Medycyna Wet. 2011, 67 (5) 347
Opis przypadku Case report
Myasthenia gravis is the most commonly diagnosed disease of the neuromuscular transmission in dogs. The hallmark seems to be a general weakness of appendicular muscles, especially during exercise, which passes after a rest, but as many as 43% of diagnosed myasthenia cases do not exhibit this symptom (10). On the molecular level, myasthenia affects the postsynaptic membrane of the motor plaque (14).
There are two forms of myasthenia: congenital and acquired. Congenital myasthenia gravis manifests itself at the age of 3-8 weeks. An insufficient number of nicotinic acetylcholine receptors in the neuromuscular junctions makes it impossible for the animal to develop physiological motor activity (12, 13). Acquired myasthe-nia is a disease with autoimmunological background. Antibodies to the main immunogenic region (MIR) on the á 61-76 end of the nicotinic acetylcholine receptor (AChR) are produced. Lymphocytes T specific for the á 100-116 end of the AChR also play a role in this immune response (3). There is a breed predilection with Akitas, terrier group, Scottish Terriers, German Short-haired Pointers and Chihuahuas (8). A familial predi-lection with Newfoundland, Mastiff and Great Dane was described, which indicates a genetic etiology of the disease (5).
Acquired myasthenia gravis is categorized into three clinical forms: focal (fatigue of the pharyngeal, laryn-geal and facial musculature), generalized (fatigue of the appendicular musculature, with or without megaoeso-phagus) and fulminating (sudden tetraparesis, tetraple-gia, megaoesophagus and respiratory insufficiency) (5). The disease sometimes occurs as a paraneoplastic syndrome. Thymoma frequently accompanies the de-velopment of myasthenia gravis in humans (75%) (7), while in dogs it was certified in less than 5% of cases (12). Very rarely, acquired myasthenia is connected with osteogenic sarcoma, cholangiocellular carcinoma, anal sac adenocarcinoma, cutaneous lymphoma (7, 9) and non-neoplastic thymic diseases, such as thymic hyper-plasia or cystic thymus (1). Myasthenia coexisting with dysautonomia in a dog was also reported (2).
Description of the case
A 7-year-old intact male German Shepherd dog was referred to the University of Warmia and Mazury for evaluation of a three-day history of abnormal gait. The initial clinical sign was the lameness of the right pelvic limb, which very rapidly progressed to paraparesis and tetraparesis. During examina-tion, the dog was unable to support its own weight or walk, even when assisted. Mentation was considered to be normal;
Familial form of autoimmune myasthenia gravis
in a dog: a case report
AGNIESZKA DRZEWIECKA, KATARZYNA PADZIOR, TADEUSZ ROTKIEWICZ, SYLWIA LEW*, MI£OS£AWA KWIATKOWSKA*
Department of Pathological Anatomy, Faculty of Veterinary Medicine, University of Warmia and Mazury, Oczapowskiego 13bl.D, 10-719 Olsztyn, Poland
*Department of Internal Medicine, Faculty of Veterinary Medicine, University of Warmia and Mazury, Oczapowskiego 14, 10-719 Olsztyn, Poland
Drzewiecka A., Padzior K., Rotkiewicz T., Lew S., Kwiatkowska M.
Familial form of autoimmune myasthenia gravis in a dog: a case report
Summary
This paper describes clinical and pathomorphological features of myasthenia gravis in a 7-year-old male German Shepherd dog. At clinical examination, many neurological deficits as well as swallowing and urinating problems were detected. Radiological examination revealed megaoesophagus and the ELISA test showed an elevated serum acetylcholine receptor (AChR) antibody titer. No improvement was observed after the administration of edrophonium hydrochloride. The diagnosis of myasthenia gravis was confirmed by electromyography. Despite the administration of pyridostygmine bromide, dexamethasone and antibiotics, no improvement followed, and the dog died. Necropsy was conducted: histopathological changes in the esophagus, cranial tibial muscle and lungs were described as muscular atrophy, muscular fibrosis and aspiration pneumonia. The dogs littermate was examined and showed an elevated AChR antibody titer too, but was asymptomatic.
Medycyna Wet. 2011, 67 (5) 348
the dog was very calm for a German Shepherd. All postural reactions, such as hemiwalking and hopping, were depressed in all four limbs, even though the dog was supported. The muscles of the pelvic limb were weak and had a reduced mass. Proprioception was absent in all four limbs. The patellar reflex and the cranial tibial muscle reflex were normal, whereas the withdrawal reflexes were diminished. Examination of the cranial nerve revealed a bilaterally decreased menace response and a decreased palpebral reflex. The owners reported that the dog had had a problem with chewing and swallowing, which was reflected in the absence of the gag reflex. Ptyalism was also observed. The dog could not urinate, so catheteriza-tion and manual expression were necessary. A blood exami-nation revealed a mild lymhocytosis; chemistry profile reve-aled slightly elevated aspartate transaminase (AST; 52 U/I) and alanine transaminase (ALT; 110 U/I); creatine kinase (CK) was elevated (829 mmol/l) as well. On the basis of the symp-toms and the blood examination, a differential diagnosis was made, including diffuse diseases of the lower motor neuron cell body and ventral root, such as polyradiculoneuritis, poly-neuropathies, disorders of the neuromuscular junction, such as myasthenia gravis, botulism, and polymyopathy.
Consequ-ently, radiographs of the backbone, hind limbs and thorax were made. A lateral thoracic radiograph revealed megaoesopha-gus and features of secondary aspiration bronchopneumonia. This finding correlated with the regurgitation of large volumes of fluids and a respiratory failure. The diagnostic hypothesis of myasthenia gravis (MG) was made, and the serum anti--AChR antibody titer was measured (ELISA) with a positive result. The titer for TE671 cells was 4300 (range is 400), and the titer for homogenized cells was 1800 (range is 300), which is typical of acquired MG. The diagnosis was confirmed by electromyography, which revealed changes characteristic of this disease. No strength improvement was noted after an intravenous administration of edrophonium hydrochloride at 0.1 mg/kg. Despite the lack of improvement, the dog was given pyridostygmine bromide at 1.1 mg/kg orally every 12 hours. In addition, dexamethasone was administered intrave-nously once a day at a dose of 0.2 mg/kg. Bronchopneumonia was treated with cefuroxime at 15 mg/kg subcutaneously twice a day, and with enrofloxacin at 5 mg/kg subcutaneously once a day. Because of anorexia, the dog was given glucose infusions. Forty-eight hours after the administration of pyri-dostygmine bromide no improvement was noted. During for-Fig. 1. The cranial tibial muscle: mild inflammatory
infiltra-tion of the muscle fiber. Van Gieson staining Fig. 2. Lamina muscularis of the oesophagus: muscular atro-phy, fibrosis and mild inflammatory infiltration. Van Gieson staining
Fig. 4. The oesophagus layers cross section: muscular atro-phy and fibrosis of lamina muscularis. Hematoxylin and eosin staining
Fig. 3. The cranial tibial muscle: muscular atrophy. Hemato-xylin and eosin staining
Medycyna Wet. 2011, 67 (5) 349 ward auscultation, lung oedema was diagnosed, and
there-fore furosemidum was administered intravenously at 4 mg/kg a day. Unfortunately, the dogs condition did not improve, and the animal died.
An autopsy confirmed megaoesophagus and aspiration pneumonia. The thymus was atrophic. Histopathological exa-mination of the lungs, oesophagus and cranial tibial muscle was conducted. Tissue was fixed in 10% neutral buffered formalin and embedded in paraffin. Sections of 5 µm were prepared and stained with hematoxylineosin (HE), iron hematoxylinpicric acidacid fuchsin (van Gieson), iron hema-toxylinlight green (Masson), periodic acidSchiff (PAS), and hematoxylinbasic fuchsinpicric acid (HBFP). In the dilated part of the oesophagus, a distinct atrophy of the mu-scular membrane, fibrosis, and mild inflammatory infiltration were described (fig. 2, 4). There was no necrosis visible. Similar changes were described in the macroscopically undilated part of the oesophagus. The cranial tibial muscle was atrophic: muscle fibers were thin and hypostained. As in the oesopha-gus, there was no necrosis, but mild inflammatory infiltration was found (fig. 1, 3).
Discussion
Acquired myasthenia gravis should be high on the list of differential diagnoses in any dog with muscle weakness. The gold standard for the diagnosis is immunoprecipitation radioimmunoassay, sensitive and specific to autoantibodies against AChR (10). ELISA, another laboratory test detecting autoantibodies, does not involve the use of radioactive substances and can be performed in every laboratory (14). Even if the level of circulating autoantibodies is not abnormally high, myasthenia gravis should not be excluded from the differential diagnosis list, because it may take months before the antibodies reach the critical level (6). This cases littermate had a high autoantibody titer. This can suggest a genetic (familial) background of the disease, but it is not evident as the littermate had no clinical signs of the disease. Familial predisposition to myasthe-nia gravis was previously described in the Great Dane breed (5). Other tests include anticholinesterase chal-lenge and electromyography, but these are far less specific (10).
There is a poor correlation between the absolute AChR antibody titer and the severity of clinical weak-ness. The presence of titin and ryanodine receptor (RyR) antibodies is associated with a later onset of the disease, while the presence of thymoma and RyR antibodies is correlated with its more severe course (11).
The available therapeutic agents, such as acetyl-cholinesterase inhibitors and corticosteroids, are still insufficiently effective, and the mortality rate remains unacceptably high (10). As a result, other therapeutic options are developed, including vaccines. In research by Galin et al. (3), peptides complementary to certain autoantigenic determinants called antigen receptor mimetics (ARM) elicited responses against certain auto-reactive T and B lymphocytes. This therapeutic vaccine increased the remission rate from 17% (spontaneous remissions) to 75%, and even when a spontaneous re-mission occurred, the vaccine accelerated the return of
the autoantibody titer to a normal level. Vaccinated ani-mals no longer required acetylcholinesterase inhibitors. The induction of antigen-specific oral tolerance (by oral administration of a B-cell epitope-free AChR fragment) was successful in experimental myasthenia gravis in rats, and it can be an ideal therapeutic device for canine MG in the future (4).
Megaoesophagus and aspiration pneumonia are common clinical and pathomorphological findings in acquired myasthenia gravis (2, 6). The second most com-mon macroscopic feature in necropsy is a thymic tumour, but it did not occur in this case. Paraffin sections can be stained according to many different histological and histochemical procedures: hematoxylineosin (HE), modified Gomori trichrome, Oil-red-O, periodic acid Schiff (PAS), cytochrome oxidase (COX), succinate dehydrogenase (SDH), and reduced nicotinamide ade-nine dinucleotide tetrazolium reductase (NADH-TR) (7). Microscopic changes are easy to describe when one of the differentiating (between connective and muscle tissue) staining methods (Masson, van Gieson) is used. When samples are frozen, an immunohistochemical analysis of the tissue is possible. Histopathological fin-dings in the muscle tissue, as well as in the oesophagus, include muscular atrophy, muscle fibers with angular profiles, and fibrosis (7).
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Corresponding author: vet. surg. Katarzyna Padzior, ul. Oczapowskiego 13/d, 19-719 Olsztyn, Poland; e-mail: kaskap17@gmail.com
