In domestic animals, many endocrine disorders are responsible for neuromuscular pathologies.
Endocrine myopathies are relatively common in older animals and can present with a variety of clinical symptoms ranging from muscle weakness to rigidity. Muscle involvement can be missed and the diagnosis of myopathy therefore a lucky guess. In other cases, the muscle weakness or rigidity may lead to a diagnosis of endocrine disorder.
Serious visible muscle atrophy in an older horse
- Interferes with carbohydrate, protein and lipid metabolism;
- Has negative influence on muscle energy metabolism;
- Classic signs of hypothyroidism not always visible.
Medical check of lame horse
Clinical signs of nerve and muscle involvement
- Intolerance to exercise
- Stiff gait
- Lameness (maybe only one limb affected)
- Facial nerve paresis
- Esophageal dysfunction
- Lower motorneurone dysfunction associated with peripheral neuropathy
HYPOTHYROID MYOPATHY – muscle lesions
In some patients, deposits of PAS positive material in muscle fibre
Central regions pale with NADH
Rapid resolution of clinical signs with a return to euthyroidism.
Prognosis in cases of Hypothyroid Neuropathy may not be so favourable.
- Alteration of energy metabolism;
- Associated with weakness and tire easily;
- Muscle and nerve biopsy appear normal;
- Weakness resolves with specific treatment.
Horse with gait anomalies
Hypoadrenocorticism (Addison’s disease)
Muscle weakness due to:
- Electrolyte imbalance;
- Circulatory insufficiency;
- Carbohydrate metabolism imbalances, due to lack of mineralcorticoids and glucocorticoids.
- Electrolyte changes: include hyponatremia, hypochloremia and hyperkalemia;
- Electrocardiogram may suggest hyperkalemia;
- ACTH stimulation test and assessment of plasma cortisol may support the diagnosis;
- Muscle biopsy is not necessary for a definitive diagnosis.
After correct diagnosis and appropriate treatment, the prognosis is good with clinical symptoms completely resolved.
Hyperadrenocorticism (Cushing’s syndrome)
- Glucocorticoids increase protein catabolism and inhibit myofibrillar protein synthesis;
- Elevated ACTH levels may have myopathic effect.
Serious lameness in pelvis
- Marked muscular atrophy especially of masticatory muscles
- Deep muscle weakness
- Stiff gait
- Proximal muscle hypertrophy and pseudomyotonia may be associated with Cushing’s syndrome in a small percentage of cases.
Muscle biopsy. Selective atrophy of type II fibres. ATPase ph 9.4
- Muscle biopsy indicates endocrine disorder because of selective atrophy of type II fibres;
- Appropriate laboratory tests for the diagnosis of Cushing’s syndrome;
- Measure plasma lactate;
- Assessment of plasma, urine and muscle carnitine.
Muscle Biopsy “Cushing’s Myopathy”
Muscle biopsy with Engel's Trichrome shows numerous angular atrophic fibres
Muscle biopsy in a subject with Cushing's (a) compared with a normal muscle biopsy (b)
“Cushing’s Myopathy” muscle biopsy (cont.)
ATPase at PH 9.4 histoenzymatic stain shows selective atrophy of type II fibres (dark-coloured)
COX histoenzymatic stain shows loss of mitochondrial activity in various fibres
- Standard therapy for Cushing’s syndrome
- Muscle weakness generally resolves when glucocorticoid levels normalise
- Supplement with L-carnitine, antioxidants and vitamin B complex
Myopathic effects of Corticosteroids
- Marked atrophy of muscle fibres (selective atrophy of type II fibres )
- Generalized weakness
- SerumCK levels normal
- +/- Signs of hypercorticism
- Lipid myopathy
- Muscle weakness and atrophy are usually observed after corticosteroid therapy;
- Flat line with ACTH stimulation;
- Typical morphological changes on muscle biopsy;
- Fluorinated corticosteroids are usually the main cause of weakness.
Selective atrophy of type I fibres
Lipid deposits in a muscle fibre Oil Red O
- Good. With gradual suspension of cortisones muscle mass and strength returns;
- In human medicine, inactivity is reported to worsen symptoms of steroid myopathy;
- Muscle activity can, in part, prevent atrophy;
- Physical therapy can be beneficial;
- Supplement with L-carnitine, Q10 coenzyme, and group B vitamins.
Corticosteroids are indicated in some Neuromuscular Pathologies but they should never be used indiscriminately.
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