Page 845 - Clinical Small Animal Internal Medicine

75 Myopathies 813

Electromyography is useful to confirm that the motor biopsy, but again, the small sample makes it highly likely
VetBooks.ir unit is affected and helps the clinician determine the that microorganisms may be missed.
Noninflammatory myopathies may have vacuoles,
extent of the disease process (focal, generalized). EMG
may be especially helpful to screen for subclinical dis-
ease; a common recommendation for EMG examination abnormal accumulations within the myofibers, loss of
normal cytoskeletal architecture (cores), necrosis and
is to assess a minimum of two limbs, usually the thoracic phagocytosis with macrophages only, may have nonspe-
and pelvic limbs on one side of the patient. Often, only cific abnormalities, or may be completely normal on
one side is examined with EMG to maintain the opposite histopathology.
limbs free of iatrogenic mechanical injury from the
exploring needle so biopsies can be obtained. Sometimes
an additional limb is examined (e.g., both pelvic limbs Therapy
and a thoracic limb), especially if the clinician wants to
assess the contralateral limb for comparison. For inflammatory myopathies, specific treatment for the
infectious process or immunosuppression is indicated, but
Histology for most noninflammatory myopathies, unless an underly-
ing or concurrent treatable disease is identified that could
A definitive diagnosis of a myopathy is based on histo- cause the myopathy (e.g., electrolyte imbalance, endocrine
logic abnormalities of the muscle. For the clinician, disease, etc.), there are no definitive treatments.
biopsy selection, handling, and processing are extremely
important. Prior to biopsy, clinicians must decide
whether they will submit fresh muscle to a specialized Nonspecific Therapy
laboratory for processing, or will take samples to submit Nutritional supplementation with L‐carnitine (50 mg/kg
in formalin for routine histopathologic evaluation. Fresh PO q12h), co‐enzyme Q10 (100 mg PO q24h), and vita-
muscle must be packed appropriately and shipped over- min B complex for “muscle support” has been recom-
night to the lab, so the clinician must schedule sampling mended for many noninflammatory myopathies, and
and shipping so that the muscle arrives in good there are only anecdotal reports of clinical improvement
condition. with these medications.
For generalized disease processes, biopsies of the
triceps, biceps femoris/quadriceps, and cranial tibial
muscles permit evaluation of both proximal and distal Inflammatory Myopathies
muscles, as well as the thoracic and pelvic limbs. If an infectious etiology (parasitic, bacterial, protozoal,
Temporalis muscle, if affected, is another muscle etc.) is identified on biopsy or suspected clinically, appro-
frequently sampled, particularly if there is a clinical priate antimicrobial therapy should be instituted. In
suspicion of MMM (although measuring serum 2M anti- small animal medicine, immune‐mediated myositides
body titer is less invasive – see Hematology/Serology are likely more common than infectious, and immuno-
earlier). If a different, specific muscle is the only one suppressive therapy is required. In some cases, immune‐
affected, then the clinician should consider sampling mediated myositis may be secondary to a chronic
that as well. By sampling multiple muscles, the clinician infection. Clinicians may choose from several different
increases the likelihood of obtaining a biopsy with path- immunomodulatory drugs, including glucocorticoids,
3
ologic changes, since only a small portion (about 1 cm ) azathioprine, and cyclosporine.
of any muscle is taken, and focal or multifocal processes
may be missed. Two laboratories process international
veterinary fresh muscle samples, and information on
shipping is available on their websites (see Resources Prognosis
below).
Muscle histopathology is broadly categorized into Prognosis will depend on the specific underlying myopa-
inflammatory (true myositides) or noninflammatory thy, and can range from grave to excellent. Some inher-
(dystrophic) myopathies (see Boxes 75.1 and 75.2). ited myopathies may have tolerable clinical signs and
Macrophages are present for myofiber phagocytosis function throughout the life of the patient (e.g., some
from any cause, but true myositis is characterized by myotonias), while others progress relentlessly to mark-
excessive cellular infiltration in the endo‐ and perimy- edly impair quality of life (e.g., muscular dystrophies).
sium, with lymphocytes, neutrophils, and eosinophils, as For most acquired myopathies, if the underlying cause
well as macrophages. If the myositis is secondary to an is identified and appropriately treated, patients may have
infection, the causative agent may be apparent in the an excellent outcome.