Clinical Vignette
Willie, a 10-year-old neutered male terrier cross, is presented for a swelling around the right femur. The owners have recognized three episodes of acute swelling, lameness, and pain in the region of the swelling over the past 2 months. Each time, the swelling subsides in 4–5 days as well as the pain. Once the swelling is diminished the pain and lameness resolve. Otherwise, Willie has been normal, maintaining a good appetite and activity (except when painful). Physical examination reveals a firm, taut, painful swelling, which primarily involves the lateral surface of the femoral area. What mechanism of disease would result in this type of history? What are your primary differential diagnoses? What diagnostics would you select?
Problem Definition and Recognition
Signs of inflammation as stated by Celsus in the first century are rubor et tumor cum calore et dolore (translated as “redness and swelling with heat and pain”). This phrase has persisted through the years and is recognized as cardinal signs of inflammation. All soft tissues of the musculoskeletal system may experience any of the above-mentioned signs with the exception of bone. Swelling may be defined as an increase in tissue volume that is not the direct result of cellular proliferation. In the musculoskeletal system, swelling occurs in the periarticular, periskeletal, and articular tissues. Bone will not experience swelling, but rather will demonstrate increases in volume as a result of cellular proliferation. As a result, any change in bone volume is more correctly described as bone enlargement. Causes of cellular proliferation leading to enlargement of any organ may be due to hypertrophy, hyperplasia, metaplasia, or neoplasia. Interestingly, bone, when injured, will experience acute inflammation, form granulation tissue, and undergo fibrous repair. Enlargement of the periarticular, periskeletal, and articular tissues may also occur. Diseases that cause these clinical findings commonly produce lameness as a concurrent and related clinical problem. Readers should consult Chapter 38 for related information.
Pathophysiology
Bone Enlargement
Bone is a dynamic tissue that may experience growing and resorption simultaneously. These two basic processes are referred to as osteogenesis, or the production of new bone, and osteolysis, or the resorption of bone. Together these two processes work concurrently to maintain the structural integrity of bone in response to injury. The osteogenic potential of bone is present throughout life but is greatest during growth and then begins a progressive decline from the point of maturity onward. This osteogenic capability may also differ within specific regions of bone. The epiphyseal and metaphyseal regions of bone have more osteogenic potential than do the diaphyseal regions of long bones. This osteogenic process is often characterized by an increase in cellularity of bone. When considering secondary bone healing, soft callus formation is observed to have the greatest increase in cellularity and vascularity, which in turn may be realized to be a period of bone enlargement. This may be observed on the endosteal, periosteal, and interfragmentary surfaces of bone. In disease processes not related to fracture healing, bone enlargement is often observed to involve primarily the periosteal surfaces. This may also be the most notable area of bone enlargement as well.
Articular Swelling or Articular Enlargement
When considering the pathophysiology of articular structures, the basic anatomy of the diarthrodial joint must first be considered. The articular cartilage, synovial fluid, synovial membrane, and articular capsule are the fundamental components of joints.
When disease exists, any or all of these components may be involved. Joint swelling occurs as a result of the synovial membrane’s response to trauma or disease. Joints, in response to inflammation, will experience an increase in synovial phagocytic activity. The synovial lining cells will hypertrophy or become hyperplastic. Villous proliferation will occur, and a fibrovascular membrane may develop. These changes may be collectively referred to as a condition called synovitis. Synovitis typically leads to a sequela called joint effusion. This is when synovial fluid volume increases as well as alterations in synovial fluid quality. Joint effusion may be characterized as serous, fibrinous, hemorrhagic, or purulent on the basis of the underlying disease process and nature of the joint fluid.
Joint enlargement typically accompanies joint swelling or chronic joint swelling. Hyperplasia, metaplasia, or neoplasia affecting the synovial membrane, joint capsule, articular cartilage, or periarticular structures may result in joint enlargement. A list of possible causes has been compiled in Table 39-1. Readers should also review Tables 38-1 to 38-3 for related information.
Periskeletal Swelling or Periskeletal Enlargement
Swelling or enlargement of the soft tissues surrounding bones and joints may occur independently or may accompany enlargement of skeletal tissues. Swelling of the surrounding muscles, fascial tissues, and other connective tissues may involve hemorrhage, edema, and inflammatory exudates (see Table 39-2). Finally as in other tissues, periskeletal tissue enlargement may result from hyperplastic, metaplastic, or neoplastic processes.
Diagnosis
History
As in all disease processes, obtaining a complete and accurate medical history is crucial in developing a diagnostic plan for musculoskeletal swelling or enlargement. Signalment, diet, activity level, husbandry, previous medical history, and length of ownership are the basis of a complete history. More specific questions with regard to location of past and current residence, travel history, onset of clinical signs, duration of swelling, anatomical locations of swelling, and progression of the swelling are all key elements in developing a complete list of rule-outs.
TABLE 39-1. Differentials for joint swelling or enlargement
| Arthritis | Feline progressive polyarthritis Nonerosive idiopathic immune mediated Infectious Lupus Rheumatoid Polyarthritis of greyhounds Degenerative joint disease Plasmacytic–lymphocytic synovitis |
| Fracture | Articular Physeal Avulsion |
| Other | Luxations Neoplasms Synovial osteochondromatosis Hemarthrosis |
Signalment. When compiling a list of differential diagnoses, breed, age, and sex predilections of certain disease processes must be considered. Certain disease processes such as craniomandibular osteopathy may occur more commonly in terrier breeds and large breeds as compared to other breeds of a similar age. Alternatively, certain disease processes may be more likely in an immature or growing animal as compared to an older or geriatric animal and vice versa (see Table 39-2).
Residence and Travel History. Infectious diseases leading to the development of musculoskeletal swelling or enlargement may be endemic to specific geographical regions. This is especially true for fungal or parasitic diseases. If a suspected infectious disease is not endemic to the area in which the animal is presented, it cannot be completely eliminated until travel history and previous area of residence are accounted for.
TABLE 39-2. Differentials for periskeletal enlargement or swelling
| Granuloma | Fibrosarcoma |
| Hemangiomatosis | Hemangiopericytoma |
| Hematoma | Hygroma |
| Lipoma | Myositis ossificans |
| Trauma | Polymyositis |
| Aneurysm (spurious) | Tumoral calcinosis |
| Sarcoma | Immune reactions |
| Infectious processes | Lymphatic obstruction |
| Nutritional myopathy (hypovitaminosis E) |
Diet. The current or a previously fed diet may be involved in the pathogenesis of musculoskeletal swelling. The use of unbalanced, oversupplemented, or alternative diets may lead to the development of hypertrophic osteodystrophy, nutritional secondary hyperparathyroidism, and/or nutritional myopathy. Fortunately, most commercially prepared diets available today are well balanced. These disease processes are therefore less likely to occur.
Activity, Husbandry, and Previous Medical History. Animals with acute onset swelling and/or lameness may have experienced trauma leading to fractures, luxations, or ligamentous injuries. Occasionally, the trauma may have been directly observed or a known event may have occurred leading to a diagnosis. If no injury can be discerned from the history, the environment in which the animal is housed or lives may provide some insight. For example, an indoor animal would be less likely to experience blunt force trauma as compared to an unconfined outdoor animal.
Some preexisting conditions may predispose joints to swelling or enlargement after a certain level of activity occurs. For example, a history of osteoarthritis coupled with recent vigorous physical activity could provide an explanation for presentation of joint swelling.
Presentation, Duration, and Progression. Acute swelling and enlargement of the musculoskeletal system is most often caused by fractures, luxations, and ligamentous injuries. The swelling is usually nonprogressive. Other injuries (i.e., bite wounds, abscesses, cellulitis) of the periskeletal tissues may also lead to significant swellings with an acute presentation. Progression of the swelling in the periskeletal tissues may be in association with continued inflammation, edema, and infection. Neoplastic processes, metabolic conditions, and other infectious diseases usually present with a history consistent with an insidious onset and gradual progression. Alternatively, some neoplasms dependent on location, tumor type, and owner perception may have a more “acute” presentation. Mast cell tumors located within the periskeletal soft tissues and muscle may result in this presentation.
Physical Examination
When examining any animal for swelling or enlargements of the bones, joints, and periskeletal structures, one must use careful, deliberate, and systematic palpation. If only one limb or a portion of the limb is affected, palpation of this area should be performed last in an attempt to allow for a more complete and accurate examination. If the swollen or enlarged limb is painful and then examined first, the remainder of the examination may be extremely difficult to complete and interpret. A similar painful response may be elicited in unaffected limbs simply because the painful limb was examined first. In addition, the clinician should be able to gain a more accurate assessment of anatomy by examination of the normal contralateral limb first. The clinician should utilize information gained from the orthopedic and physical examination to determine if the swelling or enlargement involves the bone, joint, periskeletal, periarticular, or combination of these tissues.
Differentiation of Swelling versus Enlargement. When performing the physical examination, swelling may be fluctuant with or without pitting. The other cardinal signs of redness, heat, and pain may be present. Enlargement is most notable with regard to bone, but the articular or periskeletal tissues may add to the enlargement due to cellular proliferation. Enlargements are typically appreciated as firm, thickened, discrete masses of areas or parts.
Location and Distribution. The anatomic location and distribution of swellings and enlargements are very important in formulating diagnostic plans. Some diseases may be characterized by involvement of specific bones or joints and certain areas of bone. When a single area of bone is affected, this is termed as a monostotic lesion, whereas multicentric location is referred to as polyostotic involvement. In addition, swellings and enlargements located in or confined to specific regions of long bones (e.g., the epiphysis, metaphysis, and diaphysis) may direct the diagnostic plan. The causes of bone enlargement on the basis of location and distribution are listed in Tables 39-3 and 39-4.
TABLE 39-3. Differentials for bone enlargement in young or immature dogs and cats
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