Differentials
Septic arthritis
SIGNS / SYMPTOMS
Usually only one joint involved; not migratory; patient looks toxic.
INVESTIGATIONS
Positive Gram stain from synovial fluid aspirate.
Culture of an organism from aspirate.
Elevated white cell count in blood and on microscopy of synovial fluid.
May have positive blood cultures. Imaging may show features of infection (e.g., synovial enhancement, periosteal collection, intramedullary abscess).
Juvenile arthritis
SIGNS / SYMPTOMS
Joint involvement persists for many weeks, may have small joint involvement. May not have joint pain; eye inflammation may be present. Light pink rash in systemic form. Other systemic features such as serositis may be present.
INVESTIGATIONS
Positive connective tissue testing such as positive rheumatoid factor, anti-nuclear antibody, anti-dsDNA, anticyclic citrullinated peptide (anti-CCP) IgG.
Post-infectious reactive arthropathy
SIGNS / SYMPTOMS
History of preceding viral or gastrointestinal illness.
INVESTIGATIONS
Positive infectious serology (cytomegalovirus, Epstein-Barr virus, Yersinia, hepatitis B or C, parvovirus, influenza).
In young adults, urine culture/nucleic acid amplification test may reveal recent gonorrhoea or chlamydia infection.
Lyme disease
SIGNS / SYMPTOMS
Only applies to people from Lyme endemic regions. Early Lyme disease is characterised by circular expanding rash with central clearing (erythema migrans). Later, Lyme disease can cause influenza-like symptoms; acute neurological problems, including cranial nerve palsies; arthritis usually affecting the knees; heart block.
INVESTIGATIONS
Positive Borrelia burgdoferi serology (may take 4-6 weeks to develop a serological response) or polymerase chain reaction, together with history of being in a Lyme endemic region and exposure to ticks.
Sickle cell anaemia bone crisis
SIGNS / SYMPTOMS
Family history; signs and symptoms of anaemia; not usually febrile unless infection has precipitated bone crisis. May involve small joints, back, dactylitis (painful inflammation of entire digit).
INVESTIGATIONS
Anaemia and sickle cells on blood film.
Infective endocarditis
SIGNS / SYMPTOMS
Signs of sepsis. Peripheral stigmata of endocarditis (Janeway lesions, Osler nodes, and splinter haemorrhages) are relatively uncommon in children.
INVESTIGATIONS
Positive blood culture for organism causing endocarditis.
Echocardiogram may reveal vegetations on valve leaflets, intra-cardiac abscess.
Leukaemia
SIGNS / SYMPTOMS
History may include lethargy, weight loss, night sweats, and bone pain. Bruising or spontaneous bleeding may be present.
INVESTIGATIONS
Blast cells on film. Anaemia or thrombocytopenia.
Gout and pseudogout
SIGNS / SYMPTOMS
First metacarpophalangeal joint often affected; pain excruciating and often flaky red skin over affected joint.
INVESTIGATIONS
Polarising microscopy of synovial fluid or presence of gouty tophi.
Elevated uric acid level.
Innocent murmur
SIGNS / SYMPTOMS
Otherwise normal child; quiet murmur; never purely diastolic.
INVESTIGATIONS
Normal echocardiogram.
Congenital mitral valve prolapse
SIGNS / SYMPTOMS
Mid-systolic click on cardiac auscultation.
INVESTIGATIONS
Echocardiogram reveals characteristic billowing of one or both of the mitral valve leaflets into the left atrium during/towards the end of systole.
Congenital heart disease
SIGNS / SYMPTOMS
Murmurs may be not characteristic for mitral and/or aortic incompetence. Murmurs may have been previously documented in infancy.
INVESTIGATIONS
Echocardiography will reveal abnormalities.
Hypertrophic cardiomyopathy
SIGNS / SYMPTOMS
Afebrile; may be asymptomatic.
INVESTIGATIONS
Echocardiography shows hypertrophy of the left ventricle without dilatation of cavity.
Myocarditis
SIGNS / SYMPTOMS
Usually follows a viral illness; chest pain and shortness of breath are common features.
INVESTIGATIONS
Troponin and creatine kinase elevated.
ECG may show saddle-shaped ST segments or T-wave changes.
Cardiac MRI will demonstrate myocarditic inflammation.
Cardiac muscle biopsy will demonstrate cardiac muscle inflammation.
Viral pathogens may be detected on cardiac tissue via polymerase chain reaction.
Pericarditis
SIGNS / SYMPTOMS
Pleuritic chest pain; pericardial friction rubs on auscultation.
INVESTIGATIONS
Echocardiography may show a pericardial effusion.
Systemic lupus erythematosus
SIGNS / SYMPTOMS
Malar 'butterfly' rash; joint pain typically affects hands and wrists; renal involvement is common, may have anaemia.
INVESTIGATIONS
Positive connective tissue testing such as positive anti-nuclear antibody, anti-dsDNA, and anti-Smith antibodies.
Drug intoxication
SIGNS / SYMPTOMS
History of recent ingestion; use of illicit drugs.
INVESTIGATIONS
Drug screen, including phenytoin, amitriptyline, and metoclopramide.
Wilson's disease
SIGNS / SYMPTOMS
Hepatosplenomegaly and Kayser-Fleischer rings; may have a family history.
INVESTIGATIONS
Decreased ceruloplasmin level, genetic testing.
24-hour urinary copper excretion.
Tic disorder
SIGNS / SYMPTOMS
Can be motor or phonic tics; absence of fever or any other signs of acute rheumatic fever.
INVESTIGATIONS
Psychiatric evaluation may reveal underlying cause.
Encephalitis
SIGNS / SYMPTOMS
Seizures, headache, fever; sometimes photophobia and neck stiffness.
INVESTIGATIONS
Electroencephalogram may show temporal lobe changes.
MRI brain: features depend on cause of encephalitis but may include temporal lobe haemorrhage or white matter lesions.
Polymerase chain reaction of cerebrospinal fluid to detect viral DNA/RNA.
Choreoathetoid cerebral palsy
SIGNS / SYMPTOMS
Wide spectrum of symptoms depending on severity; features include difficulty maintaining posture, scissor walking, seizures, and learning difficulties. Long-term chronic features.
INVESTIGATIONS
Clinical diagnosis. CT may help to identify cerebral haemorrhage, and MRI can be useful to look for changes in cerebral white matter in older children.
Huntington's chorea
SIGNS / SYMPTOMS
May have associated symptoms of weight loss, depression, facial tics, impairment of rapid eye movement, and dementia. More likely if a parent is affected.
INVESTIGATIONS
Genetic testing (triplet repeat).
Intracranial tumour
SIGNS / SYMPTOMS
May have headache, typically worse in the morning, with or without vomiting; may have papilloedema; cranial nerve involvement possible.
INVESTIGATIONS
CT/MRI of the brain.
Hyperthyroidism
SIGNS / SYMPTOMS
Tachycardia, tremor, weight loss.
Eye signs: exophthalmia, lid lag and retraction, proptosis.
INVESTIGATIONS
Thyroid function tests; typically elevated thyroxine (T4) and triiodothyronine (T3) with suppression of thyroid-stimulating hormone (TSH) in primary hyperthyroidism. Secondary hyperthyroidism will show elevated TSH levels.
Osteomyelitis
SIGNS / SYMPTOMS
May have history of open fracture or recent orthopaedic surgery.
INVESTIGATIONS
Imaging (plain x-ray), surgical findings, histology, and cultures.
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