Myasthenia gravis: lessons from the past 10 years

Case report: Immune remission from generalized myasthenia gravis in a dog with a thymoma and cholangiocellular carcinoma

A 9-year-old male neutered Cockapoo was presented with an acute and progressive history of exercise induced weakness involving all limbs, and bilateral decreased ability to blink. Investigations revealed generalized myasthenia gravis alongside the presence of a thymoma and a cholangiocellular carcinoma. Symptomatic treatment through pyridostigmine bromide was used to control clinical signs, and complete surgical removal of the thymoma and cholangiocellular carcinoma was performed. Serum acetylcholine receptor antibody concentration was measured serially. Clinical remission defined as resolution of clinical signs alongside discontinuation of treatment was achieved by day 251 (8.2 months). Immune remission defined as normalization of serum acetylcholine receptor antibody concentration alongside resolution of clinical signs and discontinuation of treatment was achieved by day 566 (18.5 months). Neurological examination was normal, and the owners did not report any clinical deterioration during the final follow-up appointment on day 752 (24 months), hence outcome was considered excellent. This is the first report describing the temporal evolution of serum acetylcholine receptor antibody concentration in a dog with thymoma-associated myasthenia gravis which achieved immune remission following thymectomy. Treatment was successfully discontinued without any evidence of clinical deterioration thereafter despite serum acetylcholine receptor antibody concentration not normalizing for another 315 days (10 months).

Seronegative Myasthenia Gravis with Concomitant SARS-CoV-2 Infection in a Dog

Myasthenia gravis (MG) is a disorder of neuromuscular transmission affecting the neuromuscular junction. The majority of cases involve an autoimmune attack against AChR, but a limited number of patients are seronegative for AChR antibodies. Viral infection is incriminated as a trigger for MG occurrence, and in a limited number of reports, infection with SARS-CoV-2 was found to be associated with MG expression in humans. In this report, we describe case of seronegative generalized MG in a 2-year-old crossbred female dog associated with SARS-CoV-2 infection due to close exposure to an infected owner.

The clinical utility of neostigmine administration in the diagnosis of acquired myasthenia gravis

OBJECTIVE

To assess the clinical utility of neostigmine methylsulfate administration in the diagnosis of suspected acquired myasthenia gravis (MG) in dogs and cats.

DESIGN

Retrospective study (2017-2019).

SETTING

Five university teaching hospitals and 2 private referral hospitals.

ANIMALS

Twenty-two dogs and 3 cats. Criteria for inclusion were clinical signs consistent with acquired MG, performance of a neostigmine challenge and acetylcholine receptor antibody titers.

INTERVENTIONS

None.

MEASUREMENTS & MAIN RESULTS

The route of neostigmine administration was recorded. Response to neostigmine challenge was determined via sequential evaluation of muscle strength and ambulation following administration of neostigmine methylsulfate. Response to neostigmine challenge was compared to acetylcholine receptor antibody titers, which were used as the biochemical gold standard in this study. Sixteen out of 22 dogs were diagnosed with acquired MG. Thirteen of 16 had a strong positive response to neostigmine challenge whereas 3 of 16 had no response. Two out of 3 dogs with polymyositis also had a strong positive response to neostigmine challenge. Weak positive results were seen with intracranial neoplasia (n = 1) and a dog with dilated cardiomyopathy and coxofemoral joint disease (n = 1). One cat was diagnosed with acquired MG and had a positive response to neostigmine challenge. Two cats had no response to neostigmine challenge and were diagnosed with alternate conditions. Two cats were premedicated with glycopyrrolate, one of which had a mild adverse response to neostigmine challenge (sialorrhea and mild transient tremors). Three out of 22 dogs had minimal adverse effects (sialorrhea and 1 dog with muscle tremors).

CONCLUSIONS

The neostigmine challenge appears to be safe and viable alternative to the previously utilized edrophonium challenge, particularly when weak positive responses are considered negative for acquired MG. Polymyositis cases may have a false positive response to neostigmine challenge.

Classification of myasthenia gravis and congenital myasthenic syndromes in dogs and cats

Myasthenia, a syndrome of impaired neuromuscular transmission, occurs as either an acquired or congenital condition. Myasthenia gravis (MG) is an acquired autoimmune disorder with autoantibodies against the neuromuscular junction (NMJ) of skeletal muscle whereas congenital myasthenic syndromes (CMSs) are a clinically heterogeneous group of genetic disorders affecting the NMJ with a young age of onset. Both conditions are diseases for which recognition is important with regard to treatment and outcome. We review the published literature on MG and CMSs in dogs and cats, and by comparison with published classification used in humans, propose a classification system for MG and CMSs in dogs and cats. Myasthenia gravis is first classified based on focal, generalized, or acute fulminating presentation. It then is subclassified according to the autoimmune disease mechanism or seronegativity. Autoimmune disease mechanism relates to the presence or absence of a thymoma, or administration of thiourylene medication in cats. Congenital myasthenic syndromes are classified according to the affected NMJ component, the mechanism of the defect of neuromuscular transmission, the affected protein, and ultimately the mutated gene responsible. In proposing this categorization of MG and CMSs, we hope to aid recognition of the disease groups for both conditions, as well as guide treatment, refine prognosis, and provide a framework for additional studies of these conditions.

Clinical and serologic remission of acquired myasthenia gravis in a domestic ferret (Mustela putorius furo)

CASE DESCRIPTION

A 4.5-year-old neutered male domestic ferret (Mustela putorius furo) was examined because of clinical signs compatible with neuromuscular disease.

CLINICAL FINDINGS

Results of electrophysiologic assessment, including measurement of compound muscle action potentials following repetitive nerve stimulation, and measurement of the anti-acetylcholine receptor antibody titer were consistent with a diagnosis of acquired myasthenia gravis.

TREATMENT AND OUTCOME

Medical treatment with pyridostigmine and prednisolone was instituted. The first signs of clinical improvement were observed 2 months later, followed by a slow but steady improvement over the next months. Anti-acetylcholine receptor antibody titer was measured 10 months after initiation of treatment and was markedly decreased, compared with the initial titer. Pyridostigmine and prednisolone dosages were tapered over the following 4 months without any evidence of recurrence of clinical signs. Thirty months after initial examination, the ferret was clinically normal and not receiving any treatment. A follow-up anti-acetylcholine receptor antibody titer was similar to previously published values for healthy ferrets.

CLINICAL RELEVANCE

Findings indicated that clinical and serologic remission can be achieved in ferrets with myasthenia gravis. However, owner willingness to provide extensive supportive care was vital to the outcome for this patient, as was the owner's decision to not euthanize the ferret despite an initial lack of response to treatment.

Diagnosis and Treatment of Lower Motor Neuron Disease in Australian Dogs and Cats

Diseases presenting with lower motor neuron (LMN) signs are frequently seen in small animal veterinary practice in Australia. In addition to the most common causes of LMN disease seen world-wide, such as idiopathic polyradiculoneuritis and myasthenia gravis, there are several conditions presenting with LMN signs that are peculiar to the continent of Australia. These include snake envenomation by tiger (Notechis spp.), brown (Pseudonaja spp.), and black snakes (Pseudechis spp.), tick paralysis associated with Ixodes holocyclus and Ixodes coronatus, and tetrodotoxins from marine animals such as puffer fish (Tetraodontidae spp.) and blue-ring octopus (Hapalochlaena spp.). The wide range of differential diagnoses along with the number of etiological-specific treatments (e.g., antivenin, acetylcholinesterase inhibitors) and highly variable prognoses underscores the importance of a complete physical exam and comprehensive history to aid in rapid and accurate diagnosis of LMN disease in Australian dogs and cats. The purpose of this review is to discuss diagnosis and treatment of LMN diseases seen in dogs and cats in Australia.

Acquired myasthenia gravis associated with oral sarcoma in a dog

Acquired myasthenia gravis is a common neuromuscular disorder resulting from autoantibody directed against the post-synaptic acetylcholine nicotinic receptors in skeletal muscle. Myasthenia gravis has been reported previously as a paraneoplastic syndrome. This case report presents myasthenia gravis secondary to an oral sarcoma in a juvenile Mastiffdog.

Autoimmune myasthenia gravis in a ferret

CASE DESCRIPTION

A 7-month-old neutered male ferret was evaluated for episodic pelvic limb weakness of 2 weeks' duration.

CLINICAL FINDINGS

Neurologic examination revealed flaccid tetraparesis with decreased spinal reflexes suggestive of a neuromuscular disease. Results of hematologic and CSF analyses, thoracic radiography, and abdominal ultrasonography were unremarkable. Electrodiagnostic testing revealed subtle spontaneous activity localized to pelvic limb interosseous muscles, unremarkable motor nerve conduction velocities, and lower than typical compound muscle action potential (CMAP) amplitude for tibial nerve stimulation only. A severe decremental response of the CMAP was detected with repetitive nerve stimulation (45.5% at the third ulnar nerve). An esophagogram revealed mild megaesophagus. Intravenous neostigmine methylsulfate administration resulted in immediate resolution of muscle weakness. Cross-reacting anti-acetylcholine receptor (AChR) antibodies were detected in serum (0.35 nmol/L) by use of a canine- and feline-specific muscle extract. Clinical signs and ancillary test results were diagnostic of acquired myasthenia gravis.

TREATMENT AND OUTCOME

Pyridostigmine bromide was administered (1 mg/kg [0.45 mg/lb], PO, q 8 h), resulting in complete remission of clinical signs. However, 1 month after the diagnosis, the ferret was euthanized because of recurrence of weakness despite anticholinesterase treatment.

CLINICAL RELEVANCE

To the authors' knowledge, this is the first report of acquired myasthenia gravis in a ferret and the first identification of anti-AChR antibodies in this species. Autoimmune myasthenia gravis should be considered in ferrets when weakness and flaccid paresis suggest a neuromuscular disease. Electrodiagnostic testing, anticholinesterase challenge, and AChR antibody titer determination were helpful for diagnosis of this condition.

Use of radiographic measurements in distinguishing myasthenia gravis from other causes of canine megaoesophagus

OBJECTIVES

To evaluate whether dogs with megaoesophagus due to myasthenia gravis display less oesophageal dilatation radiographically than dogs with other causes of megaoesophagus.

METHODS

Thoracic radiographs of 66 dogs with megaoesophagus in which concurrent acetylcholine receptor antibody titre was known were analysed retrospectively. Maximum oesophageal diameter was transformed to a "relative oesophageal diameter" using a ratio with thoracic inlet diameter. Dogs were divided into two groups according to "MG" or "non-MG" antibody status and median relative oesophageal diameter values were compared between groups and with age, weight and sex. A receiver operating characteristic plot was used to evaluate a suitable relative oesophageal diameter cut-off.

RESULTS

Twenty dogs were diagnosed with myasthenia gravis and 46 with other causes of megaoesophagus. Thoracic inlet size correlated significantly with bodyweight and surface area (r(2)=0.627 and 0.669, respectively). Median values of relative oesophageal diameter for the MG group and non-MG group were 0.58 and 0.66, respectively, and these showed a small, but significant, difference (P=0.029), although there was complete overlap in the range of relative oesophageal diameter values between groups. There was no significant association between relative oesophageal diameter and sex, age or weight or significant difference in age, sex or weight between the two groups. An increased odds ratio for myasthenia gravis existed in golden retrievers and German shepherd dogs.

CLINICAL SIGNIFICANCE

Relative oesophageal diameter appears to be of limited diagnostic utility in distinguishing dogs with megaoesophagus due to myasthenia gravis from those with megaoesophagus due to other causes.

Myasthenia Gravis and Hypothyroidism in a Dog With Meningomyelitis

A 12-year-old, spayed female miniature poodle was evaluated because of a 4-day history of paraparesis, dysuria, and tenesmus. Neurological assessment suggested peripheral nervous system dysfunction, predominantly pelvic limb weakness with a possible concurrent sixth lumbar (L6) to second sacral (S2) myelopathy. Further studies supported the diagnoses of myasthenia gravis, hypothyroidism, and meningomyelitis. To the authors’ knowledge, this is the first reported case of concurrent myasthenia gravis and meningomyelitis in the dog. It was unclear whether the identified conditions evolved from a shared etiopathogenesis or were merely coincidental.

Myasthenia gravis in a polar bear (Ursus maritimus)

A 14.6-yr-old, female, multiparous polar bear (Ursus maritimus) acutely developed an apparent hind limb weakness. Physical examination and diagnostic tests including a hemogram, serum biochemistry, electrolytes, radiographs, and myelogram did not provide a definitive diagnosis. No improvement in condition was noted during 4 days of supportive care, and the bear was euthanized. An ovoid mass was present in the anterior mediastinum, and a thymoma was confirmed histologically. Compared with control polar bears, elevated serum acetylcholine receptor (AChR) antibodies (0.13 +/- 0.06 nmol/L vs. 0.86 nmol/L) were detected by immunoprecipitation radioimmunoassay, which is consistent with myasthenia gravis (MG) in other species. Although the AChR antibody test has not been validated in the polar bear, we are confident in the postmortem diagnosis of MG, which is commonly associated with thymoma in other species.

Acquired myasthenia gravis associated with a non-invasive thymic carcinoma in a dog

An 8 1/2-year-old neutered male Beagle was diagnosed with acquired myasthenia gravis associated with a non-invasive thymic carcinoma. The thymic mass was surgically excised and the dog was treated with pyridostigmine, prednisolone and azathioprine. Serial acetylcholine receptor antibody titres were increased initially but slowly declined to normal values over a period of 24 weeks. Improved exercise tolerance was seen following therapy, however, oesophageal dysfunction persisted. The dog was euthanased 26 weeks after initial presentation due to a complicating illness. A necropsy showed no regrowth or metastasis of the thymic carcinoma.

Evaluation of jitter by stimulated single-fiber electromyography in normal dogs

Single-fiber electromyography (SFEMG), a technique used to investigate neuromuscular transmission, has been described previously in the pelvic limb of dogs. Because preferential involvement of isolated muscle groups can occur in disorders of neuromuscular transmission, SFEMG was done in the peroneus longus (PL), extensor carpi radialis (ECR), and orbicularis oculi (OO) muscles of 10 adult, clinically normal dogs. Jitter was calculated as the mean absolute value of the consecutive differences in latency of 50 single muscle fiber action potentials after stimulation of intramuscular nerve bundles at the level of the motor point in at least 20 muscle fibers per muscle. Bilateral recordings were performed in 3 dogs. Mean jitter values were determined for each muscle, and differences among muscle groups and among dogs were compared. The upper limits of mean consecutive difference (mean plus 3 standard deviations) for the PL, ECR, and OO muscles were 21.94, 22.53, and 23.39 micros, respectively, and the upper limit of mean consecutive difference for individual muscle fibers in the respective fiber pools was 28.62, 36.39, and 35.68 micros. Jitter values for the ECR and OO were significantly higher than the jitter value for the PL muscle (P < .05). Significant differences among muscles or dogs or between sides were not observed for the ECR. Significant differences among dogs were observed for OO jitter values and were attributed to extremely low jitter values in 1 dog. Significant differences were demonstrated between sides for the PL and were attributed to small sample size. Results of this study provide normative data that can be used in the application of the stimulated SFEMG technique to dogs with suspected disorders of neuromuscular transmission.

Myasthenia gravis and disorders of neuromuscular transmission

Myasthenia gravis is a disorder of neuromuscular transmission that occurs in congenital and acquired autoimmune forms. Acquired myasthenia gravis is probably the most common neuromuscular disorder in dogs that can be diagnosed and treated. An early, accurate diagnosis and appropriate therapy is of utmost importance to a good clinical outcome in this disorder. This article focuses on the diagnosis and treatment of acquired myasthenia gravis in dogs and cats with brief discussions of other disorders of neuromuscular transmission, including congenital myasthenia gravis, tick paralysis, botulism, and organophosphate intoxication.

Muscle and nerve biopsy

Biopsy of muscle and nerve is an essential component of the diagnostic plan for animals with suspected neuromuscular disease. This article includes descriptions of the biopsy procedures and information regarding appropriate biopsy site selection and correct handling and processing of the tissues. The normal and pathological appearance of muscle and nerve using routine histochemical and histological techniques are described and illustrated emphasizing the basic myopathic and neuropathic responses of the neuromuscular system.

Titin and ryanodine receptor autoantibodies in dogs with thymoma and late-onset myasthenia gravis

Similar to human autoimmune myasthenia gravis (MG), canine MG occurs spontaneously and is associated with autoantibodies against the nicotinic acetylcholine receptor (AChR). In addition to AChR, human MG patients with thymoma or late-onset MG have antibodies against titin and ryanodine receptor (RyR). The objective of this study was to establish if dogs with confirmed MG (AChR antibody titer >0.6 nmol/l) also developed titin and RyR antibodies and identify possible associations with thymoma, late age of onset, or severity of clinical signs. Sera from dogs (n=430) with previously diagnosed autoimmune MG (N=415), other immune-mediated neuromuscular disorders including polymyositis (PM) and masticatory muscle myositis (N=5), and control dogs (N=10) were evaluated for the presence of titin antibodies in ELISA using MGT-30 as antigen, a peptide representing the main immunogenic region (MIR) for human titin antibodies. Titin antibody positive sera were further examined for RyR antibodies in Western blots using a RyR fusion protein (pc2-RyR) as antigen, which covers the MIR for human MG sera. Titin antibodies were found in sera of 80/430 dogs. Thymoma was present in 11/80 and age of onset was after 4 years in 66/80 titin positive dogs. Two of the titin positive dogs had PM. RyR antibodies were found in 13/80 sera (8/13 thymoma, 12/13 age of onset after 4 years, and 1/13 PM). Neither titin nor RyR antibodies were found in sera of healthy control dogs. Acute fulminating MG was described in five dogs with both titin and RyR antibodies. From these studies we conclude that titin and RyR antibodies in canine and human MG have a similar association with thymoma, late-onset MG, and possibly with more severe forms of MG.

Myasthenia gravis associated with cutaneous lymphoma in a dog

A middle-aged golden retriever presenting with exercise intolerance and multifocal cutaneous nodules was diagnosed as having non-epitheliotropic cutaneous lymphoma with concurrent myasthenia gravis. Treatment with corticosteroids induced short-term remission of the lymphoma and alleviation of myasthenic signs.

Acquired myasthenia gravis: what we have learned from experimental and spontaneous animal models

Acquired myasthenia gravis (MG) is a disorder of neuromuscular transmission in which muscle weakness results from an autoantibody mediated depletion of acetylcholine receptors (AChR) at the neuromuscular junction. Experimental autoimmune myasthenia gravis, described in rodents and rabbits, has provided a good model of the effects of the autoimmune response against AChR and has shown that the specificities of the immune response in MG are those that would be obtained by immunization with native AChR. It has provided little information, however, about what initiates and sustains the immune response in MG. Acquired MG occurs spontaneously in dogs and may be the most common neuromuscular disorder that can be diagnosed in this species. As in human MG, an autoimmune response against AChR has been demonstrated and AChR autoantibodies have been implicated in the pathogenesis. The variability in clinical presentation, methods of diagnosis, and occurrence with other autoimmune diseases and neoplasia are identical to that of humans. Future studies of spontaneous canine autoimmune MG may provide clues to the determination of what factors initiate and sustain the autoimmune response to AChR, and in the study of specific suppression of the autoimmune response against AChR.