Recommended Diagnostic Work-up for Atrophy
Equine Muscle Atrophy
What is muscle atrophy?
Muscle atrophy is not a single disease, it is simply the loss of muscle mass and it can have a range of clinical presentations and underlying causes. This complexity can make it a challenge to diagnose its underlying cause and to treat appropriately.
What causes muscle atrophy?
Muscular atrophy can be grouped into three broad categories based on the underlying reason for the atrophy. Muscle atrophy can be caused by:
(1) a problem in the nerve innervating the muscle (neurogenic),
(2) damage to muscle fibers (myogenic) or
(3) a systemic disease that causes generalized weight loss.
Within each category, there can be specific diseases affecting the nerves, muscles or body weight, as outlined below:
(A) Focal atrophy –involves a specific muscle (i.e. one site is atrophied)
Trauma to a nerve
Compression of the nerves by tumors or scar tissue
Equine Protozoal myeloencephalitis (EPM)
(B) Generalized Muscle Atrophy (typically many sites are atrophied)
Equine motor neuron disease (EMND)
Polysaccharide storage myopathy (PSSM)- homozygotes with two copies of the mutation can have topline atrophy
Inflammatory myositis- Lyme’s disease
Immune mediated myositis (IMM)
Rimmed vacuolar myopathy
IMM and calciphylaxis
Vitamin E deficient mitochondrial myopathy
(3) Systemic disease
Chronic granulomatous disease (sarcoidosis)
Lymphosarcoma or other tumors
What is the best approach to find the cause of muscle atrophy?
It is important to determine the cause of muscle atrophy in order to determine the most appropriate treatment plan. First, a detailed history should be taken, including speed of onset, duration, previous episodes of atrophy or tying up, appetite, diet (including vitamin/mineral supplementation), changes in attitude, and frequency of lying down. Second, a physical exam of the horse should be completed, attempting to determine if the muscle loss is due to a (1) neuropathic, (2) myopathic or (3) systemic disease process. Standing the horse square on level ground, evaluate the degree of muscle loss from the side and behind. In some cases of mild atrophy, it may be easier to feel the degree of atrophy than see it. An oral exam should be completed, examining the teeth, masseter and temporal muscles. All muscle groups should be examined for tone, pain and twitching. A neurologic exam can determine if there is weakness or ataxia.
Finally, certain diagnostic tests may be appropriate based on the history and clinical signs and suspected type of myopathy. A diagnostic tree for muscle atrophy is provided below for suggestions.
Potentially helpful diagnostic tests include;
1. Ultrasound: This helps determine the extent and depth of focal muscle atrophy. Compare the left and right sides in the same spot to assess degree of muscle involvement.
2. Electromyography (EMG). If available, this can identify muscles with neurogenic atrophy through the presence of positive sharp waves and abnormal electrical potentials
3. CK and AST activity. Rule out myodegenerative processes such as IMM or severe rhabdomyolysis
4. Serum vitamin E – Rule out EMND, vitamin E deficient myopathy
5. GYS1 genetic test for type 1 PSSM. A genetic test is available for horses with severe chronic topline atrophy or drafts with generalized atrophy
6. ACTH or dexamethasone suppression test if Cushings-related muscle loss is suspected.
7. Muscle biopsy provides information about the type, cause and severity of theatrophy at a cellular level, and likelihood muscle will regenerate as outlined below.
How does a muscle biopsy help a horse with muscle atrophy? What muscle should be biopsied?
Muscle biopsies can distinguish neurogenic and myogenic atrophy, provide information of the severity of muscle degeneration and regeneration, amount of fibrosis which will prevent atrophy and help determine the cause. The information gained from a muscle biopsy can guide and monitor treatment programs. If submitting a muscle biopsy, it is critical to biopsy the correct muscle(s) for the suspected type of atrophy.
1) For generalized atrophy and suspected EMND, a biopsy of the sacrocaudalis muscle is recommended.
2) For atrophy of the topline muscles and suspected immune-mediated myopathies, two biopsies are recommended;
a. Fresh semimembranosus biopsy
b. Formalin-fixed TruCut samples (14 gauge) of the epaxial muscles.
3) For evaluation of focal muscle atrophy, a fresh open biopsy of the specific muscle group affected is highly recommended.
4) For exertional myopathies and suspected PSSM-related atrophy semimembranosus muscle.
What determines how fast a muscle will atrophy?
Some disease processes cause very rapid muscle atrophy. A third of the muscle mass may disappear in a day with immune-mediated myositis and rimmed vacuolar myopathy.
If very severe, acute rhabdomyolysis can result in muscle atrophy within a week of onset. Muscle wasting over a period of a month occurs with vitamin E deficient myopathy, EPM and trauma to a nerve. Slow progressive atrophy occurs with EMND and systemic diseases
Why is it important to determine if muscle atrophy is symmetrical?
Muscle atrophy can be focal, affecting only one muscle or muscle group or widespread over the horse’s body. Focal atrophy may be the result of damage to the nerve serving that muscle or due to an inflammatory myopathy in that muscle. Rapid atrophy primarily affecting the topline is characteristic of immune-mediated myositis in Quarter Horse related breeds. Slow onset atrophy of the topline in these breeds is more characteristic of homozygous type 1 PSSM. With systemic diseases topline atrophy can be prominent but there should also be generalized loss of fat elsewhere in the body. EMND affects all muscles of the body but often shows prominent atrophy of the quadriceps and weight shifting due to inability to fix the stifles.
What is the best treatment for muscle atrophy?
The best treatment plan will depend on the underlying cause of your horse’s atrophy. In some cases this may involve oral treatment with medications such as vitamin E or it may involve a corticosteroid for immune-mediated diseases. To help muscle recover its full form, additional therapies such as physical rehabilitation therapy, therapeutic ultrasound and electrical muscle stimulation may be indicated. Your veterinarian will guide treatment based on the underlying cause of atrophy.
Will my horse recover fully?
Muscle has a remarkable ability to regenerate and many horses do return to a normal muscle mass and full function, depending on its cause. Nerve damage may take up to a year to know whether it will fully regenerate which will be followed by a return of muscle mass. Immune mediated muscle atrophy can be dramatic but muscle mass often returns with treatment. In some cases, atrophy may have progressed to the point where connective tissue has replaced the muscle fibers, in which case the prognosis for full recovery of muscle mass is more guarded. The replacement of muscle fibers with connective tissue is often apparent on a muscle biopsy.