1 Disorders of Muscle

Neuromuscular disease

A patient is ataxic and has abnormal postural reactions and reflexes. This is most likely primarily *muscular or neuromuscular* disease.

Weakness:

• stiff gait

• lameness

• trembling

• exercise intolerance

• ventral neck flexion

Patients with generalized muscle disease frequently present with what? Name some examples:

1. Acquired

2. Inherited

Name the two types of muscle disorders:

a. inflammatory (immune mediated and infectious)

d. traumatic

e. endocrine (esp. cushings)/metabolic

Select the muscle disorders which are often characterized as "acquired":

a. inflammatory (immune mediated and infectious)

b. muscular dystrophy

c. CNM of labrador retrievers

d. traumatic

e. endocrine (esp. cushings)/metabolic

b. muscular dystrophy

c. CNM of labrador retrievers (centronuclear myopathy)

Select the muscle disorders which are often characterized as "inherited":

a. inflammatory (immune mediated and infectious)

b. muscular dystrophy

c. CNM of labrador retrievers

d. traumatic

e. endocrine (esp. cushings)/metabolic

Acquired inflammatory myopathies (though I think dermatomyositis also has an inherited component)

Masticatory muscle myositis (MMM), extraocular myositis, dermatomyositis, protozoal myositis, fibrotic myopathy, and canine/feline idiopathic polymyositis are all examples of what?

Masticatory Muscle Myositis:

• Temporalis atrophy

• prominent zygomatic arch

• masseter atrophy

• trismus (dec. range of motion/resistance of mouth)

What is this? How do you know?

Masticatory Muscle Myositis (MMM)

What is a common immune-mediated disorder involving only the muscles of mastication in dogs?

Masticatory muscle myositis (MMM): ONLY muscles of mastication involved = focal

Polymyositis: atrophy may be prominent in temporalis and masseter muscles, but will ALSO have other signs like stiff stilted gait and mild to severe weakness

What is the difference between masticatory muscle myositis and polymyositis?

Masticatory Muscle Myositis (hint: 2M is a specific type of muscle fiber found predominantly in the masticatory muscles of dogs)

With this acquired inflammatory disease, IgG is directed against the unique myosin components of 2M muscle fibers, commonly in large breeds (GSD, doberman, retrievers)

1. Temporalis

2. Masseter

3. Pterygoid

4. Digastricus

Name the 4 muscles of mastication in dogs:

Masticatory muscle myositis

OTHER DIFFERENTIALS FOR A PAINFUL DOG:

1. retrobulbar abscess

2. dental disease

3. TMJ disease

A doberman comes in, O complains dog isn't eating. Dog presents with pain upon opening the mouth and painful palpation of masticatory muscles. Later, the same dog presents with marked atrophy and fibrosis of masticatory muscles with eyes sunk into the orbits. What are you thinking? Name some other differentials for this painful dog.

1. Trigeminal nerve disorder

2. Diffuse polymyositis or polyneuropathy

3. Systemic disease: hypothyroidism, HAC, cancer, cachexia

What if a doberman comes in with clinical signs of Masticatory muscle myositis but is NOT painful, what are differentials for this non painful dog?

1st choice: Circulating 2M autoantibodies prior to steroid treatment.

• 90% positive in acute cases

2nd choice: Immunohistochemistry on muscle biopsies of the masseter and temporalis mnuscles

• NOT frontalis = wrong dx

How can you diagnose masticatory muscle myositis?

Masticatory muscle myositis

Thoughts?

• Glucocorticoids (immunosuppressive doses because MMM is an acquired inflammatory disease).

  • Note: glucocorticoids are a TYPE of corticosteroids

• Commonly prednisone on tapering dose.

• Add azathioprine or cyclosporine if unresponsive.

How do you treat masticatory muscle myositis?

Dogs

Bilateral

Masticatory muscle myositis ONLY AFFECTS *dogs or cats* and is bi/unilateral?

Will cause fractures and dislocation of the jaw, leading to unnecessary trauma

Forced tearing/stretching and opening of the fibrosed muscles under anesthesia USED TO BE recommended for masticatory muscle myositis treatment. Why is it not anymore?

Extraocular Myositis: bilateral exophthalmus and eyelid retraction in YOUNG

What is this?

Canine Idiopathic Polymyositis

This is a diagnosis of EXCLUSION, where you must rule out everything else first when you have diffuse inflammation of skeletal muscle in dogs

TRUE: primary autoimmune OR secondary to systemic immune-mediated disease

TRUE OR FALSE: Canine idiopathic polymyositis can be caused by primary or secondary disease processes.

Canine idiopathic polymyositis

A dog presents with mild to severe weakness, stiff stilted gait that worsens with exercise, atrophy prominent in temporalis and masseter muscles, megaesophagus, and dysphagia. Top differential?

Elevated CK (2-100 fold) and AST is helpful, but MUSCLE BIOPSY IS DEFINITIVE DIAGNOSIS

How do you diagnose canine idiopathic polymyositis?

Boxer

Which dog breed is commonly affected with canine idiopathic polymyositis as a secondary to paraneoplastic lymphoma?

(remember: paraneoplastic is a syndrome not caused by the direct spread or compression of the tumor but are instead triggered by the body's immune system reacting to the lymphoma)

• Treat or rule out underlying disease

• Otherwise, prednisone

• If megaesophagus = bailey chair (that dog high chair from lecture to help prevent regurg)

How can you treat canine idiopathic polymyositis?

CANINE idiopathic polymyositis

Which is more common: *feline or canine* idiopathic polymyositis?

Rule out other causes of common acquired diffuse weakness: *hypokalemia (K) and thiamine deficiency*

Feline idiopathic polymyositis is RARE. Because you know this, what is important to rule out?

DIFFUSE muscle weakness which is mild to severe and sometimes ventroflexion

What is the NUMBER ONE SYMPTOM of feline and canine idiopathic polymyositis?

Mild hypokalemia

70% of cats affected with idiopathic polymyositis suffer from what (I think meaning this is a concurrent problem?)

Predni*solo*ne (because remember prednisone is used in dogs but is not as bioavailable in cats)

If a cat actually IS affected by feline idiopathic polymyositis (because the potassium and thiamine is correct), how can you treat?

Cats: cervical ventroflexion

Hypokalemic myopathy MORE commonly affects what animal? How? (What clinical sign?)

Collies and Shelties

Which dog breeds are more commonly predisposed to dermatomyositis?

Dermatomyositis = do skin and muscle biopsy

A 3 month old collie comes in. O is complaining about skin lesions on the face. You find there is dermatitis but also vasculitis and polymoysitis with erythema, crusts, ulcers, and alopecia on the face and body. Thoughts?

Oral tetracyclines (broad spectrum antibiotics) and niacinamide (B3). Potentially glucocorticoids

How can you treat dermatomyositis?

Fibrotic Myopathy

This nonpainful disorder is characterized by the formation of a fibrous band within the muscle typically because of repetitive muscle injury. Commonly GSD

Gracilis and Semitendinosus muscles (she also had semimembranosus hilighted in class)

What muscles are most commonly affected by fibrotic myopathy?

Ultrasound

How can you diagnose fibrotic myopathy?

Acquired endocrine/metabolic myopathies (disease of muscle)

Glucocorticoid excess/deficiency (hyper/hypoadrenocorticism), hypothyroid dogs, hypokalemic CKD cats, postpartum hypocalcemia, and thiamine deficiency in cats are all examples of what?

Nutrition associated hypokalemic muscle weakness (post-partum)

Though it is more common in cats, dogs can also be present with cervical ventroflexion. What most likely causes it?

Inherited myopathies

Muscular dystrophy, centronuclear myopathy (CNM) of labrador retrievers, myotonia, and metabolic myopathies are all examples of what?

Golden retriever muscular dystrophy

What do you think is happening here because the puppy is so young?

Muscular dystrophy

(note: Hypertrophic muscular dystrophy in cats is characterized by muscle weakness and an unusual increase in muscle size, particularly in the limbs and tongue, which is how it can make sense that with a dystrophic disease there is hypertrophy of the muscles)

This X linked disorder commonly affects siamese cats, with clinical signs like muscular hypertrophy, protruding tongue, and stiff gait appear at 6 months of age. *CK will be elevated*.

Hypokalemia

What is your top differential for a cat who is experiencing ventral neck flexion, weakness, and stiff gait?