Musculoskeletal System Pathology Review

Test Your Musculoskeletal System Knowledge

1. Muscle Responses to Injury

  • Muscles can respond to injury in various ways, each leading to potential diseases:
    • Denervation Atrophy
    • Examples:
      • Laryngeal Hemiplegia: Caused by damage to the left recurrent laryngeal nerve.
      • “Sweeney”: Caused by injury to the supraspinatus nerve due to trauma.
      • Equine Motor Neuron Disease: Causes symmetrical atrophy of muscles.
      • Equine Protozoal Myeloencephalitis: Leads to symmetrical atrophy.
    • Disuse Atrophy
    • Causes
      • Painful lameness, bone fractures, or diseases that require limb immobilization.
      • Example: An animal in a cast which limits muscle use, leading to gradual atrophy.
      • Example: An animal in a restricted environment (e.g., a cage with little mobility).
    • Malnutrition/Cachexia Atrophy
    • Causes:
      • Insufficient quantity and/or quality of nutrients in the diet.
      • Chronic illnesses, such as neoplasms and chronic parasitism (e.g., Haemonchus contortus in goats).
    • Endocrine Disease-Causing Atrophy
    • Examples:
      • Dogs: Hypothyroidism and Hyperadrenocorticism (Cushing’s syndrome).
      • Horses: Aging horses with pituitary dysfunction or tumors (equine Cushing’s syndrome).
    • Hypertrophy
    • Types:
      • Physiologic hypertrophy due to conditioning through exercise.
      • Genetic mutations, such as those in the myostatin gene—evident in breeds like Whippets or Belgian blue bulls.
      • Examples of genetic hypertrophy: Hyperkalemic Periodic Paralysis (HYPP) leads to prominent muscle mass. X-linked Muscular Dystrophy concerns muscular dystrophy often in cats, less in dogs.
      • Use of feed additives that promote muscle building (specific types only).
    • Necrosis
    • Causes:
      • Trauma, ischemia, inherited diseases (like muscular dystrophies), inflammatory myopathies, extreme exertion (such as caused by capture myopathy).
      • Examples:
      • Downer Syndrome in large animals.
      • Deep Pectoral Myopathy in chickens.
      • Nutritional Myopathy (white muscle disease due to Selenium and/or Vitamin E deficiency).
      • *Ionophore Toxicity—can lead to muscle necrosis as well.
    • Importance: The range of causes indicates the complexity of diagnosing skeletal muscle issues, and should encourage thinking beyond covered cases.

2. Myasthenia Gravis Comparison (Dogs and Cats)

  • Main Difference in Cats: No megaesophagus present.
  • Congenital Myasthenia Gravis:
    • An autosomal recessive condition without autoantibodies to ACh receptors.
    • Symptoms include generalized weakness with possible megaesophagus leading to aspiration pneumonia.
  • Acquired Myasthenia Gravis:
    • Immune-mediated, with a small percentage associated with thymomas (tumors of the thymus).
    • Presence of autoantibodies against acetylcholine receptors.
    • Symptoms similar to congenital, including generalized weakness and megaesophagus leading to aspiration pneumonia.

3. Case Study of a Sudden Death in Young Beef Bull

  • Field Necropsy Findings:
    • Muscles appeared variably dark red (hemorrhagic, emphysematous with crepitus) accompanied by muscle edema.
  • Differential Diagnosis:
    • Main suspect is one of the Clostridium species causing myositis.
  • Questions for the Farmer:
    • Vaccination history and if such occurrences have been recorded before.
    • Consider that spores can be ingested from soil and manure, reaching the bloodstream and leading to muscle damage.
  • Pathophysiology:
    • An anaerobic environment, favored by localized trauma and subsequent hypoxia/anoxia, triggers spore activation leading to significant tissue necrosis and hemorrhage.
  • Testing Recommendations:
    • Submit fresh muscle for culture or fluorescent antibody testing to confirm the presence of pathogens. Histological analysis can provide additional information about hemorrhage and necrosis.

4. Spinal Column Curvature Terms

  • Abnormal Dorsal Curvature:
    • - Term Used: Kyphosis.

5. Limb Absence Terms

  • Absence of One or More Limbs:
    • Various terms:
    • Amelia: Complete absence of limbs.
    • Peromelia: Absence or malformation of limbs.
    • Phocomelia: Limbs partially developed.
    • Hemimelia: Absence of part of the limb.
    • Notomelia: Absence of the spine or back segment.

6. Green Muscle Diseases

  • Two Types and Differentiation:
    • Deep Pectoral Myopathy:
    • Associated with compartment syndrome, often due to trauma (e.g., vigorous wing beating in birds).
    • Green appearance due to breakdown products of hemoglobin and myoglobin.
    • Eosinophilic Myositis:
    • Caused by parasitic infections (e.g., Sarcocystis).
    • Occurs upon cyst rupture leading to eosinophilic reaction, imparting green discoloration.
  • Importance: Sarcocystis is incidental unless cysts rupture, unlike zoonotic threats such as Trichinella spiralis.

7. Bone Composition

  • Bone Structure:
    • Aggregate of organic & inorganic materials:
    • Osteoid (Organic): 90% Type I collagen produced by osteoblasts.
    • Hydroxyapatite Crystals (Inorganic): Composed largely of calcium and phosphate, also includes minor inorganic constituents such as magnesium, sodium, zinc, and fluoride.

8. Osteopetrosis vs. Osteoporosis

  • Osteopetrosis:
    • Congenital osteoclast defect leading to skeletal limb dysplasia (e.g., in Angus cattle); exhibits thickened bones with thin cortices and accumulation in diaphysis & metaphysis.
    • Seen in chickens due to an osteoblast defect from Avian Leukosis virus infection.
  • Osteoporosis:
    • Nutritional imbalance/metabolic disease leading to thin cortices, porous bones, and decreased bone mass due to nutritional deficiencies (calcium, phosphorus, and copper).

9. Rickets vs. Osteomalacia

  • Rickets:
    • Affects growing young animals, resulting in soft bones due to decreased mineralization (defective endochondral ossification). Notable growth plate enlargement leads to “rachitic rosary” appearance.
    • Common cause: Dietary deficiency of Vitamin D or phosphorus.
  • Osteomalacia:
    • Occurs in adults, characterized by soft bones and similar mineralization issues to rickets, but no involvement of growth plates. Diagnosis often requires similar dietary evaluations.

10. Nutrition for Guinea Pigs

  • Important Nutritional Requirement:
    • Vitamin C: Canonical requirement for Guinea pigs; lack can impede collagen cross-linking leading to hemorrhaging due to weak collagen fibers.

11. Differential Diagnosis of Muscle Pallor in Pigs

  • Differential Considerations:
    • Primary differentials:
    • Vitamin E/Selenium deficiency.
    • Ionophore toxicity.
    • Testing Recommendations:
    • Collect liver samples for trace mineral panel or serum if the animal is still alive.
    • If feed additives suspected, analyze these for known ionophores (e.g., Monensin). Historical information also essential due to autolysis effects on postmortem samples.

12. Hypertrophic Osteopathy vs. Metaphyseal Osteopathy

  • Hypertrophic Osteopathy:
    • Characterized by periosteal proliferation primarily in long bone diaphysis and metaphyses, associated with thoracic masses or lesions.
  • Metaphyseal Osteopathy:
    • Affects young, growing dogs, typically large breeds; manifests as metaphyseal necrosis and inflammation, chiefly in the radius & ulna.

13. Risk Factors for Osteochondrosis (OCD)

  • Risk Factors:
    • Genetics.
    • Trauma.
    • Nutritional factors.
    • Growth rate.

14. Degenerative Joint Disease (DJD) Terms and Features

  • Fibrillation:
    • The initial degeneration of articular cartilage characterized by softening and roughening areas.
  • Eburnation:
    • The most painful stage where bone appears polished and ivory-like due to articular cartilage loss.
  • Osteophytes:
    • Bony outgrowths that develop around joints with DJD, forming at the margins of articular cartilage.
  • Erosion:
    • Thinning of cartilage due to progression from fibrillation leading to eburnation and potential subchondral bone changes.

15. Spondylosis vs. Spondylitis and Diskospondylitis

  • Spondylosis:
    • Degenerative condition resulting from DJD leading to osteophyte proliferation and potential ankylosing spondylosis.
  • Spondylitis:
    • Inflammation of vertebrae, also termed vertebral osteomyelitis; often results from bacterial infections.
  • Diskospondylitis:
    • Involves inflammation of the intervertebral disk and adjacent vertebrae; commonly due to systemic infections.