Lecture 1- Disorders of Muscles

How does weakness from primary muscle disease differ from neurologic weakness in C/F patients

Patients with muscle disease are usually not ataxic and have normal postural reactions and spinal reflexes

What exams are critical in any weak small animal patient

A thorough physical exam and a complete neurologic exam

What are common clinical signs of generalized muscle disease in dogs and cats

Stiff or stilted gait, lameness, trembling, ventral neck flexion, exercise intolerance, muscle swelling and pain, muscle atrophy or fibrosis

What are the major categories of acquired myopathies

Inflammatory, traumatic, endocrine or metabolic

What are the major categories of inherited myopathies

Muscular dystrophy and centronuclear myopathy of Labrador Retrievers

List the main acquired inflammatory myopathies in this lecture

Masticatory muscle myositis, extraocular myositis, canine idiopathic polymyositis, feline idiopathic polymyositis, dermatomyositis, protozoal myositis, fibrotic myopathy

What is the pathophysiology of masticatory muscle myositis

Immune-mediated disease with IgG directed against the unique 2M myosin fibers of masticatory muscles

Which muscles are affected in masticatory muscle myositis

Temporalis, masseter and other muscles of mastication

Which breeds are predisposed to masticatory muscle myositis

Large breeds including German Shepherd Dogs, Dobermans and retrieving breeds

What are early clinical signs of masticatory muscle myositis

Pain on opening the mouth, painful palpation of masticatory muscles and head, possible swelling of temporalis and masseter muscles

What are late clinical signs of masticatory muscle myositis

Marked atrophy and fibrosis of masticatory muscles, sunken eyes and inability to open the mouth to full range of motion

What is a typical owner complaint in late masticatory muscle myositis

Difficulty opening the dog's mouth or inability to eat normally

What are painful differentials for a dog with painful jaw opening and masticatory muscle swelling

Retrobulbar mass or abscess, dental disease, temporomandibular joint disease or middle ear or bullae disease

What are non-painful differentials for masticatory muscle atrophy

Trigeminal nerve disorder, diffuse polymyositis or polyneuropathy, systemic disease such as hypothyroidism, hyperadrenocorticism or cancer cachexia

What CBC changes may be seen in masticatory muscle myositis

CBC is often normal but may show mild anemia, neutrophilia or peripheral eosinophilia

What chemistry changes may be seen in masticatory muscle myositis

CK, AST and globulins may be mildly increased

How useful is circulating 2M antibody testing in masticatory muscle myositis

Positive in about 85-90 percent of acute cases and should be done before steroid therapy

What tests can definitively diagnose masticatory muscle myositis

Muscle biopsy with immunohistochemistry and electromyography

What is the standard prednisone protocol for masticatory muscle myositis

Prednisone 1-2 mg/kg orally every 12 hours for 3-4 weeks then taper over 4-6 months toward a low effective dose every other day

What is the goal of long-term prednisone therapy in masticatory muscle myositis

Low effective dose given every other day which may be indefinite

When should additional immunosuppressants be added in masticatory muscle myositis

If the patient is not responsive to prednisone alone

Name two adjunct immunosuppressants used for masticatory muscle myositis

Azathioprine and cyclosporine

How should diet be managed in chronic fibrosed masticatory muscle myositis

Provide dietary modification for patients with limited mouth opening

What historical treatment for masticatory muscle myositis is now contraindicated

Forced stretching or tearing open fibrosed muscles under anesthesia

What is the prognosis for masticatory muscle myositis

Guarded

What is the classic clinical presentation of extraocular myositis

Bilateral exophthalmus with eyelid retraction, possible chemosis, no third eyelid prolapse and impaired vision

What is the profile of canine idiopathic polymyositis

Presumed autoimmune diffuse inflammation of skeletal muscles which may be primary or secondary to systemic immune-mediated disease, protozoal infection or neoplasia

Which dogs are predisposed to canine idiopathic polymyositis

Adult large breeds including German Shepherd Dogs, Boxers, Newfoundlands and Vizslas

What are typical clinical signs of canine idiopathic polymyositis

Mild to severe weakness, stiff or stilted gait worsened by exercise, painful muscles or non-painful atrophy, prominent temporalis and masseter atrophy, regurgitation with megaesophagus, dysphagia and weak bark

What are neurologic exam findings in canine idiopathic polymyositis

Usually normal unless there is concurrent polyneuritis

What serum enzyme changes are seen in canine idiopathic polymyositis

CK elevated two- to one hundred-fold and AST elevated

What serum protein change is common in canine idiopathic polymyositis

Elevated gamma globulins

What does EMG show in canine idiopathic polymyositis

Abnormalities affecting multiple muscle groups

What is required for definitive diagnosis of canine idiopathic polymyositis

Muscle biopsy

Which systemic diseases should be investigated in canine idiopathic polymyositis

Systemic lupus erythematosus, toxoplasmosis, neosporosis and neoplasia including paraneoplastic lymphoma

What diagnostic tests are recommended to search for systemic disease in canine idiopathic polymyositis

CBC, chemistry, urinalysis, joint taps, protozoal testing, ANA testing, thoracic and abdominal radiographs, ultrasound and fine needle aspirates

What is the prednisone protocol for canine idiopathic polymyositis

Prednisone 1-2 mg/kg orally every 12 hours with taper at 14 and 28 days if responding aiming for a low effective dose every other day

When should additional immunosuppressants be considered in canine idiopathic polymyositis

If the dog does not respond or relapses during prednisone taper

How is megaesophagus managed in canine idiopathic polymyositis

Elevated feeding, small frequent meals and monitoring for aspiration

What is the prognosis for primary autoimmune canine idiopathic polymyositis

Good if there is no severe megaesophagus or aspiration pneumonia

What is the profile of feline idiopathic polymyositis

Rare cause of diffuse weakness in cats where hypokalemia and thiamine deficiency must be ruled out

What are typical clinical signs of feline idiopathic polymyositis

Sudden onset diffuse muscle weakness with ventral neck flexion, inability to jump and muscle pain

What are neurologic exam findings in feline idiopathic polymyositis

Generally normal

What serum enzyme changes are seen in feline idiopathic polymyositis

Elevated CK and AST

What electrolyte abnormality is common in feline idiopathic polymyositis

About 70 percent of affected cats are mildly hypokalemic

What additional tests are recommended in feline idiopathic polymyositis

Toxoplasma gondii testing, FeLV and FIV testing, thoracic radiographs for thymoma, muscle biopsy and EMG with multifocal abnormalities

What underlying causes must be treated first in feline idiopathic polymyositis

Correct hypokalemia, treat thiamine deficiency and consider toxin or drug-induced polymyositis

What empirical treatment may be used for suspected toxoplasmosis in feline polymyositis

Clindamycin

What glucocorticoid protocol is used for feline idiopathic polymyositis

Prednisolone 4-6 mg/kg per day orally tapering over 8-12 weeks

Why is prednisolone preferred over prednisone in cats

Better oral bioavailability in feline patients

What is the prognosis for feline idiopathic polymyositis

About one-third recover spontaneously but recurrence is common

Which species are mainly affected by hypokalemic myopathy

Cats mainly but also dogs

What are clinical signs of hypokalemic myopathy

Weakness, stiff or stilted gait, exercise intolerance, cervical ventroflexion and muscle pain

How is hypokalemic myopathy diagnosed

Compatible clinical signs with serum potassium less than 4.0 mEq/L

How is hypokalemic myopathy treated

Potassium supplementation and correction of the underlying cause of hypokalemia

What is a typical oral potassium supplementation protocol in hypokalemic myopathy

Potassium gluconate Tumil-K one-quarter teaspoon per 4.5 kg body weight orally twice daily in food

What is the profile of dermatomyositis

Uncommon inherited autoimmune disease characterized by dermatitis, vasculitis and polymyositis

Which breeds are predisposed to dermatomyositis

Familial disease in Rough and Smooth Collies and Shelties with sporadic cases in Welsh Corgis, Australian Cattle Dogs and Border Collies

At what age do dermatomyositis skin lesions typically appear

Three to six months of age and may fluctuate

What are typical skin lesions in dermatomyositis

Erythema, crusts, ulcers, scales and alopecia of inner pinnae, head, face and body

How is dermatomyositis diagnosed

High clinical suspicion, skin and muscle biopsies and EMG showing spontaneous myofiber electrical activity

How are dermatomyositis skin lesions medically managed

Oral tetracyclines and niacinamide or pentoxifylline

How is dermatomyositis-associated myositis managed

Glucocorticoids

What environmental modifications help manage dermatomyositis

Low stress environment and minimizing sun exposure

What is the prognosis for dermatomyositis

Variable but overall good with intermittent flares

What is the profile of fibrotic myopathy

Non-painful disorder characterized by formation of a fibrous band within a muscle likely due to repetitive injury

Which breeds are predisposed to fibrotic myopathy

Any breed with German Shepherd Dogs overrepresented

What are clinical signs of fibrotic myopathy

Mechanical lameness with characteristic gait, neurologically normal and a palpable tight usually non-painful band in the affected muscle

Which pelvic limb muscles are commonly affected in fibrotic myopathy

Gracilis, semitendinosus, quadriceps, biceps femoris and semimembranosus

Which thoracic limb muscles are commonly affected in fibrotic myopathy

Supraspinatus and infraspinatus

How is fibrotic myopathy diagnosed

Based on characteristic lameness, palpation of a fibrotic band and ultrasound of the affected muscle

What are treatment options for fibrotic myopathy

Physical conditioning, rehabilitation and pain control with surgical approaches described but high recurrence

What is the prognosis for fibrotic myopathy

Guarded, usually not cured but quality of life and function can be good

List common endocrine and metabolic causes of myopathy

Glucocorticoid excess or deficiency, hypothyroidism in dogs, hypokalemia in cats with chronic kidney disease, post-partum hypocalcemia and thiamine deficiency in cats

What is tetanus in relation to muscle disease

Sustained contraction of extensor muscles due to Clostridium tetani neurotoxin affecting nerves and not a primary muscle disease

Which species are more commonly affected by tetanus

Dogs more than cats

When do clinical signs of tetanus typically appear after wound infection

Five to ten days

How is tetanus treated

Wound debridement, penicillin, antitoxin and control of muscle spasms

What is risus sardonicus in tetanus

Contracture of facial muscles producing a characteristic grimace

What is the sawhorse stance in tetanus

Generalized rigid extension of limbs and body

List the inherited myopathies discussed

Muscular dystrophy, centronuclear myopathy, myotonia and inherited metabolic myopathies

What is the inheritance pattern of centronuclear myopathy in Labrador Retrievers

Autosomal recessive

What is the inheritance pattern of muscular dystrophy in dogs

X-linked

Which genders are affected by centronuclear myopathy

Both males and females

Which genders are predominantly affected by muscular dystrophy

Predominantly males

What is the age of onset for centronuclear myopathy

Three to four months of age

What is the age of onset for muscular dystrophy

Birth to a few weeks of age

What are clinical signs of centronuclear myopathy in Labradors

Stiff or stilted gait, bunny hopping, muscle atrophy and poor conformation

What are clinical signs of muscular dystrophy in dogs

Stiff or stilted gait, dysphagia, ptyalism and tongue hypertrophy

How are tendon reflexes affected in centronuclear myopathy

Often reduced

How are tendon reflexes affected in muscular dystrophy

Normal until end-stage disease

What are CK levels like in centronuclear myopathy

Normal to mildly elevated

What are CK levels like in muscular dystrophy

Markedly elevated

What therapy is recommended for centronuclear myopathy

L-carnitine supplementation and avoidance of cold, stress and excitement

What therapy is available for muscular dystrophy

No effective therapy

What is the prognosis for centronuclear myopathy

Guarded with disease often stabilizing by one year of age and acceptable quality of life

What is the prognosis for muscular dystrophy

Poor due to progressive disease

Which feline breeds are reported with X-linked muscular dystrophy

Domestic shorthair, Maine Coon and Siamese cats

At what age do clinical signs of feline muscular dystrophy appear

Five to six months of age

What are clinical signs of feline muscular dystrophy

Marked muscular hypertrophy, protruding tongue, stiff or stilted gait and bunny hopping