Neurological paraneoplastic syndromes are a rare group of disorders that occur in 1-2% of people with malignancy. They are usually caused by an immune response, triggered by and directed against a tumour, that cross-reacts with protein expressed by the peripheral or central nervous system. Any part of the nervous system can be affected and patients often develop severe and permanent disability. Diagnosis can be difficult as in two-thirds of patients the neurological problems appear up to 5 years before the tumour manifests. However, certain of these syndromes are often associated with specific serum autoantibodies that can be useful both in diagnosis of the neurological syndrome and in focusing the search for a particular tumour. Thus, these antibodies can allow earlier identification and treatment of cancer and, potentially, a reduction in morbidity and mortality. It was only in the 1980s that the first anti-neuronal autoantibodies were characterized and their associations with clinical syndromes and tumours defined. Further antibodies have been isolated over the past 20 years and novel pathogenic mechanisms for several syndromes have been recognized. For example, voltage-gate ion channels seem to be a common target for autoantibodies involved in peripheral nerve diseases such as the Lambert-Eaton myasthenic syndrome and neuromyotonia (Isaacs' syndrome). However, the place of most paraneoplastic antibodies in the pathogenesis of central syndromes is yet to be fully elucidated.

[Isaacs' syndrome, stiff person syndrome and Satoyoshi disease: pathomechanisms and treatment]

Neurological disorders with characteristic clinical manifestations of painful muscle cramp and stiffness are not infrequent. The immune-mediated mechanism with specific antibodies among these diseases is particularly important for treatment. Isaacs' syndrome (acquired neuromyotonia) is an antibody-mediated potassium channelopathy. The suppression of voltage-gated potassium channel (VGKC) by antibodies induces peripheral nerve hyperexcitability. Antibodies may decrease VGKC density by cross-linking F (ab)2 fractions and increasing the degradation rate of VGKCs. Stiff person syndrome (SPS) and its variants show characteristic symptoms and signs of central nervous system hyperexcitability due to antibodies to the GABA-ergic system such as glutamic acid decarboxylase (GAD), amphiphysin 1 and gephyrin. The role of GAD is the subject of debate, however, recent studies reveal the intrathecal synthesis of GAD which is specific for SPS and appears to impair GABA synthesis. Satoyoshi disease is characterized by painful muscle cramp, baldness, intractable diarrhea, bone and joint deformity, and endocrine disturbances. Muscle cramp may be due to inhibition of the spinal interneuron and hyperexciatability of the anterior horn cell. In patients with Satoyoshi disease, sera reacted with an 85 kDa protein of human brain lysate. In all these disorders, suppression or removal of specific antibodies is critical, however, the effects are short-lived, and supplemental treatment to reduce the hyperexcitability of the peripheral or central nervous system will be needed.

Myokymia, neuromyotonia, dermatomyositis, and voltage-gated K+ channel antibodies

A young woman presented with facial myokymia in association with dermatomyositis. There was no evidence of peripheral neuropathy. Needle electromyography showed prominent myokymic discharges and brief neuromyotonic discharges in addition to many small-amplitude, short-duration motor unit potentials. Myokymia and dermatomyositis both responded to immunosuppressive treatment. The presence of antibodies to voltage-gated potassium channels and the association with dermatomyositis indicated an autoimmune cause for myokymia, which may have been due to reversible peripheral nerve hyperexcitability.

Isaacs' syndrome as a potassium channelopathy of the nerve

Isaacs' syndrome (acquired neuromyotonia) is an antibody-mediated potassium channel disorder (channelopathy). The target channel proteins of the antigens are voltage-gated potassium channels (VGKCs), especially dendrotoxin-sensitive fast potassium channels. The suppression of voltage-gated outward K(+) current by antibodies induces hyperexcitability of the peripheral nerve. Patch clamp studies show that antibodies may not directly block the kinetics of VGKCs but may decrease channel density. Electrophysiological, pharmacological, and immunological findings indicate that the site of origin of spontaneous discharges is principally in the distal portion of the motor nerve and/or within the terminal arborization. The spectrum of potassium channelopathies is expanding. The existence of antibodies against VGKCs should be considered in patients who present with generalized nerve hyperexcitability of undetermined etiology.

Acute oxaliplatin-induced peripheral nerve hyperexcitability

PURPOSE

Oxaliplatin is a novel platinum compound with clinical activity in several malignancies. Neurotoxicity is dose-limiting and occurs in two distinct forms, an acute neurologic symptom complex that occurs within hours or days of therapy and a chronic, cumulative sensory neuropathy.

PATIENTS AND METHODS

Patients were treated in a phase I study designed to establish the maximum-tolerated dose of capecitabine given with oxaliplatin. Because of the unusual neurosensory toxicity of oxaliplatin, detailed neurologic examination, needle electromyography (EMG), and nerve conduction studies (NCS) were performed before and the day after oxaliplatin in a subset of 13 patients. Carbamazepine therapy was tried in 12 additional patients to determine whether the neurologic effects might be relieved.

RESULTS

All patients experienced acute, reversible neurotoxicities with oxaliplatin. Symptoms included paresthesias, dysesthesias, cold hypersensitivity, jaw pain, eye pain, pain in the arm used for drug infusion, ptosis, leg cramps, and visual and voice changes. Serial EMG and NCS revealed striking signs of hyperexcitability in motor nerves after oxaliplatin. In patients who achieved therapeutic levels, carbamazepine did not alter the clinical or electromyographic abnormalities.

CONCLUSION

The acute neurotoxicity seen with oxaliplatin is characterized by peripheral-nerve hyperexcitability, and the findings are similar to the clinical manifestations of neuromyotonia. Carbamezepine, which provides symptomatic relief in acquired neuromytonia, did not seem to be beneficial. Efforts to identify a successful neuroprotectant strategy would have a major impact on improving patient quality of life and the ability to deliver full doses of oxaliplatin.

[A case of Isaacs' syndrome associated with dextrocardia]

INTRODUCTION

Isaacs syndrome is a disorder of unknown etiology characterized by muscular rigidity, cramps and myokymias. Described by Isaacs in 1961 and called by him as continuous muscular activity syndrome. There are few reports in Latino american countries.

CLINICAL CASE

A 31 year-old man with sustained muscular contractions in lower limbs and diffuse myokymias since he was eighteen-year old. Dextrocardia was disclosed on clinical examination.

CONCLUSION

We report the case of a patient with Isaacs syndrome and dextrocardia.