Muscle Disease Overview in Animals

General Approach to Muscle Disease

• Muscle disease encompasses various disorders impacting skeletal muscle.
• The presentation and diagnostic approach vary across species.
• A thorough understanding of muscle anatomy and physiology is crucial for accurate diagnosis and management.
• Common clinical presentations include pain, heat, swelling, muscle cramping, abnormal limb positions, weakness, fatigue, poor performance, muscle fasciculation, atrophy, sweating, and myoglobinuria.
• Diagnostic considerations include the onset and nature of clinical signs, activities prior to symptom onset, exercise tolerance, gait abnormalities, muscle loss (atrophy or wasting), progression, and other signs of illness.

Learning Objectives

• Relate muscle anatomy and physiology to clinical presentations and investigations of muscle diseases.
• Understand common clinical presentations in animals with muscle disease.
• Outline the diagnostic method for skeletal muscle disorders using a problem-based clinical approach.
• Explain the potential long-term implications of myopathies and factors influencing disease prognosis.

Role of Skeletal Muscle

• Maintains posture.
• Generates movement (power, strength, speed).
• Enables precise coordination.
• Acts as an active metabolic organ.

Muscle Anatomy

• The provided images highlight the structures involved.

Physiology

• The presentation briefly mentions Lance Armstrong (and his racing cyclist history) and horse VO2 max levels.
• VO2 max in Thoroughbreds is approximately 140-180ml/kg/min.
• 55% of a Thoroughbred's composition is muscle.

What Can Go Wrong With Muscles?

• Skeletal muscle diseases arise from disorders within the neuromuscular input, muscle cell membrane, myofibre proteins, and metabolic functions.
• This section highlights acquired versus hereditary factors.

Approach to Investigating Muscle Disease / Clinical History

• Signalment (species, breed, age, sex).
• List of common myopathies (e.g. equine: exertional rhabdomyolysis, metabolic disorders, atypical myopathy).
• Factors such as diet (nutritional deficiencies - Selenium/Vitamin E deficiency), access to pasture, exercise and husbandry, breed history, and other recent diseases.

Establishing The Problem

• Onset of clinical signs (acute vs. chronic).
• Activities prior to symptom onset.
• Detailed nature of clinical signs (e.g. weakness, exercise intolerance, gait abnormalities, muscle loss, progression, other signs of illness—comorbidities).

Clinical Signs Associated With Muscle Disease

• Pain, heat, and/or swelling on palpation of muscle.
• Muscular cramping.
• Abnormal limb position (e.g. muscle tears).
• Gait abnormalities.
• Weakness.
• Fatigue or poor performance.
• Muscle fasciculation.
• Muscle atrophy.
• Sweating.
• Myoglobinuria.

Problem-Based Approach (Examples)

• Muscular cramping with exercise: Overexertion, electrolyte/energy depletion, exertional rhabdomyolysis, tying up in horses, racing greyhounds, myotonia congenita, delayed relaxation of contracted muscles, fainting goats.

• Persistent or permanent gait abnormalities: Muscle injury (e.g., tears, strains), fibrotic myopathy (can occur in any species).

• Episodic or progressive weakness: Neurological or neuromuscular causes (e.g., upper/lower motor neuron lesions, spinal trauma), equine motor neuron diseases, myasthenia gravis, disorders of cell membrane function (e.g. Selenium/Vitamin E deficiency, hypokalemia. hypothyroidism).

• Muscle wasting/atrophy: Neurogenic atrophy, disuse atrophy, equine motor neuron disease, immune-mediated polymyositis, infectious myositis (e.g., Lyme disease).

• Acute rhabdomyolysis: Exertional rhabdomyolysis, infectious myonecrosis, toxicities (e.g. hypoglycin A, lonophore poisoning), circulatory disturbances, post-anaesthetic myopathy.

Diagnosis of Muscle Disease

• Serum Muscle Enzyme Activity: Creatine kinase (CK), aspartate transaminase (AST) - elevated levels suggest muscle damage.
• Urinalysis: Myoglobinuria indicating muscle damage, haemoglobinuria
• Specific blood tests: Vitamin/selenium assays for possible deficiencies (white muscle disease).
• Immunology testing: Myasthenia Gravis (anti-AChR antibody titre).
• Genetic tests: DNA extraction for conditions such as Polysaccharide Storage Myopathy (PSSM1), Hyperkalaemic Periodic Paralysis (HYPP), Myosin heavy chain myopathy (MYHM), Glycogen Branching Enzyme Deficiency (GBED), Malignant hypothermia.
• Diagnostic imaging: Ultrasound, MRI for assessing structure, injuries, and infection foci.
• Muscle biopsy: Histopathology, biochemical analysis (often used in horses).

Muscle Biopsies

• Differences between postural and locomotor muscles. Location & fibre type relevant for diagnoses.

Muscle Histopathology

• Images demonstrating normal healthy muscle tissues contrast with those found in various muscle diseases.

Long-Term Sequelae of Muscle Disease

• Fibre necrosis (death of muscle fibres).
• Loss of muscle fibre number.
• Fibrosis of muscle tissue.
• Reduced muscle contractile and expansive functions.

Prognosis for Patients with Muscle Disease

• Generally favourable with appropriate management.
• Factors influencing prognosis include: underlying conditions, severity of muscle pathology, clinical signs, use of animal (e.g., athlete vs. companion), breeding implications of hereditary conditions.

Summary

• Understanding muscle anatomy and physiology are crucial for investigating, diagnosing, managing, and assessing prognosis in animals with myopathies.
• Thorough clinical history, species, and signalment are vital factors to establish cause.
• Myopathies cause a wide range of clinical signs from poor performance to severe pain.
• Diagnostic tests (Enzymes, biopsies, imaging).