Thymoma-associated exfoliative dermatitis with post-thymectomy myasthenia gravis in a cat

Thymoma-associated exfoliative dermatitis was suspected in a cat with a cranial mediastinal mass. The dermatopathy resolved with surgical removal of a thymoma. The cat manifested neurologic signs consistent with myasthenia gravis 7 wk after surgery. Exfoliative dermatitis and post-thymectomy myasthenia gravis in the same cat has not been reported previously.     

Metastatic thymoma and acquired generalized myasthenia gravis in a beagle

A 16-year-old, spayed female beagle was diagnosed with metastatic thymoma causing a probable paraneoplastic syndrome of generalized acquired myasthenia gravis. Anticholinesterase treatment was initiated; however, 5 days later the dog died.     

Video‐Assisted Thoracoscopic Resection of Noninvasive Thymomas Using One‐Lung Ventilation in Two Dogs

Objective— To report the technique and outcome of video‐assisted thoracoscopic surgery (VATS) for resection of cranial mediastinal thymoma in 2 dogs. Study Design— Case report. Animals— Eleven‐year‐old Labrador Retrievers (n=2). Methods— Two dogs had VATS resection of thymoma. Preoperative computed tomography (CT) scans revealed well‐circumscribed cranial mediastinal masses with dimensions of 4.5 cm × 4.2 cm × 3.7 cm and 2.1 cm × 2.1 cm × 4.1 cm (at the time of resection) without CT evidence of vascular invasion. One‐lung ventilation (OLV) was achieved using a bronchoscopically placed double‐lumen endobronchial tube. A 3‐portal technique was used for VATS access to the thorax. Thymomas were dissected from the tissues of the cranial mediastinum with the aid of a harmonic scalpel and retrieved in specimen retrieval bags. Results— Two cranial mediastinal thymomas were resected successfully, with their capsules intact, using a VATS technique. One dog developed aspiration pneumonia postoperatively which resolved with treatment and remains healthy 18 months postoperatively with no radiographic evidence of tumor recurrence. The second dog was diagnosed with myasthenia gravis, megaesophagus, and aspiration pneumonia preoperatively and despite recovering well from the procedure had a second episode of aspiration pneumonia 5 days postoperatively and was euthanatized. Conclusions— VATS resection of modestly sized noninvasive thymoma is possible in dogs. Careful case selection and preoperative imaging are the keys. Clinical Relevance— Thoracoscopic resection of modestly sized mediastinal masses forms part of an increasingly expansive set of indications for minimally invasive surgery in select veterinary patients.     

Acquired myasthenia gravis in a cat with thymoma

A 4-year-old castrated Abyssinian cat was evaluated for profound neuromuscular weakness. Results of electromyography and repetitive nerve stimulation tests were normal. Thoracic radiography revealed a cranial mediastinal mass, which was excised and identified as a thymoma. Serum acetylcholine receptor antibodies were detected at high concentration, supporting a diagnosis of acquired myasthenia gravis. Clinical signs of disease responded to treatment with pyridostigmine and corticosteroids.     

Clinical Forms of Acquired Myasthenia Gravis in Dogs: 25 Cases (1988–1995)

The purpose of this project was to investigate the clinical forms of acquired myasthenia gravis in dogs. The medical records from 25 dogs with seropositive acquired myasthenia gravis were reviewed, and the following data were recorded for each patient: signalment, history, clinical findings; results of IV edrophonium chloride administration, repetitive nerve stimulation, and presence or absence of muscle membrane staining by immunocytochemical methods; serum acetyl-choline receptor antibody concentration; treatment; and outcome. Several clinical forms of acquired myasthenia gravis were identified. Nine of the 25 patients (36%) had no historical or clinical evidence of appendicular muscle weakness, and were designated as focal myasthenics. These dogs exhibited focal weakness in one or more of the following muscle groups: facial {3 of 9), pharyngeal (3 of 9), and laryn-geal (3 of 9). The remaining 16 dogs (64%) exhibited appendicular muscle weakness. Four of these 16 dogs had acute onset and rapid development of clinical signs, and were designated as acute fulminating myasthenics. The remaining 12 dogs were classified as generalized myasthenics. All 4 dogs with acute fulminating myasthenia gravis had megaesophagus, 2 had facial muscle weakness, and 1 had pharyngeal muscle weakness. Ten of the 12 dogs with generalized myasthenia gravis had megaesophagus, 4 had facial muscle weakness, 4 had pharyngeal muscle weakness, and 3 had laryngeal muscle weakness. Historical or clinical evidence of exercise-associated appendicular weakness was found in only 6 of the 12 (50%) dogs with generalized myasthenia gravis, and in none of the dogs with acute fulminating myasthenia gravis. Seven of the 12 dogs with generalized myasthenia gravis had weakness primarily (n = 1) or exclusively (n = 6) of the pelvic limbs. Two of the 4 dogs with acute fulminating myasthenia gravis had primarily pelvic limb weakness. Twelve of the 25 dogs (48%) died or were euthanized shortly after admission to the hospital due to aspiration pneumonia. The dogs with acute fulminating myasthenia gravis had a markedly higher 1-year mortality rate in comparison with the other 2 groups. The use of immunosuppressive therapy had a significant positive effect on patient survival, regardless of the type of myasthenia gravis. This investigation demonstrates that acquired myasthenia gravis in dogs is a disorder with a wide spectrum of clinical forms, similar to the analagous disorder in people.     

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.     

Focal myasthenia gravis in a dog.

A 10-month-old American cocker spaniel was evaluated for megaesophagus, aspiration pneumonia, but no appendicular muscle weakness. During hospitalization, weakness of the facial muscles developed, this resolved with anticholinesterase administration. Serum antibodies against acetylcholine receptors were documented, confirming the diagnosis of focal myasthenia gravis. Diagnosis, management, and medical treatment are discussed.     

Radiation treatment for thymoma in a dog

A 10-year-old Toy Poodle was admitted for evaluation of a chronic cough caused by a cranial mediastinal mass. The mass was diagnosed, by needle biopsy, as a thymoma. Detection of serum acetylcholine receptor antibodies suggested that the dog had subclinical myasthenia gravis. The dog underwent orthovoltage radiation treatment, which resulted in an approximate 60% reduction in tumor mass and freedom from clinical signs for 6 months. Concurrent use of prednisolone may have been associated with disappearance of the acetylcholine receptor antibodies.     

Congenital myasthenia gravis in Smooth-Haired Miniature Dachshund dogs

Three 8-week-old Miniature Smooth Haired Dachshund littermates were diagnosed with myasthenia gravis based on clinical signs, results of electrophysiological testing, and response to the short-acting anticholinesterase drug, edrophonium chloride. Congenital myasthenia gravis was confirmed by the demonstration of decreased acetylcholine-receptor density in external intercostal muscle in the absence of demonstrable serum antiacetylcholine receptor antibody or antibodies complexed to acetylcholine receptors in muscle biopsy samples. Unlike most previously reported cases of congenital myasthenia gravis that are relentlessly progressive, clinical signs resolved spontaneously by 6 months of age.     

Canine myasthenia gravis

Decreased acetylcholine receptor content of the neuromuscular junction underlies the muscular weakness seen in both acquired and congenital myasthenia gravis. Immune mediated mechanisms account for the acquired defect while decreased membrane insertion of receptor leads to the congenital disorder. The congenital form typically presents in the first few weeks of life with generalised episodic weakness as the predominant sign. Although treatment with cholinesterase inhibitors may help temporarily, the prognosis is poor. Acquired myasthenia gravis can occur between eight months and 13 years of age and may present as megaoesophagus alone or as exercise-related weakness in conjunction with megaoesophagus. Although treatment of the immune mediated form is often successful and spontaneous remissions also occur, aspiration pneumonia secondary to megaoesophagus remains a major cause of mortality in affected dogs. Resolution of megaoesophagus must therefore be a major goal for future treatment regimes. The remarkable similarity between canine and human acquired myasthenia gravis suggests that the canine thymus gland warrants greater attention pathologically and perhaps therapeutically considering the value of thymectomy in human patients.     

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.     

Myasthenia gravis associated with thymic neoplasia in a cat

Objective: To describe acute myasthenia gravis (MG) in the postoperative period following removal of a thymoma in an adult cat. Case summary: A 6‐year‐old spayed female domestic short haired cat weighing 6.4 kg was referred for workup and treatment of a cranial mediastinal mass. Thoracoscopic biopsies confirmed the diagnosis of thymoma. Median sternotomy was performed and approximately 95% of the mass was surgically excised. Postoperatively the cat became remarkably weak and hypercapneic. A presumptive diagnosis of MG was made following a positive response to edrophonium injection (Tensilon test). An elevated serum acetylcholine receptor antibody level was consistent with a diagnosis of MG. Initial treatment consisted of neostigmine, followed by corticosteroids and pyridostigmine. The cat responded well to therapy. New or unique information provided: This report describes a rare syndrome of postoperative weakness due to development of MG following incomplete removal of a thymoma. Post‐thymectomy weakness associated with MG has been reported in dogs and is not well described in cats.     

Benign cranial mediastinal lesions in three cats

Cranial mediastinal lesions were detected in three cats, associated with respiratory impairment (case one), spontaneous pneumothorax (case two) and myasthenia gravis (case three), respectively. On gross and histological examination, the first case was considered either a lymphangioma or a branchial cystic mass of the thymic region of the mediastinum; a cystic lesion was suggested by sonographic detection of multiple anechoic cavitations within a circumscribed mass, while fine needle aspiration cytology excluded lymphosarcoma. The second case was diagnosed histologically as a cystic thymoma, but the third case was not examined microscopically. The masses were amenable to surgical excision in the first two cats, while this proved unnecessary in the third case because of resolution following treatment with dexamethasone. Corticosteroid responsiveness was unhelpful in distinguishing between these benign lesions and lymphosarcoma, as in two cases there was a partial or complete response to dosing with prednisolone or dexamethasone. These cases are presented to emphasise that conditions other than lymphosarcoma can produce cranial mediastinal lesions in cats, and that the prognosis for surgical treatment of lymphangiomas, multilocular thymic cysts and cystic thymomas can be excellent.     

Autoimmune form of myasthenia gravis in a juvenile Yorkshire Terrier x Jack Russell Terrier hybrid contrasted with congenital (non-autoimmune) myasthenia gravis of the Jack Russell

A case of juvenile onset myasthenia gravis is described in a Yorkshire Terrier x Jack Russell bitch in which there were serum autoantibodies to acetylcholine receptors and reduction in the muscle's content of acetylcholine receptors. Approximately 25% of the receptors remaining were complexed with antibody. After 8 months of treatment with the anticholinesterase pyridostigmin, the animal made a complete recovery and the serum level of antibody decreased. The condition of this dog in many respects resembled acquired myasthenia gravis of adult onset and is contrasted with the seven cases of congenital myasthenia gravis which we have studied in the Jack Russell Terrier. In the congenital form of myasthenia gravis, reduction of acetylcholine receptors in muscle is found without any demonstrable autoantibodies to acetylcholine receptors, either in serum or muscle.     

Acquired myasthenia gravis in a poodle

An 11-year-old, spayed female, teacup poodle was evaluated for a chronic cough, lethargy, hindlimb weakness, and reluctance to exercise. Thoracic radiographs revealed megaesophagus and aspiration pneumonia. Serum antibodies against acetylcholine receptors confirmed the diagnosis of myasthenia gravis. The unusual clinical history and case outcome are discussed.     

Treatment of a Myasthenic Dog with Mycophenolate Mofetil

A ten‐year‐old, male castrated Springer Spaniel was presented for dysphagia, ptyalism, and regurgitation. Evidence of megaesophagus and mild aspiration pneumonia were apparent on thoracic radiographs. A diagnosis of focal acquired myasthenia gravis was suspected and subsequently confirmed with a positive serum acetylcholine (ACh) receptor antibody concentration (3.87 nM/L). A gastrostomy tube was placed shortly after presentation; food and drugs (including azathioprine) were administered through the tube. After transient improvement, the dog suddenly deteriorated clinically, experiencing frequent episodes of regurgitation and developing severe aspiration pneumonia. Mycophenolate mofetil (MMF), a novel immunosuppressive drug with relative specificity for lymphocytes, was instituted every twelve hours via the gastrostomy tube. Within four days of beginning MMF therapy, both clinical evidence of pharyngeal/esophageal dysfunction and radiographic evidence of megaesophagus had resolved. Initially, clinical side‐effects of combined MMF/AZA administration were not apparent, but the patient experienced several vomiting episodes during the third week of treatment. The vomiting resolved after decreasing the dose of both drugs. The patient made a full recovery, and a one‐month follow‐up ACh receptor antibody concentration was normal (0.26 nM/L). After one month of combination therapy, the patient was weaned off of AZA and maintained on MMF as the sole immunosuppressive drug. The dog was subsequently weaned off of MMF on two occasions. Mycophenolate mofetil was reinstituted after the first discontinuation due to the development of profound appendicular muscle weakness two days after stopping MMF; the weakness resolved within 24 hours of reinstituting MMF. A positive ACh receptor antibody concentration (0.89 nM/L) after the second MMF weaning prompted the second reinstitution of MMF. Two months following this second MMF reinstitution, the dog was again serologically negative (0.51 nM/L) for myasthenia gravis. At the time of last followup, the dog remained in clinical remission eight months after initial presentation. The use of MMF to treat acquired myasthenia gravis in dogs has not been reported previously. The literature concerning MMF and its potential use in treating patients with autoimmune diseases is discussed.     

Adult onset acquired myasthenia gravis in three Great Dane littermates

Acquired canine myasthenia gravis is an autoimmune disease in which autoantibodies are directed against muscle postsynaptic nicotinic acetylcholine receptors. Three adult great dane littermates were evaluated over a four month time period for an acute onset of generalised neuromuscular signs. All three dogs had elevated serum acetylcholine receptor antibody titres, which were considered diagnostic for acquired myasthenia gravis. Identification of three littermates with acquired myasthenia gravis in a breed with a low relative risk of developing the disease suggests a familial and possibly a genetic predisposition to myasthenia gravis in this family of dogs.     

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.     

Diseases of the horse: the centennial of a great book

A 7-year-old Saint Bernard developed muscular weakness 1 year after right forelimb amputation and adjuvant cisplatin chemotherapy for osteogenic sarcoma. Clinical and laboratory findings supported a diagnosis of myasthenia gravis, and the dog had clinical improvement in response to prednisone treatment. Two additional dogs with myasthenia gravis and osteogenic sarcoma were identified by review of the medical records of the University of California Veterinary Medical Teaching Hospital. Findings indicated that myasthenia gravis or other neuromuscular transmission disorders may be associated with muscular weakness in dogs with osteogenic sarcoma.     

Canine thymoma

Thymoma is an uncommon canine neoplasm of thymic epithelial cells. It is seen in various breeds but may occur more frequently in German Shepherd Dogs. Middle-aged or older dogs can be affected and no sex predilection exists. A paraneoplastic syndrome of myasthenia gravis, nonthymic malignant tumors, and/or polymyositis occurs in a significant number of dogs with thymoma. Clinical signs are variable and are related to a space-occupying cranial mediastinal mass and/or manifestations of the paraneo-plastic syndrome. Dyspnea is the most common presenting clinical sign. Thoracic radiographs usually show a cranial mediastinal mass. Lymphoma is the main differential diagnosis. A definitive diagnosis may be made by closed biopsy but is more likely to be confirmed by thoracotomy. Thymomas may be completely contained within the thymic capsule or may spread by local invasion or metastasis. A staging system allows for an accurate prognosis and a therapeutic plan. Surgical removal of encapsulated thymomas may result in long-term survival or cure. Invasive or metastatic thymomas carry a guarded prognosis. Manifestations of the paraneoplastic syndrome complicate treatment. Adjuvant radiation and chemotherapy may be of value for advanced cases; however, adequate clinical trials have not been done in the dog.