The basic abnormality in myasthenia gravis (MG) is a reduction in acetylcholine receptors (AChRs) at neuromuscular junctions due to the effects of autoantibodies that are directed against the AChRs in most patients, or against neighboring proteins involved in the clustering of AChRs (MuSK, LRP-4, or agrin). Clinically, MG is characterized by muscle weakness and fatigue, often in typical patterns. The diagnosis may be missed early, and depends on the recognition of clinical manifestations, the measurement of autoantibodies, and/or electrophysiological features. The treatment of MG involves the enhancement of neuromuscular transmission by anticholinesterase drugs (pyridostigmine), and by immunotherapy. Therapy should be designed to improve the clinical features quickly, and keep the symptoms in abeyance over the long term. Rapid improvement can be achieved when necessary by the administration of intravenous immunoglobulin or plasma exchange. Intermediate rates of improvement over months involve the use of adrenal corticosteroids, the calcineurin inhibitors cyclosporine or tacrolimus, and in some patients, the B-cell inhibitor rituximab. For long-term treatment, mycophenolate and azathioprine are the most effective agents. A thymectomy may also have long-term beneficial effects. The majority of MG patients can live normal lives, but most patients require lifelong treatment. The physician's skill in managing the immunotherapeutic agents and avoiding adverse side effects is of paramount importance in the treatment of MG.

Myasthenia gravis (first of two parts).

Organophosphate-induced delayed polyneuropathy (OPIDP) is a rare toxicity resulting from exposure to certain organophosphorus (OP) esters. It is characterised by distal degeneration of some axons of both the peripheral and central nervous systems occurring 1-4 weeks after single or short-term exposures. Cramping muscle pain in the lower limbs, distal numbness and paraesthesiae occur, followed by progressive weakness, depression of deep tendon reflexes in the lower limbs and, in severe cases, in the upper limbs. Signs include high-stepping gait associated with bilateral foot drop and, in severe cases, quadriplegia with foot and wrist drop as well as pyramidal signs. In time, there might be significant recovery of the peripheral nerve function but, depending on the degree of pyramidal involvement, spastic ataxia may be a permanent outcome of severe OPIDP. Human and experimental data indicate that recovery is usually complete in the young. At onset, the electrophysiological changes include reduced amplitude of the compound muscle potential, increased distal latencies and normal or slightly reduced nerve conduction velocities. The progression of the disease, usually over a few days, may lead to non-excitability of the nerve with electromyographical signs of denervation. Nerve biopsies have been performed in a few cases and showed axonal degeneration with secondary demyelination. Neuropathy target esterase (NTE) is thought to be the target of OPIDP initiation. The ratio of inhibitory powers for acetylcholinesterase and NTE represents the crucial guideline for the aetiological attribution of OP-induced peripheral neuropathy. In fact, pre-marketing toxicity testing in animals selects OP insecticides with cholinergic toxicity potential much higher than that to result in OPIDP. Therefore, OPIDP may develop only after very large exposures to insecticides, causing severe cholinergic toxicity. However, this was not the case with certain triaryl phosphates that were not used as insecticides but as hydraulic fluids, lubricants and plasticisers and do not result in cholinergic toxicity. Several thousand cases of OPIDP as a result of exposure to tri-ortho-cresyl phosphate have been reported, whereas the number of cases of OPIDP as a result of OP insecticide poisoning is much lower. In this article, we mainly discuss OP pesticide poisoning, particularly when caused by chlorpyrifos, dichlorvos, isofenphos, methamidophos, mipafox, trichlorfon, trichlornat, phosphamidon/mevinphos and by certain carbamates. We also discuss case reports where neuropathies were not convincingly attributed to fenthion, malathion, omethoate/dimethoate, parathion and merphos. Finally, several observational studies on long-term, low-level exposures to OPs that sometimes reported mild, inconsistent and unexplained changes of unclear significance in peripheral nerves are briefly discussed.

Organophosphate-Induced Delayed Polyneuropathy

Publisher Summary This chapter describes the production and assay of antibodies to acetylcholine receptors All the basic methods necessary to assay and purify acetylcholine receptors (AChRs) and AChR subunits for use as immunogens to produce antisera and monoclonal antibodies (mAbs) to AChRs and to induce experimental autoimmune myasthenia gravis are discussed in the chapter AChR is routinely identified and quantitated, by the binding of radioactively labeled krait or cobra venom toxins, which are competitive inhibitors of acetylcholine binding An easy approach to study the specificity of antibodies to intact AChR is to test their reaction, with 123 I-toxin-labeled AChR, from various species by radioimmunoassay The competitive binding to native AChR method uses mAbs to AChR that have been mapped, by their ability to react, with denatured subunits to help map the subunit specificity of antibodies, that react only, with native AChR, and to distinguish between mAbs to various antigenic determinants on the same subunit It is demonstrated that the antigenicity of the native AChR molecule is dominated, by a small region, on the α-subunit

Production and assay of antibodies to acetylcholine receptors.

Neuromuscular diseases have been in the limelight for many years, mainly because of the growing interest in muscle and the application of new techniques, such as muscle histochemistry and electron microscopy. This book should be of great interest to muscle histochemists, and should appeal to clinical neurologists and neuropathologists who have more than a fleeting interest in muscle disease. In addition, technicians can use it as a manual for appropriate technical procedures. The authors are known authorities in the field of neuromuscular disease. The book is divided into three sections, each containing illustrations, basic pathology, and case histories—a beautiful integration of the clinical with the pathologic. The first section deals with the muscle biopsy procedure, histochemical stains and reactions, and details of normal and pathologic changes of biopsied muscle. The second section deals with pathologic and clinical aspects of individual neuromuscular disease drawn mainly from the authors' experience, and a

Muscle Biopsy: A Modern Approach

Summary Myasthenia gravis (MG) is a well-recognised disorder of neuromuscular transmission that can be diagnosed by the presence of antibodies to the acetylcholine receptor (AChR). However, some patients (about 15%) with generalised MG do not have detectable AChR antibodies. There is some evidence, however, that this "seronegative" MG is an antibody-mediated disorder. Plasma from patients with the disorder seems to contain various distinct humoral factors: IgG antibodies that reversibly inhibit AChR function; a non-IgG (possibly IgM) factor that indirectly inhibits AChR function; and an IgG antibody against the muscle-specific kinase (MuSK). The presence of antibodies against MuSK appears to define a subgroup of patients with seronegative MG who have predominantly localised, in many cases bulbar, muscle weaknesses (face, tongue, pharynx, etc) and reduced response to conventional immunosuppressive treatments. Moreover, muscle wasting may be present, which prevents complete response to these therapies.

https://www.thelancet.com/pdfs/journals/laneur/PIIS1474-4422(03)00306-5.pdf

Seronegative generalised myasthenia gravis: clinical features, antibodies, and their targets

Dystrophic muscle shows increase in fibre density, abnormally low jitter in some recordings and more often increased jitter. The cross section of the motor unit has normal length. There are no signs of abnormal volume conduction characteristics. The increased fibre density is believed to be due to localised increase in the number of muscle action potential generators. The findings are compatible with a remodeling of the motor unit due to fibre loss and a reparative process with fibre regeneration and reinnervation.

https://jnnp.bmj.com/content/jnnp/46/11/981.full.pdf

The motor unit in muscular dystrophy, a single fibre EMG and scanning EMG study.

Some years ago we described the purification of the nicotinic acetylcholine receptor (nAChR) from the electric organ of Torpedo marmorata using biospecific chromatography on a-neurotoxin from Naja naja siamensis bound to a Sepharose matrix.], 2 In subsequent papers we described the immunogenic properties of nAChR and characterized, by electrophysiological techniques, a muscle paralysis observed in immunized rabbits.\", The work has now been further extended. New neurophysiological data, including single-fiber EMG, measurement of miniature end-plate potentials ( MEPPs) and end-plate potentials (EPPs) as well as morphological data have been obtained. Antibody titers have been correlated with the decrement in the amplitude of the compound muscle potential. Altogether 33 rabbits have been immunized. All of them developed the same kind of paralysis.

Experimental Myasthenia in Rabbits: Biochemical, Immunological, Electrophysiological, and Morphological Aspects

Neurophysiological investigations and determinations of cholinesterase activity on plasma and erythrocytes were carried out on 11 Swedish spraymen exposed to bromophos, diazinon, dursbane, and malathion. Plasma cholinesterase activity was significantly reduced after work, while erythrocyte cholinesterase activity was unchanged. In none of the workers with a decreased plasma cholinesterase activity after work could any related acute neuromuscular disturbance be detected when the men were tested with repetitive nerve stimulation and with single fiber electromyography. Signs of subclinical neuropathy were present as a slight reduction in sensory conduction velocity and increased fiber density in some workers.

Effect of occupational exposure to organophosphorus insecticides on neuromuscular function

In a study of 71 patients with myasthenia gravis, the local fiber density of muscle fibers within motor units was estimated with single fiber electromyography (SFEMG). The fiber density was increased significantly in the patient group, compared to normal controls. This increase was not correlated to disease severity or duration, but was significantly higher in patients treated with cholinesterase inhibitors as compared to untreated patients. The study indicates a slight reorganization of the motor unit in untreated patients with myasthenia gravis and more pronounced changes in patients treated with cholinesterase inhibitors.

Signs of reinnervation in myasthenia gravis.

The effect of small doses of succinylcholine on nerve muscle impulse transmission in situ was studied with single fibre electromyography. Extracellular muscle fibre action potentials were recorded at voluntary activation or on electrical stimulation. The results showed that the effect of succinylcholine was mainly localized in the the motor end-plate. An initial shortening of the neuromuscular delay, due to a slight depolarization, was followed by an increase in the neuromuscular transmission time of up to 3 ms, probably owing to an increased rise-time of the end-plate potential. In some experiments a neuromuscular block appeared. After this, the neuromuscular transmission time was normalized within 30 s. The jitter, a measure of the variability in the neuromuscular delay, could usually be accurately measured only during the recovery phase after the maximal effect of succinylcholine. During the recovery phase it was considerably increased for a time period of up to 3 min. This jitter increase was most probably due to a reduced end-plate potential, that is, a desensitization to acetylcholine in the motor end-plate.

Motor end-plates, even in the same motor unit, showed a great difference in sensitivity to succinylcholine. Tachyphylaxis also developed after the small doses administered, and lasted for at least 15 min.

ZUSAMMENFASSUNG
Die Wirkung kleiner Succinylcholindosen auf die Nerv-Muskel-Impulstransmission in situ wurde mit Hilfe der Einzelfaser-Elektromyographie unter-sucht. Extrazellulare Muskelfaser-Aktionspotentiale wurden bei willkurlicher Aktivierung bzw. bei elek-trischer Stimulation registriert. Die Ergebnisse zeigten, das die Wirkung von Succinylcholin haupsachlich in der motorischen Endplatte lokalisiert war. Eine initiale Verkurzung der neuromuskularen Verspatung entsprechend einer leichten Depolarisation wurde von einer Zunahme der neuromuskularen Ubertragungszeit bis auf 3 ms, wahrscheinlich verursacht durch eine verlangerte Anstiegszeit des Endplattenpotentials, gefolgt. Bei einigen Untersuchungen trat neuromuskulare Blockade auf. Danach normalisierte sich die neuromuskulare Transmission innerhalb von 30 Sekunden. Die “Verzitterung”, ein Mas far die Schwankung der neuromuskularen Verspatung, konnte in der Regel nur wahrend der Erholungsphase nach der Maximal-wirkung von Succinylcholine genau gemessen werden. Wahrend der Erholungsphase war diese fur eine Zeitspanne bis zu 3 Minuten betrachtlich verlangert. Diese Verlangerung war hochstwarscheinlich durch eine Verringerung des Endplattenpotentials hervorgerufen, welche eine Desensitivisierung der motorischen Endplatte gegen Azetylcholin darstellt.

Motorische Endplatten zeigten sogar in derselben motorischen Einheit grose Unterschiede in der Empfindlichkeit gegen Succinylcholin. Nach der Verabreichung von kleinen Dosen entwickelte sich eine Tachyphylaxie, welche mindestens 15 Minuten anhielt.

Per Hilton-Brown

Papers

The motor unit in muscular dystrophy, a single fibre EMG and scanning EMG study.

Experimental Myasthenia in Rabbits: Biochemical, Immunological, Electrophysiological, and Morphological Aspects

Effect of occupational exposure to organophosphorus insecticides on neuromuscular function

Signs of reinnervation in myasthenia gravis.

Effect of Succinylcholine on Single Motor End‐plates in Man