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Myopathies

Marguerite F. Knipe, DVM, DACVIM (Neurology)

School of Veterinary Medicine, University of California Davis, Davis, CA, USA

Etiology/Pathophysiology Signalment

Myocytes are specialized cells, with contractile elements Inherited myopathic processes (Box 75.1) typically
(actin and myosin) comprising the majority of the intra- exhibit clinical signs in young animals (<6 months old)
cellular space, along with the other organelles (mito- and may be static or progressive. Acquired myopathies
chondria, lysosomes, etc.) Disruption of myocytes (Box 75.2) may develop at any age, and are often progres-
(myofibers) will result in decreased ability to contract the sive unless the underlying cause is addressed.
muscle with appropriate strength. Myopathies occur Some myopathies have (to date) only been described in
secondary to either intrinsic pathology within the certain breeds (Box 75.3), and presentation by one of
myofiber itself (noninflammatory myopathy) or infiltra- these breeds with appropriate clinical signs should raise
tion and damage by inflammatory cells (myositis) from suspicion for one of these breed‐associated myopathies.
infectious or immune‐mediated disease. Any disruption
of the muscle membrane or intracellular structures
results in myofiber dysfunction, whether through accu- History and Clinical Signs
mulation of storage products, through cellular infiltra-
tion, or dysfunction of the sarcolemma, other cytoskeletal Functional manifestations of myopathies may include
proteins, or membrane ion channels. gait abnormalities (lameness, stiff or stilted gait), exer-
cise intolerance, focal or generalized weakness, dyspho-
nia, dysphagia, regurgitation, possibly megaesophagus,
Epidemiology muscle tremors and/or fasciculations, and postural alter-
ations (e.g., cervical ventroflexion, especially in cats).
The wide range of etiologies for myopathies makes The history and clinical signs of patients with myopa-
delineating epidemiology difficult for each specific dis- thies are often the same as with other neuromuscular
ease. About one‐third of canine generalized inflamma- diseases (neuropathies and junctionopathies), so noting
tory myopathies (myositides) are associated with these signs in the patient’s history often just localizes a
infectious agents, although a causal relationship will be problem to the neuromuscular system, and not necessar-
difficult to determine unless histopathologically con- ily specifically to a nerve, muscle, or junction problem.
firmed. There is also an association with canine polymy- Specific physical manifestations of myopathies may
ositis and skeletal muscle lymphoma, whether through include decreased muscle mass (atrophy or hypotrophy)
malignant transformation of infiltrating lymphocytes, a or even hypertrophied muscles in some diseases (myoto-
paraneoplastic syndrome, or as primary skeletal muscle nia congenita, some muscular dystrophies). Very young
lymphoma. patients with muscle diseases may have limb deformities

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical