About 10 days ago, we noticed that the right side of our 4 ...

Hi thereCustomer

Sally asked me to stop in to see if I can help you further.

There are a number of different problems that can cause muscle atrophy as you are describing. Some of these problems cause Bilateral atrophy (on both sides) and some cause Unilateral atrophy. Even though you are only seeing this on one side at the moment, it has been so sudden in onset that it could just be that the second side is lagging, so I would not rule out the bilateral problems just yet.

The top concerns I would have are as follows:

• 1. Masticatory muscle myositis (MMM) which usually affects both sides. It often causes pain on opening the mouth - but NOT always. http://www.petplace.com/dogs/masticatory-muscle-myositis/page1.aspx
• 2. A tumour of the trigeminal nerve sheath. This nerve goes to those affected muscles, and this problem would only occur on one side. Sometimes dogs get inflammation of this nerve for unknown reasons. Here is more: http://www.petplace.com/dogs/trigeminal-neuritis-in-dogs/page1.aspx
• 3. Cushing's disease - very unlikely since your dog has no other symptoms (panting, vastly increased urination so that he couldn't make it through the night, increased appetite, potty belly and hair loss). This causes problems on both sides as well. http://www.petplace.com/dogs/hyperadrenocorticism-cushing-s-syndrome-in-dogs/page1.aspx
• 4. Polymyositis -bilateral muscle atrophy, this inflammation of the muscles can be caused by infection with toxoplasma or neospora organisms, which can be tested for by a blood sample http://www.petplace.com/dogs/polyneuropathy-in-dogs/page1.aspx
• 5. Hypothyroidism - bilateral muscle atrophy (unlikely because would be very gradual onset http://www.petplace.com/dogs/hypothyroidism-in-dogs/page1.aspx)
• 6. Trauma causing inability to open jaw fully or trauma damaging the nerve.

If your boy came to see me, I would want to do some blood-work to check his thyroid, check for abnormalities seen with Cushing's disease, check an MMM titre, and check for antibody levels against toxoplasma and neospora. Muscle/nerve biopsies would probably be the highest yield test, ideally combined with electromyography.

Dr. Diane Shelton is an internist in CA whose lab does all the testing for myasthenia gravis, masticatory myositis and other neuromuscular disease. Her address: Comparative Neuromuscular Lab, Office of Vet Services, University of California, San Diego, Basic Science Bldg Rm 1057, La Jolla, CANNN-NN-NNNN
Her web site: http://medicine.ucsd.edu/vet_neuromuscular

I will copy here from a lecture she gave at a conference in 2002 to the American College of Veterinary Internal Medicine:

Immune-Mediated Neuromuscular Disorders

ACVIM 2002

G. Diane Shelton, DVM, PhD, DACVIM
La Jolla, CA

INTRODUCTION

Immune-mediated disorders are among the most common causes of neuromuscular weakness in the canine population. This discussion will focus primarily on the two most common of these disorders, acquired myasthenia gravis (MG) and masticatory muscle myositis (MMM), with brief discussions of the less frequently occurring polymyositis (PM), extraocular myositis (EOM) and dermatomyositis (DM).

ACQUIRED MYASTHENIA GRAVIS

Acquired myasthenia gravis (MG) is a neuromuscular disorder characterized by focal or generalized muscle weakness resulting from immune-mediated destruction of acetylcholine receptors (AChRs) at the neuromuscular junction. Acquired MG is probably the most common neuromuscular disorder we can diagnose in the dog. Recent studies have identified a high relative risk for acquired MG in the Akita, Scottish Terrier, German Shorthaired Pointer, and Chihuahua breeds of dogs, and Abyssinian and Somali cats. Familial MG has also been identified in Newfoundlands and Great Danes.

Clinical Presentations

Acquired MG may be divided into focal (weakness localized to pharyngeal and esophageal musculature), generalized (generalized weakness and esophageal/pharyngeal weakness), acute fulminating (severe generalized weakness), and paraneoplastic forms. The palpebral reflex may be weak, fatigable, or absent. Neurological evaluation will usually be normal if performed before weakness develops. Tendon reflexes are usually normal but may be fatigable. In acute fulminating MG, dogs exhibit rapid development and progression of generalized appendicular weakness and may present non-ambulatory in lateral recumbency. This form of MG may mimic lower motor neuron diseases such as tick paralysis, botulism, and polyradiculoneuritis. Due to the diversity of clinical presentations, acquired MG should be on the list of differential diagnoses of most lower motor neuron disorders.

Diagnosis

The "gold standard" for the diagnosis of acquired MG in both dogs and cats is the documentation of antibodies against muscle type nicotinic AChRs by immunoprecipitation radioimmunoassay (test available through the Comparative Neuromuscular Laboratory, University of California, San Diego; Website: http://medicine.ucsd.edu/vet_neuromuscular; Email:XXX@XXXXXX.XXX). This assay has proven to be both sensitive and specific. While false positive results are extremely rare, approximately 2% of dogs with generalized MG may be "seronegative." An edrophonium chloride challenge (0.1 mg/kg IV) may be used for a presumptive diagnosis. A dramatic increase in muscle strength is a positive response. As false positive and negative results may occur with this test, it should not be used to "make or break" the diagnosis. Although associated with an anesthetic risk in a possibly critical patient, electrophysiological evaluation of a decrement in the muscle action potential following repetitive nerve stimulation has also been used in the diagnosis of MG. Thoracic radiographs should be evaluated for the presence of megaesophagus and a cranial mediastinal mass. An electrocardiogram should also be evaluated if bradycardia is present since 3^rd degree heart block has been documented in a subpopulation of canine MG patients. MG has also been associated with other disorders including hypothyroidism, other autoimmune diseases, thymoma, thymic cysts, cholangiocellular carcinoma, and osteogenic sarcoma.

Treatment

There is considerable controversy over the appropriate therapy for MG. Because of a lack of appropriately controlled and well-designed clinical trials in veterinary medicine, optimum treatments for acquired MG have not been established. After study of virtually thousands of cases of canine MG over a 15-year period, this author makes the following recommendations:

Altered feeding procedures including elevation of food and water or placement of a PEG tube for maintenance of hydration, nutrition, and drug delivery if esophageal dilatation is present.

Anticholinesterase drugs to prolong the action of acetylcholine at the neuromuscular junction and enhance neuromuscular transmission. The most commonly used drug is pyridostigmine bromide (Mestinon, 1-3 mg/kg administered orally BID or TID).

IF AN OPTIMAL RESPONSE TO THE ABOVE THERAPY IS NOT OBTAINED, corticosteroid therapy may be initiated. Immunosuppressive dosages are not advised, as there may be a worsening of muscle weakness. Low-dose alternate day prednisone therapy (0.5 mg/kg every other day) is suggested.

In cases of severe generalized MG (acute fulminating), intensive care is usually required including ventilatory support. In human patients, expensive short-term treatment modalities such as plasma exchange (plasmapheresis) and intravenous immunoglobulin may be used; however, because of the expense and technical difficulty of performing these therapies, they are not routinely used in veterinary medicine. In the absence of aspiration pneumonia, high-dose intravenous methylprednisolone therapy may be of benefit in severe cases of MG. In one human study (Arsura 1985), 2 g of methylprednisolone was given IV every 5 days to 15 patients that had severe generalized MG. Satisfactory improvement without worsening of muscle weakness occurred in 10 of 15 patients.

Compared to the rate of spontaneous remission rates in dogs treated with only altered feeding procedures and Mestinon, the benefit of other immunosuppressive agents including azathioprine is not known.

Spay myasthenic female dogs and cats as soon as possible after MG is under control, because heat cycles and pregnancy can exacerbate active MG.

Vaccinations may exacerbate active MG.

Avoid use of drugs that may affect neuromuscular transmission such as ampicillin, aminoglycosides, antiarrhythmic agents, phenothiazines, anesthetics, narcotics, and muscle relaxants. Also, organophosphate dips may result in a cholinergic crisis since they could be additive with pyridostigmine.

An early, accurate diagnosis, appropriate therapy, and dedicated owners are keys to a good clinical outcome in MG. In the absence of severe aspiration pneumonia, pharyngeal weakness, or acute fulminating MG, the prognosis for survival and complete remission should be good. If thymoma is present, the prognosis is guarded unless there is complete surgical removal of the neoplasia. Concurrent hypothyroidism should be treated if present. Follow up AChR antibody titers should be evaluated every 6-8 weeks since they return to normal range with clinical remission of the disease.

MASTICATORY MUSCLE MYOSITIS

After acquired MG, masticatory muscle myositis (MMM) is the second most common neuromuscular disorder occurring in the dog. Several large and small breeds of dogs may be affected. As with MG, an early and accurate diagnosis is important for a favorable clinical outcome.

Clinical Presentations

Clinical signs include muscle atrophy and/or swelling restricted to the muscles of mastication, jaw pain, exophthalmos in the acute stage and enophthalmos in the chronic stage with marked muscle atrophy, and abnormalities of jaw movement including trismus or in rare cases inability to close the jaw. Inability to open the jaws under anesthesia is a classical finding in MMM.

Diagnosis

Evaluation of a muscle biopsy specimen from the temporalis or masseter muscle is essential for confirming the diagnosis and determining prognosis. The degree of inflammation, amount of fibrosis and myofiber destruction can all be evaluated. One common mistake is the platysma muscle is biopsied and not the temporalis muscle. The platysma muscle is not affected in MMM. Demonstration of autoantibodies against masticatory muscle type 2M fibers in an immunocytochemical assay is also diagnostic of MMM but cannot be used for prognosis. (this test also available through the Comparative Neuromuscular Laboratory, University of California, San Diego). The serum creatine kinase (CK) concentration may be normal or mildly elevated. Electromyography may demonstrate abnormalities restricted to the masticatory muscles but is not necessary for the diagnosis. Radiographic evaluation for temporomandibular joint abnormalities and probing behind the last upper molar for the presence of a retroorbital abscess should also be performed while the animal is under anesthesia. A retroorbital abscess is commonly confused with MMM.

Treatment

Immunosuppressive therapy must be used. Prednisone (1-2 mg/kg BID) should be initiated until the serum CK returns to the normal range (if elevated) and jaw function returns to normal. The dosage should then be decreased until the lowest alternate day dosage is obtained that will keep the dog free of clinical signs. Alternate day therapy should be maintained for 4-6 months. Azothioprine therapy may be added in cases that do not respond optimally to corticosteroids alone or where the side effects of the corticosteroids cannot be tolerated. In the absence of marked fibrosis and myofiber destruction, the prognosis should be good for return of muscle mass and function. Inflammation and myofiber destruction can be particularly severe in the Rottweiler, Doberman, and Samoyed breeds of dogs. Early diagnosis and treatment would be particularly important in these breeds to prevent irreversible myofiber damage. With inadequate drug dosages and duration of therapy, clinical signs will return and may be more difficult to manage.

POLYMYOSITIS

Polymyositis occurs less frequently than MMM and can look clinically similar to MG with generalized weakness and esophageal dilatation. Underlying causes include immune-mediated and paraneoplastic disorders, and as a complication of infectious agents. Although poorly characterized in veterinary medicine, adverse reactions to drugs may also result in an inflammatory myopathy.

Clinical Presentation

Various breeds of dogs may be affected with polymyositis (PM) with no obvious age or sex predisposition. Newfoundland and Boxer dogs may be over-represented. Clinical signs may include a stiff stilted gait, muscle pain, muscle weakness, and exercise intolerance. Generalized muscle swelling or atrophy may be present that includes the muscles of mastication. Regurgitation of food and/or water, or difficulty swallowing may be present as a result of pharyngeal or esophageal weakness.

Diagnosis

The muscle biopsy is the single most important test to confirm a diagnosis of PM. If confirmed, further testing for infectious agents including antibody titers for Toxoplasma gondii, Neospora caninum, and tick-related diseases (Lymes disease, Ehrlichiosis, and Rocky Mountain Spotted fever) should be performed. Bacterial myositides are rare but may occur. Serum CK should also be evaluated, but may or may not be elevated. A normal serum CK concentration should not rule out an inflammatory myopathy.

Treatment

Specific therapy for any identified infectious agent or neoplasia should be initiated. Elevate food and water in cases with dysphagia and regurgitation. If hydration and caloric intake cannot be maintained, a PEG tube should be placed. If an immune-mediated disorder is most likely, immunosuppressive therapy should be initiated (See discussion under MMM). Residual muscle atrophy may be present in chronic conditions with extensive myofiber loss, fatty infiltration and fibrosis. Early diagnosis and appropriate therapy is essential to a good clinical outcome. Serum CK concentration should be monitored if elevated and used as a guide to therapeutic response. Supplementation with L-carnitine (50 mg/kg BID) may be beneficial in improving muscle strength during corticosteroid treatment. Adequate activity and exercise is also important in preventing additional steroid related muscle atrophy.

EXTRAOCULAR MYOSITIS

The clinical presentation of extraocular myositis (EOM) may be similar to an acute stage of MMM with bilateral exophthalmos and impaired vision. Golden Retrievers may be particularly susceptible. Serum CK concentration should be normal or mildly elevated. The immunocytochemical assay for antibodies against masticatory muscle type 2M fibers should be negative. An orbital sonogram may demonstrate swollen extraocular muscles. A biopsy from the masticatory muscles should be normal. Mononuclear cell infiltration in EOM is restricted to the extraocular muscles. Immunosuppressive dosages of corticosteroids, as for MMM, should result in decreased swelling of the extraocular muscles. Prognosis should be favorable if diagnosed correctly and treated adequately. With inadequate therapy and chronic disease, fibrosis of the extraocular muscles with resulting strabismus may occur.

DERMATOMYOSITIS

Characteristic skin lesions should be present in breeds predisposed to this disorder (Rough coated collies, Shetland sheepdogs, Australian cattle dogs). Concurrent muscle atrophy, pain or abnormal gait may be present similar to PM. Age of onset has been reported to be 3-5 months for familial dermatomyositis. Skin biopsy should be diagnostic and demonstrate characteristic lesions. Muscle biopsy should demonstrate an inflammatory myopathy, however, this has not been a consistent finding. The temporalis and cranial tibial muscles have been the most useful in the identification of cellular infiltrates. Treatment consists of immunosuppressive therapy as in other immune-mediated muscle disorders. Appropriate breeding programs should be initiated aiming to eliminate this disorder from the line. An autosomal dominant inheritance pattern has been reported in the Rough Coated Collies and Shetland Sheepdogs.

REFERENCES

1. Shelton GD, Lindstrom JM (2001). Spontaneous remission in canine myasthenia gravis: implications for assessing human MG therapies. Neurology 57:2139-2141.

2. Shelton GD (2002). Myasthenia gravis and disorders of neuromuscular transmission. In: The Veterinary Clinics of North America. Small Animal Practice. Neuromuscular Diseases (Shelton GD, ed). W.B. Saunders Co., Philadelphia, 32:189-206. (Recent review of myasthenia gravis in dogs and cats).

3. Podell M (2002). Inflammatory myopathy. In: The Veterinary Clinics of North America. Small Animal Practice. Neuromuscular Diseases (Shelton GD, ed). W.B. Saunders Co., Philadelphia, 32:147-167. (Recent review of inflammatory myopathies in dogs and cats).

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

G. D. Shelton, DVM, PhD, DACVIM
Pathology, SVM
University of California - San Diego
La Jolla, CANNN-NN-NNNN

So, there are a number of different things that could be causing this... and sometimes despite tests we just call it "idiopathic" which means we don't know what is causing it! But I would definitely go ahead with the biopsy and at the very least, MMM titre as a starting place. I hope that these tests will allow your vet to make a diagnosis and start treatment.

If this has been helpful, please accept my answer and leave feedback. If you need more information, just click on reply and I will be happy to help you!

Fiona

Expert:	Dr.Fiona
Pos. Feedback:	99.9 %
Accepts:	1652

Dog Veterinarian

16 years experience as a companion animal veterinarian in British Columbia, California and Ontario

Ask this Expert a Question >