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Orlando Paciello » 7.Endocrine Myopathies

Endocrine Myopathies

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

Endocrine Myopathies

Hypothyroidism

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
Myalgia
Lameness (maybe only one limb affected)
Cramps
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

Treatment

Rapid resolution of clinical signs with a return to euthyroidism.

Prognosis in cases of Hypothyroid Neuropathy may not be so favourable.

Hyperthyroidism

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.

Hypoadrenocorticism

Diagnosis

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.

Hypoadrenocorticism

Prognosis

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

Hyperadrenocorticism

Clinical signs

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.

Horse with hirsutism

Muscle biopsy. Selective atrophy of type II fibres. ATPase ph 9.4

Hyperadrenocorticism

Diagnosis

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

Treatment

Standard therapy for Cushing’s syndrome
Muscle weakness generally resolves when glucocorticoid levels normalise
Supplement with L-carnitine, antioxidants and vitamin B complex

Steroid myopathy

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

Steroid myopathy

Diagnosis

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.

Histopathology

Selective atrophy of type I fibres

Lipid deposits in a muscle fibre Oil Red O

Prognosis

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.