Arthritis of joints involves inflammation of the synovial membrane. Joints become swollen, tender and warm, and stiffness limits their movement. With time, multiple joints are affected (polyarthritis). Most commonly involved are the small joints of the hands, feet and cervical spine, but larger joints like the shoulder and knee can also be involved. Synovitis can lead to tethering of tissue with loss of movement and erosion of the joint surface causing deformity and loss of function.

RA typically manifests with signs of inflammation, with the affected joints being swollen, warm, painful and stiff, particularly early in the morning on waking or following prolonged inactivity. Increased stiffness early in the morning is often a prominent feature of the disease and typically lasts for more than an hour. Gentle movements may relieve symptoms in early stages of the disease. These signs help distinguish rheumatoid from non-inflammatory problems of the joints, such as osteoarthritis. In arthritis of non-inflammatory causes, signs of inflammation and early morning stiffness are less prominent with stiffness typically less than one hour, and movements induce pain caused by mechanical arthritis.

The pain associated with RA is induced at the site of inflammation and classified as nociceptive as opposed to neuropathic. The joints are often affected in a fairly symmetrical fashion, although this is not specific, and the initial presentation may be asymmetrical.

As the pathology progresses the inflammatory activity leads to tendon tethering and erosion and destruction of the joint surface, which impairs range of movement and leads to deformity. The fingers may suffer from almost any deformity depending on which joints are most involved. Specific deformities, which also occur in osteoarthritis, include ulnar deviation, boutonniere deformity (also "buttonhole deformity", flexion of proximal interphalangeal joint and extension of distal interphalangeal joint of the hand), swan neck deformity (hyperextension at proximal interphalangeal joint and flexion at distal interphalangeal joint) and "Z-thumb." "Z-thumb" or "Z-deformity" consists of hyperextension of the interphalangeal joint, fixed flexion and subluxation of the metacarpophalangeal joint and gives a "Z" appearance to the thumb. The hammer toe deformity may be seen. In the worst case, joints are known as arthritis mutilans due to the mutilating nature of the deformities.

The rheumatoid nodule, which is sometimes in the skin, is the most common non-joint feature and occurs in 30% of people who have RA. It is a type of inflammatory reaction known to pathologists as a "necrotizing granuloma". The initial pathologic process in nodule formation is unknown but may be essentially the same as the synovitis, since similar structural features occur in both. The nodule has a central area of fibrinoid necrosis that may be fissured and which corresponds to the fibrin-rich necrotic material found in and around an affected synovial space. Surrounding the necrosis is a layer of palisading macrophages and fibroblasts, corresponding to the intimal layer in synovium and a cuff of connective tissue containing clusters of lymphocytes and plasma cells, corresponding to the subintimal zone in synovitis. The typical rheumatoid nodule may be a few millimetres to a few centimetres in diameter and is usually found over bony prominences, such as the elbow, the heel, the knuckles, or other areas that sustain repeated mechanical stress. Nodules are associated with a positive RF (rheumatoid factor) titer, ACPA, and severe erosive arthritis. Rarely, these can occur in internal organs or at diverse sites on the body.

Several forms of vasculitis occur in RA, but are mostly seen with long-standing and untreated disease. The most common presentation is due to involvement of small- and medium-sized vessels. Rheumatoid vasculitis can thus commonly present with skin ulceration and vasculitic nerve infarction known as mononeuritis multiplex.

Other, rather rare, skin associated symptoms include pyoderma gangrenosum, Sweet's syndrome, drug reactions, erythema nodosum, lobe panniculitis, atrophy of finger skin, palmar erythema, and skin fragility (often worsened by corticosteroid use).

There are five main types of psoriatic arthritis:

- Oligoarticular: This type affects around 70% of patients and is generally mild. This type does not occur in the same joints on both sides of the body and usually only involves fewer than 3 joints.

- Polyarticular: This type accounts for around 25% of cases, and affects five or more joints on both sides of the body simultaneously. This type is most similar to rheumatoid arthritis and is disabling in around 50% of all cases.

- Arthritis mutilans (): Affects less than 5% of patients and is a severe, deforming and destructive arthritis. This condition can progress over months or years causing severe joint damage. Arthritis mutilans has also been called chronic absorptive arthritis, and may be seen in rheumatoid arthritis as well.

- Spondyloarthritis (): This type is characterized by stiffness of the neck or the sacroiliac joint of the spine, but can also affect the hands and feet, in a similar fashion to symmetric arthritis.

- Distal interphalangeal predominant (): This type of psoriatic arthritis is found in about 5% of patients, and is characterized by inflammation and stiffness in the joints nearest to the ends of the fingers and toes. Nail changes are often marked.

Pain, swelling, or stiffness in one or more joints is commonly present in psoriatic arthritis. Psoriatic arthritis is inflammatory, and affected joints are generally red or warm to the touch. Asymmetrical oligoarthritis, defined as inflammation affecting one to four joints during the first six months of disease, is present in 70% of cases. However, in 15% of cases, the arthritis is symmetrical. The joints of the hand that is involved in psoriasis are the proximal interphalangeal (PIP), the distal interphalangeal (DIP), the metacarpophalangeal (MCP), and the wrist. Involvement of the distal interphalangeal joints (DIP) is a characteristic feature and is present in 15% of cases.

In addition to affecting the joints of the hands and wrists, psoriatic arthritis may affect the fingers, nails, and skin. Sausage-like swelling in the fingers or toes, known as dactylitis, may occur. Psoriasis can also cause changes to the nails, such as pitting or separation from the nail bed, onycholysis, hyperkeratosis under the nails, and horizontal ridging. Psoriasis classically presents with scaly skin lesions, which are most commonly seen over extensor surfaces such as the scalp, natal cleft and umbilicus.

In psoriatic arthritis, pain can occur in the area of the sacrum (the lower back, above the tailbone), as a result of sacroiliitis or spondylitis, which is present in 40% of cases. Pain can occur in and around the feet and ankles, especially enthesitis in the Achilles tendon (inflammation of the Achilles tendon where it inserts into the bone) or plantar fasciitis in the sole of the foot.

Along with the above-noted pain and inflammation, there is extreme exhaustion that does not go away with adequate rest. The exhaustion may last for days or weeks without abatement. Psoriatic arthritis may remain mild or may progress to more destructive joint disease. Periods of active disease, or flares, will typically alternate with periods of remission. In severe forms, psoriatic arthritis may progress to arthritis mutilans which on X-ray gives a "pencil-in-cup" appearance.

Because prolonged inflammation can lead to joint damage, early diagnosis and treatment to slow or prevent joint damage is recommended.

Symptoms of JIA are often nonspecific initially, and include lethargy, reduced physical activity, and poor appetite. The first manifestation, particularly in young children, may be limping. Children may also become quite ill, presenting with flu-like symptoms that persist. The cardinal clinical feature is persistent swelling of the affected joint(s), which commonly include the knee, ankle, wrist, and small joints of the hands and feet. Swelling may be difficult to detect clinically, especially for joints such as those of the spine, sacroiliac joints, shoulder, hip, and jaw, where imaging techniques such as ultrasound or MRI are very useful.

Pain is an important symptom. Morning stiffness that improves later in the day is a common feature (this implies inflammatory-type joint pain versus mechanical-type joint pain). Late effects of arthritis include joint contracture (stiff, bent joint due to fibrosis) and joint damage. Children with JIA vary in the degree to which they are affected by particular symptoms. Symptoms may also differ between sexes, affecting girls and boys differently among different geographic locations. This is predicted to be due to biological differences in different geographic regions. Children may also have swollen joints (inflammatory swelling, or in chronic arthritis due to synovial proliferation and thickening, and periarticular soft-tissue swelling).

Symptoms of inflammatory arthritis include stiffness, pain, and swelling of the joints, restricted motions, and reduced physical strength. Other symptoms may include systemic complaints including fatigue.

Polyarticular JIA affects five or more joints in the first 6 months of disease. This subtype can include the neck and jaw, as well as the small joints usually affected. This type of JIA is more common in girls than in boys. It accounts for about 40% of JIA cases.

Usually, the smaller joints are affected in polyarticular JIA, such as the fingers and hands, although weight-bearing joints such as the knees, hips, and ankles may also be affected. The joints affected are usually symmetrical, meaning that it affects both joints on both sides of the body (such as both wrists.) Children with polyarticular JIA are also at risk for developing chronic iridocyclitis or uveitis and should also be monitored by an optometrist or ophthalmologist.

Rheumatoid factor may be positive, i.e. seropositive, in children with polyarticular JIA occurring between 9 and 16 years of age and is associated with" HLA DR4" and "HLA DW4". This group has poorer prognosis with about 50% progressing to severe disabiling arthritis, persisting into adulthood. It is generally seronegative in JIA occurring below 10 years of age with a milder disease process and responds better to treatment. Seropositivity is rare in children with systemic JIA. Due to the greater number of joints affected by polyarticular JIA, as well as the tendency to worsen over time, polyarticular JIA must be treated aggressively.

Three types of juvenile arthritis exist—juvenile rheumatoid arthritis (JRA), juvenile chronic arthritis (JCA), and juvenile idiopathic arthritis (JIA), of which JRA is the most common.

JRA again can be divided into three main forms: The classification is based upon symptoms, number of joints involved and the presence of certain antibodies in the blood.

1. Polyarticular arthritis is the first type of arthritis, which affects about 30–40% of children with arthritis and is more common in girls than boys. Typically five or more joints are affected (usually smaller joints such as the hands and feet but many also affect the hips, neck, shoulders and jaw).

2. Oligoarticular (aka pauciarticular) arthritis can be early or late onset and is the second type of arthritis, affecting about 50% of children with juvenile arthritis. This type affects fewer than four joints (usually the large joints such as knees, ankles or wrists) and may cause eye inflammation in girls with positive anti-nuclear antibodies (ANA). Girls younger than eight are more likely to develop this type of arthritis.

3. Systemic disease is the least common form, with 10–20% of children (boys and girls equally) being affected with limited movement, swelling and pain in at least one joint. A common symptom of this type is a high, spiking fever of or higher, lasting for weeks or months, and a rash of pale red spots on the chest, thighs or other parts of the body may be visible.

Treatments for inflammatory arthritis vary by subtype, though they may include drugs like DMARDs (disease-modifying anti-rheumatic drugs) and tumor necrosis factor inhibitors.

Systemic JIA is characterized by arthritis, fever, which typically is higher than the low-grade fever associated with polyarticular and a salmon pink rash. It accounts for 10-20% of JIA and affects males and females equally, unlike the other two subtypes of JIA, and affects adolescents. It generally involves both large and small joints. Systemic JIA can be challenging to diagnose because the fever and rash come and go. Fever can occur at the same time every day or twice a day (often in late afternoon or evening) with a spontaneous rapid return to baseline (vs. septic arthritis of continuous fever). The rash often occurs with fever. It is a discrete, salmon-pink macules of different sizes. It migrates to different locations on skin, rarely persisting in one location more than one hour. The rash is commonly seen on trunk and proximal extremities or over pressure areas.

Arthritis is often absent in the first weeks or even 6–8 months into the illness.

Systemic JIA may have internal organ involvement such as hepatosplenomegaly, lymphadenopathy, serositis, hepatitis, or tenosynovitis.

A polymorphism in macrophage migration inhibitory factor has been associated with this condition.

Pain, which can vary in severity, is a common symptom in virtually all types of arthritis. Other symptoms include swelling, joint stiffness and aching around the joint(s). Arthritic disorders like lupus and rheumatoid arthritis can affect other organs in the body, leading to a variety of symptoms. Symptoms may include:

- Inability to use the hand or walk

- Stiffness, which may be worse in the morning, or after use

- Malaise and fatigue

- Weight loss

- Poor sleep

- Muscle aches and pains

- Tenderness

- Difficulty moving the joint

It is common in advanced arthritis for significant secondary changes to occur. For example, arthritic symptoms might make it difficult for a person to move around and/or exercise, which can lead to secondary effects, such as:

- Muscle weakness

- Loss of flexibility

- Decreased aerobic fitness

These changes, in addition to the primary symptoms, can have a huge impact on quality of life.

Juvenile arthritis, also known as Childhood arthritis (JA) is any form of chronic arthritis or arthritis-related conditions which affects individuals under the age of 16.

Juvenile arthritis is a chronic autoimmune disease.

Infectious arthritis is another severe form of arthritis. It presents with sudden onset of chills, fever and joint pain. The condition is caused by bacteria elsewhere in the body. Infectious arthritis must be rapidly diagnosed and treated promptly to prevent irreversible joint damage.

Psoriasis can develop into psoriatic arthritis. With psoriatic arthritis, most individuals develop the skin problem first and then the arthritis. The typical features are of continuous joint pains, stiffness and swelling. The disease does recur with periods of remission but there is no cure for the disorder. A small percentage develop a severe painful and destructive form of arthritis which destroys the small joints in the hands and can lead to permanent disability and loss of hand function.

Systemic-onset juvenile idiopathic arthritis (also known as systemic juvenile idiopathic arthritis (sJIA) or the juvenile onset form of Still's disease) is a type of juvenile idiopathic arthritis (JIA) with extra-articular manifestations like fever and rash apart from arthritis. It was originally called systemic-onset juvenile rheumatoid arthritis or Still's disease.

Predominantly extra-articular manifestations like high fevers, rheumatic rash, enlargement of the liver and spleen, enlargement of the lymph nodes, and anemia. Others manifestations include inflammation of the pleura, inflammation of the pericardium, inflammation of the heart's muscular tissue, and inflammation of the peritoneum are also seen.

It is sometimes called "juvenile-onset Still's disease", to distinguish it from adult-onset Still's disease. However, there is some evidence that the two conditions are closely related.

It may be associated with bilateral edema in lower limbs, pain and joint swelling. Sometimes there is previous history of inflammatory joint problems and bilateral edema of lower limbs.

Alphavirus Polyarthritis Syndrome has an incubation period of 3–21 days, depending on the specific virus, with either a gradual or sudden onset of fever, arthralgias, headache, and lymphadenitis and conjunctivitis in some forms. A maculopapular rash may present 4–8 days post symptom onset and may be accompanied by an increase in fever. Joint pains associated with this condition may recur for many months after initial illness for up to a year.

The signs and symptoms of ankylosing spondylitis often appear gradually, with peak onset being between 20 and 30 years of age. Initial symptoms are usually a chronic dull pain in the lower back or gluteal region combined with stiffness of the lower back. Individuals often experience pain and stiffness that awakens them in the early morning hours.

As the disease progresses, loss of spinal mobility and chest expansion, with a limitation of anterior flexion, lateral flexion, and extension of the lumbar spine, are seen. Systemic features are common, with weight loss, fever, or fatigue often present. Pain is often severe at rest but may improve with physical activity, but inflammation and pain to varying degrees may recur regardless of rest and movement.

AS can occur in any part of the spine or the entire spine, often with pain referred to one or the other buttock or the back of the thigh from the sacroiliac joint.

Arthritis in the hips and shoulders may also occur. When the condition presents before the age of 18, it is more likely to cause pain and swelling of large lower limb joints, such as the knees. In prepubescent cases, pain and swelling may also manifest in the ankles and feet where heel pain and enthesopathy commonly develop. Less commonly ectasia of the sacral nerve root sheaths may occur.

About 40% of people with AS will also experience inflammation of the anterior chamber of the eye, causing eye pain, redness, floaters and sensitivity to light. This is thought to be due to the association that both AS and uveitis have with the inheritance of the HLA-B27 antigen.

Inflammation of the prostate occurs with increased frequency in men.

Cardiovascular involvement may include inflammation of the aorta, aortic valve insufficiency or disturbances of the heart's electrical conduction system. Lung involvement is characterized by progressive fibrosis of the upper portion of the lung.

These symptoms could consist of asthenia, fever, anorexia, and weight loss. It mostly occurs during a severe disease flare.

The second most common clinical finding of this disease is joint pain with or without arthritis, after chondritis.

All synovial joints may be affected.

At presentation, around 33% of people have joint symptoms that involve Polyarthralgia and/or polyarthritis or oligoarthritis that affects various parts of the body and often appears to be episodic, asymmetric, migratory and non-deforming. The most common sites of involvement are the metacarpophalangeal joints, proximal interphalangeal joints and knees. After which is followed by the ankles, wrists, metatarsophalangeal joints and the elbows. Any involvement of the axial skeleton is considered to be very rare. Tests for rheumatoid factor are negative in affected persons with RP, unless there is a co-morbidity with RA.

Less often it has been reported that persons may experience arthralgia, monoarthritis, or chronic polyarthritis that mimics rheumatoid arthritis, leading to a difficult diagnosis for this disease. The appearance of erosions and destruction, however, is exceedingly rare and this may point instead to rheumatoid arthritis as a cause.

Diseases and inflammation of tendons have been reported in small numbers of people with RP. During the course of the disease, around 80% of people develop joint symptoms.

Palindromic rheumatism is a rare type of inflammatory arthritis which causes sudden inflammation in one or several joints, lasts a few hours or up to a few days, and then goes away completely. The problem usually involves 2 or 3 joints, which have onset over hours and last days to weeks, before subsiding. However episodes of recurrence form a pattern, with symptom-free periods between attacks lasting for weeks to months. The large joints are most commonly involved. Constitutionally, there may or may not be a fever, and swelling of the joints. The soft tissues are involved with swelling of the periarticular tissues, especially heel pads and finger pads. Nodules may be found in the subcutaneous tissues.Attacks may become more frequent with time but there is no joint damage after attacks.

It typically affects people between the ages of 20 and 50. One study showed an average age of onset of 49.

Occurs in 5-10% of patients who have psoriasis.Classic presentation involves the DIP(distal inerphalangeal joints).Morning stiffness is present.Deformity of involved joints,dactylitis and nail involvement are common.Well demarcated red plaques with silvery scaling - the classic lesions of psoriasis are seen on the dorsum of the hand.

When monoarthritis is caused by "pseudogout" (calcium pyrophosphate deposition disease, CPPD), the inflammation usually lasts days to weeks, and involves the knees in half of all attacks. Like gout, attacks can occur spontaneously or with physical trauma or metabolic stress. Patients may feel well in between pseudogout attacks, and 5% present with pseudo-rheumatoid symptoms.

Palindromic rheumatism is a disease of unknown cause. It has been suggested that it is an abortive form of rheumatoid arthritis (RA), since anti-cyclic citrullinated peptide antibodies (anti-CCP) and antikeratin antibodies (AKA) are present in a high proportion of patients, as is the case in rheumatoid arthritis. Unlike RA and some other forms of arthritis, palindromic rheumatism affects men and women equally. Palindromic rheumatism is frequently the presentation for Whipple disease which is caused by the infectious agent "Tropheryma whipplei" (formerly "T. whippelii").

Anti-citrullinated protein antibody is frequently associated.

Polyarthritis is most often caused by an auto-immune disorder such as rheumatoid arthritis, amyloidosis, psoriatic arthritis, and lupus erythematosus but can also be caused by infection with an alphavirus such as chikungunya virus and Ross River virus. This condition is termed alphavirus polyarthritis syndrome. Sindbis virus, which is endemic in Northern Europe, Africa, the Middle East, and Australia, is the most widely distributed of the alphaviruses causing polyarthritis, though infection is usually mild or asymptomatic.

Mortality is increased in people with AS and circulatory disease is the most frequent cause of death. AS patients have an increased risk of 60% for cerebrovascular mortality, and an overall increased risk of 50% for vascular mortality. About one third of those with Ankylosing spondylitis have severe disease, which reduces life expectancy.

As increased mortality in ankylosing spondylitis is related to disease severity, factors negatively affecting outcomes include:

- Male sex

- Plus 3 of the following in the first 2 years of disease:

- Erythrocyte sedimentation rate (ESR) >30 mm/h

- Unresponsive to NSAIDs

- Limitation of lumbar spine range of motion

- Sausage-like fingers or toes

- Oligoarthritis

- Onset <16 years old

The following conditions are typically included within the group of "seronegative spondylarthropathies":

Some sources also include Behcet's disease and Whipple's disease.