Rheumatology

Image study

Plain radiography X-rays can show anatomical changes that reflect important pathological processes Although most changes have low specificity, combinations of features and targeting of certain bones or joints result in characteristic patterns that have high diagnostic specificity. Joints to be X-rayed are usually selected on the basis of involvement identified at clinical assessment.

ErosionsCartilage and bone erosion is a hallmark of major inflammatory arthropathies. Intracapsular bone erosion firstoccurs at the joint margin (‘marginal erosion’) wherebone is exposed directly to inflammatory synoviumwithout the protection of overlying cartilage. Loss ofthe sharp cortical line is the first radiographic sign andprecedes more definite scalloping of the bony contourCartilage erosion also starts at the margin andslowly works centrally, resulting in loss of ‘joint space’.

Both RA and seronegative spondyloarthritis (especiallypsoriatic arthritis) can cause marginal erosions. In RAthere is no bone or periosteal reaction, resulting in atrophic‘non-proliferative’ erosions often with juxta-articular osteopenia and soft tissue swelling. By contrast, in seronegative spondyloarthritis new bone formation and periosteal reaction with retained bone density are more common, resulting in ‘proliferative’ erosions .Accompanying ossifying enthesopathy

In the first 1–2 weeks of acute septic arthritis the Xrayis often normal, apart from osteopenia and soft tissueswelling. However, erosion proceeds rapidly and resultsin generalised loss of joint space with loss of corticalintegrity centrally (central erosion) as well as marginally.In chronic gout, bony defects develop slowly as massivecrystal deposits (‘tophi’) cause pressure necrosis tosurrounding bone. Such ‘pressure erosions’ .occur at extracapsular as well as intracapsular sites .

Calcification Calcification of fibrocartilage and hyaline cartilage (chondrocalcinosis) is most commonly due to calcium pyrophosphate crystals. Calcification at extracapsular sites is mainly apatite. Spotty, multiple calcifications of soft tissues (calcinosis) mainly target peripheral and intermediate sites such as finger pulps, wrists and forearms, and are a feature of connective tissue disease.


Radionuclide bone scan This is a useful investigation in patients who have bone pain. It involves gamma-camera imaging following an intravenous injection of 99mTc-bisphosphonate. Early post-injection images reflect vascularity and can show increased perfusion of inflamed synovium, Pagetic bone, or primary or secondary bone tumours Delayed images taken a few hours later reflect bone remodelling as the bisphosphonate localises to sites of active bone turnover. Scintigraphy has a high sensitivity for detecting important bone and joint pathology that is

Computerised tomography (CT)and magnetic resonanceimaging (MRI)

These techniques give detailed information on anatomy, allowing three-dimensional visualisation of anatomically complex structures such as the spinal canal and facet joints which may be inadequately assessed by plain X-rays. Drawbacks of CT are limited soft tissue resolution and a high radiation dose, and MRI is frequently preferred. It provides detailed information on both structure and physiology of cartilage, bone and other locomotor tissues,

Ultrasonography

Ultrasonography is inferior to CT or MRI for definition of deep structures and abnormalities within bone, but is a useful outpatient investigation for confirmation of small joint synovitis/erosion, for anatomical confirmation of periarticular lesions, and for assistance in guiding

DEXA

Bone mineral density Measurement of bone mineral density (BMD) plays a pivotal role in the investigation and management of osteoporosis. The investigation .of choice is dual energy X-ray absorptiometry (DEXA) of the spine and hip.

Blood tests

C-reactive protein (CRP)and erythrocyte sedimentation rate (ESR) Infections, inflammation and malignancy can trigger an acute phase response (APR) with alterations in C-reactive protein and erythrocyte sedimentation rate CRP is the single most useful marker of the APR..

Full blood count Changes in the FBC can occur in inflammatory rheumatic diseases but are non-specific (e.g. neutrophilia in vasculitis,acute gout and sepsis; neutropenia in lupus). Many disease-modifying antirheumatic drugs (DMARDs) have marrow toxicity and require regular monitoring of the FBC.

Autoantibodies

Autoantibody tests are a useful adjunct to clinical evaluation in the diagnosis of rheumatic diseases but false positive results are common. Those most commonly used in rheumatology are described below; antiphospholipid antibodies, which occur in systemic lupus erythematosus


Rheumatoid factor Rheumatoid factor (RF) is an antibody directed against the Fc fragment of human IgG and was so named because it was first identified in patients with RA. RFs may be of any immunoglobulin class but IgM is most commonly tested. RF also occurs in a wide variety of other conditions and in some normal adults it therefore has low diagnostic specificity for RA and also lacks sensitivity, since about 30% of patients with typical signs of RA are negative for RF (so-called seronegative RA). The principal use of RF testing is for prognosis, since a high RF titre at presentation associates with more severe disease.


Antibodies to cyclic citrullinated peptides (anti-CCP antibodies) Anti-CCP antibodies bind to peptides in which the amino acid arginine has been converted to citrulline by peptidylarginine deiminase, an enzyme abundant in inflamed synovium. They have similar sensitivity to RF for RA (70%) but much higher specificity (> 95%).

Antinuclear antibodies (ANA)These are directed against one or more components ofthe nucleus. gives the many causes of a positiveANA. Low titre ANA is common in normal individuals.The higher the ANA titre, the greater its diagnostic significance,but high titres do not imply more severe disease.The most common indication for ANA testing is inthe diagnosis of SLE. ANA has high sensitivity for SLE(virtually 100%) but low specificity (10–40%); a negativeANA virtually excludes SLE but a positive result doesnot confirm it.


(p-ANCA). c-ANCA are associated with antibodiesto proteinase-3 (PR3), and occur in > 90% of patientswith Wegener’s granulomatosis with renal involvement.antibodies, is associated with microscopic polyarteritis and Churg–Strauss vasculitis

Biochemistry

Routine biochemistry is useful in the assessment of metabolic bone disease, muscle diseases and gout. Serum levels of uric acid are usually raised in gout but a normal level does not exclude the diagnosis .


Serum creatinine kinase (CK) levels are useful in thediagnosis of myopathy or myositis, Several bone diseases, including Paget’s disease,renal bone disease and osteomalacia give a characteristicpattern on biochemical testing which can be usefuldiagnostically The best markers of bone resorptionare N-telopeptide (NTX) and C-telopeptide(CTX)

Joint aspiration

Joint aspiration with examination of synovial fluid (SF) is pivotal in patients suspected of having septic arthritis, crystal arthritis or intra-articular bleeding. It should be done in all patients with acute monoarthritis, and samples sent for microbiology and clinical chemistry

Tissue sampling

Bone biopsy is helpful in the differential diagnosis of bone diseases when less invasive tests have proved inconclusive. If a systemic disease is suspected, the biopsy should be taken from the iliac crest using a large diameter (8 mm) trephine needle under local anaesthetic.


Synovial biopsy may be required in patients with chronic inflammatory monoarthritis or tenosynovitis to identify specific causes such as chronic mycobacterial infection or pigmented villonodular synovitis It may be obtained arthroscopically or using ultrasound guidance under local anaesthetic. Muscle biopsy is useful in the investigation of myopathy, myositis and systemic vasculitis