Swellings and Enlargements (Soft and Hard Tissue)
Carter E. Judy • John Maas
Swellings and enlargements consist of soft tissue (e.g., tendon) or hard tissue (e.g., osseous) and can occur anywhere on an animal's body. Typically, swellings and enlargements that are clinically relevant are located on the limbs.
Swellings and enlargements can be further divided into two principal groups: those associated with a specific musculoskeletal anatomic structure and those that are not. For example, a soft fluctuant swelling in the region of the left carpus may be caused by an abnormality of the antebrachialcarpal joint (e.g., septic synovial effusion) or may not involve the joint at all (e.g., subcutaneous abscess). Although lameness can be associated with such a swelling, clearly it is important to determine the cause of the abnormality because the one involving the joint may require the more immediate treatment.
Mechanisms of Swellings and Enlargements
The mechanism by which swelling or enlargement develops depends on the tissue involved. Soft tissue swelling is often produced by trauma, inflammation, infection, or neoplasia; it can consist of interstitial fluid (e.g., edema), fluid within an open space (e.g., synovial hernia), or a localized accumulation of cells or fibrous tissue. Localized edematous swelling is commonly caused by inflammation and/or obstruction of venous blood or lymph flow. Generalized edema is usually the result of increased hydrostatic pressure caused by circulatory failure or an altered capillary-tissue osmotic gradient stemming from hypoalbuminemia. Fluctuant swellings such as hematoma, synovial effusion, a purulent abscess, or a plasma-filled cyst contain free fluid. Granulation tissue, fibrous scar tissue, and tumor cells are the most common constituents of firm soft tissue swellings. Rupture of supporting or confining structures (e.g., prepubic tendon rupture) can result in unusual forms of soft tissue swelling caused by herniation of internal organs.
Many factors influence new bone formation. Trauma and infections initiate bony enlargement (e.g., callus) by disrupting the periosteum, producing inflammation and eventually ossification. The extent of periosteal new bone formation depends on the cause of the stimulus and size of the affected area. Remodeled bone may also arise from nontraumatic events, usually associated with altered metabolism or neoplasia. Bony enlargements associated with the metaphysis and physis in young, growing animals are usually secondary to a combination of nutritional and traumatic factors. For example, dietary calcium, phosphorus, and vitamin D imbalance can lead to abnormal bone growth. A bony swelling develops gradually and may become noticeable only after it enlarges, interferes with normal function, or becomes a source of lameness.
Approach to Diagnosis of Swellings and Enlargements in Horses
For causes of swellings and enlargements in horses see Box 13.6.
1. History. A history should determine the number of horses involved, duration of clinical signs, and possibility that traumatic events or environmental factors are responsible for causing a swelling or enlargement. In addition, changes over time in the appearance and size of the swelling or enlargement can be informative.
■ BOX 13.6
Causes of Swellings and Enlargements in Horses Soft Tissue
Septic (infectious) arthritis
Secondary (degenerative) joint disease
Sprain
Strain
Hygroma
Tenosynovitis
Osteochondrosis
Suspensory desmitis or sesamoiditis
Infections of the foot
Insect bites or snakebites
Cellulitis
Abscess
Herniation
Neoplasia
Capped hock
Hematoma
Phycomycosis
Hard Tissue
Secondary (degenerative) joint disease
Fracture
Sequestrum
Osteomyelitis (periosteal new bone formation)
Epiphysitis
Luxation or subluxation
Osteochondroma
Osteomalacia (rickets)
Bucked shins (dorsal metacarpal disease)
Hypertrophic osteopathy
Ankylosis or arthrogryposis
Calcinosis caused by plant poisoning
Selenium toxicity
2. Inspection and palpation.
The location of the swelling and its proximity to anatomic structures often reveal the tissue involved and the probable cause of the condition. For example, swelling around a joint may indicate arthritis, periarthritis, or hygromas. Tendon swelling may indicate tendonitis or ruptured tendons. Swelling over ligaments may indicate rupture, subluxation, or inflammation around a ligament. Muscle swelling results from abscesses or fascial tears. Subcutaneous swelling may indicate hematomas, edema from inflammation around a ligament, or cellulitis. Bony enlargements can often be localized to the shaft of a bone (e.g., periosteal callus) or ends of a bone (e.g., metaphyseal flaring). Periarticular new bone may be readily apparent (e.g., ringbone) or may not be found even on deep palpation. New bone formation can also be found associated with the axial skeleton and head. Palpation of a swelling can determine its consistency and association with anatomic structures. Osseous swelling indicates calcification, proliferation of bone, or fracture. Firm soft tissue swelling indicates inflammation, abnormal proliferation of soft tissue (e.g., granulation, tumor), or herniation. Warmth, redness, and pain associated with swelling indicate active inflammation. While new bone is forming, the swelling may be soft and sensitive to palpation. Cold and insensitivity to palpation suggest inadequate blood supply and possibly ischemia (e.g., gangrene). Lameness caused by an injury or condition that results in a hard swelling or enlargement may be accentuated by performing a stress test, such as trotting the horse in hand after direct pressure on the swelling. Intraarticular anesthesia may substantially reduce lameness caused by joint effusion associated with periarticular new bone.3. Radiography, ultrasonography, and alternative imaging techniques. In addition to identifying definitively the nature of an osseous swelling or enlargement, radiography can gauge the severity and progression of the disease and help establish a therapeutic plan and prognosis.
Ultrasound can often determine the position (e.g., depth, area) and volume of a soft tissue swelling and the optimum site for aspiration or biopsy. Thermography may help identify subtle heat production secondary to inflammation and increased blood flow, before onset of a swelling, allowing for early treatment. Nuclear scintigraphy may help localize the cause of swellings and identify whether they are bony or soft tissue in origin. CAT scanning is useful for evaluating bony swellings, especially of the head when swellings of the mandible and maxilla may be related to infected teeth. MRI may prove useful for accurate imaging of soft tissue masses that cannot be accurately characterized with other diagnostic techniques.4. Cytology, microbiology, and histology. A fine-needle aspiration, using aseptic techniques, should be performed to obtain samples for microbiologic culture (e.g., bacterial and fungal) of soft tissue swellings. If the material is very viscous, a large-gauge needle may be required. Fluid collected for cytology should be placed in tubes containing ethylenediaminetetraacetic acid (EDTA) to prevent clotting before analysis. Tissue samples obtained by biopsy should be placed in 10% buffered formalin.