Joint effusion implies abnormal accumulation of fluid inside the synovial space, which may occur due to numerous conditions, including trauma, infections, both systemic and joint-related diseases, as well as tumors. Limited mobility of the affected joint and pain are the two most important complaints. The diagnosis is made by clinical observation coupled with synovial fluid examination and MRI. Treatment depends on the underlying cause.
Joint effusion almost universally includes pain, tenderness, overlying skin erythema and swelling . Arthrogenic muscle inhibition that manifests with muscle weakness has been reported in many patients as a result of joint effusion , most commonly affecting the quadriceps muscle. Moreover, gait disturbances due to effusion in the knee joint has also been reported as a possible symptom . Limited range of motion is an important diagnostic clue. Exacerbation of symptoms is rather common in conditions such as rheumatoid arthritis and gout, whereas fever and chills may be noted in patients in whom an infection is the underlying cause. In the case of tumors, night pain, sweating and unintentional weight loss may be observed .
The first step in assessing joint effusion is a meticulously obtained patient history and a complete physical examination with a particular emphasis on the affected joint. Information regarding events prior to the appearance of symptoms, such as presence of trauma (which can be confirmed by the appearance of abrasions and ecchymoses on the involved joint) or preexisting conditions should be obtained in detail. Identifying whether monoarticular or polyarticular disease is present can significantly aid in the diagnostic workup, as monoarticular involvement usually results from trauma or tumors, whereas multiple joints are affected in gout, infections or systemic diseases . Laboratory studies may reveal elevated inflammatory parameters such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), fibrinogen and leukocyte count. Rheumatoid factor (RF) may be elevated in the setting of rheumatoid arthritis or rheumatic fever, uric acid levels are increased in patients with gout and antinuclear antibodies (ANA) are present in many patients suffering from SLE . To confirm joint effusion and make a definite diagnosis, arthrocentesis with subsequent synovial fluid examination is required. The differential diagnosis can be significantly narrowed based on macroscopic appearance, viscosity, presence of cells and culture results and effusions are classified into  :
In addition to synovial fluid examination, imaging studies such as MRI are very useful in evaluating the status of the affected joint , and can rule out other conditions with similar clinical presentation, such as tibial fractures or meniscus injury .
Initial management focuses on alleviating symptoms through administration of NSAIDs and cryotherapy, but the mainstay of treatment is identification and management of the underlying cause. Intravenous antimicrobial therapy is necessary in infectious arthritis and drugs such as ceftriaxone, cefotaxime and vancomycin are considered as first-line agents, depending on culture results . NSAIDs are used in gout (colchicine or allopurinol, a drugs used to excrete excess amounts of uric acid), Reiter's syndrome and other inflammatory diseases, whereas rheumatoid arthritis often requires administration of disease-modifying anti-rheumatic drugs (DMARDs). Patients in whom trauma was the cause require a multidisciplinary approach from the fields of surgery, orthopedics and rehabilitation medicine because of the delicate nature of joint injury and its often lengthy recovery process . Aspiration of synovial fluid has been initially thought of as a procedure of choice, since rapid aspiration leads to immediate relief in most patients . Frequent recurrence is noted, however, and without resolution of the underlying cause, joint effusions may appear rather frequently.
Similarly to etiology and pathophysiological mechanisms, the prognosis of joint effusion depends on the underlying disease that caused it. The severity of fluid accumulation (it was shown that as little as 5 mL of fluid are sufficient to cause symptoms) , promptness of the diagnosis and treatment are some of the most important factors that determine patient outcomes.
Numerous diseases have been implicated as potential causes of joint effusions  :
Joint-related complains are quite common in medical practice, but the exact prevalence and incidence rates of joint effusion are not known. In general, arthritis is more commonly encountered in women (some conditions, however, such as Reiter's syndrome and gout are more frequently encountered in men) and elderly patients, whereas obesity has shown to be an important risk factor for numerous joint-related diseases. Risk factors such as trauma and a myriad of other illnesses have been well-established.
The pathogenesis of joint effusions and the content of accumulated fluid varies on the underlying etiology. Patients who suffer from trauma often develop hemorrhagic effusions as a result of injury to the articular structures, whereas accumulation of inflammatory cells (primarily neutrophils and macrophages) and formation of pus as a reaction to the microbial pathogen leads to the appearance of symptoms and effusion. Inflammatory cells are also mobilized and brought into the synovial fluid in the autoimmune diseases or gout, in which urate crystals precipitate acute attacks of pain and effusion . These pathophysiological mechanisms universally cause pain, while a neurological decline in activation of muscles, known as arthrogenic muscle inhibition (AMI), has been well-documented, particularly for the quadriceps muscle in the setting of knee injury .
Preventive strategies for joint effusion comprise adequate treatment of diseases that predispose patients to this event, appropriate exercise that does not result in muscle and joint overuse and adequate dietary habits to avoid excessive body weight. Because joint effusions may recur, ensuring long-term follow-up of patients is necessary in preventing disability that may occur as a result of inadequate treatment.
Joint effusion is a term that indicates increased concentrations of fluid in the synovial space and may develop in diseases of various origin. Trauma is often considered as the most common predisposing condition, while infections such as gonorrhea and tuberculosis, several autoimmune joint diseases such as rheumatoid arthritis, osteoarthritis and Reiter's syndrome, but also gout and malignant tumors are described as potential causes . The pathogenesis model slightly differs depending on the cause. In the case of trauma or malignant tumors, mechanical damage to the synovial membrane and surrounding structures induces fluid accumulation, whereas autoimmune and infectious diseases trigger various non-inflammatory and inflammatory changes that allow influx of cells and disrupt normal concentrations of proteins and glucose . It is not uncommon for effusions to be hemorrhagic, especially in trauma. The main complaints of patients suffering from joint effusions is pain and swelling, whereas diverse accompanying symptoms and signs may be observed depending on the underlying etiology. For this reason, a detailed patient history and a thorough physical examination can substantially narrow the list of possible causes. Involvement of one or more joints, which movement aggravates or reduces pain, history of trauma or the presence of symptoms such as fever, chills, night pain, weight loss, skin abrasions and ecchymoses are vital diagnostic clues . Reduced muscle strength and significant gait alterations can be observed in some patients but conflicting reports exist regarding their clear association  . In addition to patient history and physical exam, imaging studies such as magnetic resonance imaging (MRI) can be used , but synovial fluid examination through arthrocentesis is necessary. Examination of parameters such as viscosity, macroscopic appearance (color, turbidity), cellular presence and cultivation to exclude infections are vital in distinguishing between diseases . Treatment principles significantly depend on the underlying cause. Antibiotics are used for infections, non-steroidal anti-inflammatory drugs (NSAIDs) for suppression of inflammation in autoimmune conditions, whereas appropriate orthopedic and rehabilitation procedures are necessary for patients in whom trauma was the cause. Aspiration of joint fluid was proposed to be one of the most efficient therapeutic methods, but because of frequent recurrences that occur as early as 1 week , management of the underlying disorder is imperative in order to achieve good long-term outcomes.
Joint effusion is a manifestation of numerous diseases and represents pathological accumulation of fluid inside the joint. It can appear in conditions such as trauma, joint infections (gonorrhea and tuberculosis being one of the most common), conditions that primarily affect the joints such as rheumatoid arthritis, gout, pseudogout, osteoarthritis and Reiter's syndrome, but also tumors and systemic diseases such as Crohn disease, systemic lupus erythematosus and psoriatic arthritis. Joint effusion stems from either mechanical disruption of structures that comprise the joint, such as ligaments, tendons and synovial membranes or activation of inflammatory cells in the case of infection or autoimmune diseases. As a result, fluid accumulation occurs and causes symptoms such as pain, swelling, tenderness and limited range of motion. Patients may report fever and chills in the setting of an infection, while pain during the night and unexplained weight loss may suggest the development of a tumor. Because numerous diseases may cause joint effusion, the physician is obliged to obtain a detailed patient history that can reveal previous trauma or existing conditions that can predispose patients to this condition. Additionally, a meticulous physical examination with a particular emphasis on the painful joint is equally important and various probes may determine the exact site of injury. To confirm joint effusion as the cause of symptoms, however, aspiration of synovial fluid from the affected joint by a procedure called arthrocentesis and subsequent examination is essential. Magnetic resonance imaging (MRI) is a useful diagnostic method as well, but its role primarily includes exclusion of other conditions that may present with similar symptoms, such as fractures or injuries to other joint structures. Treatment principles somewhat depend on the underlying cause. Infections are treated with intravenous antibiotics, whereas non-steroidal anti-inflammatory drugs (NSAIDs) and disease-modifying anti-rheumatic drugs (DMARDs) are used for alleviation of symptoms in the majority of conditions, including gout, rheumatoid arthritis and other inflammatory joint diseases. Aspiration of accumulated fluid was initially thought to be an effective therapeutic method, but frequent recurrence has been observed, which is why the most important part of joint effusion management is recognition and treatment of the underlying cause.