Write a short note on seronegative spondyloarthritis, including ankylosing spondylitis and reactive arthritis.

Definition & Concept

The seronegative spondyloarthritides are a family of inflammatory arthritides sharing: absence of rheumatoid factor ("seronegative"), association with HLA-B27, axial involvement (sacroiliitis and spondylitis), enthesitis (inflammation at tendon and ligament insertions), asymmetrical large-joint oligoarthritis of the lower limbs, dactylitis, extra-articular features (uveitis, mucocutaneous, cardiac), and familial clustering.

Members: ankylosing spondylitis (the prototype), reactive arthritis, psoriatic arthritis, enteropathic arthritis (with inflammatory bowel disease), and undifferentiated spondyloarthritis.

Ankylosing Spondylitis

Definition: a chronic inflammatory disease of the axial skeleton, principally the sacroiliac joints and spine, leading to syndesmophyte formation, ankylosis and progressive spinal fusion. It affects young men predominantly (3:1), with onset typically in the late teens to early thirties. Over 90% are HLA-B27 positive.

Clinical features:

• Inflammatory back pain — insidious onset before age 40, duration over 3 months, morning stiffness exceeding 30 minutes, improvement with exercise and NOT with rest, and night pain that wakes the patient in the second half of the night. This pattern distinguishes it from mechanical back pain.
• Alternating buttock pain from sacroiliitis.
• Reduced spinal mobility: modified Schober's test (less than 5 cm increase on forward flexion), reduced lateral flexion, reduced chest expansion (below 5 cm) from costovertebral involvement, and increased occiput-to-wall distance.
• Progressive deformity: loss of lumbar lordosis, thoracic kyphosis, and the "question mark" posture.
• Enthesitis — Achilles tendonitis, plantar fasciitis; peripheral arthritis of hips and shoulders (hip involvement predicts poorer function).

Extra-articular — the "A" list:

• Anterior uveitis — acute, unilateral, recurrent, painful, in up to 40%; the commonest extra-articular feature
• Aortic regurgitation and aortitis; AV conduction block
• Apical pulmonary fibrosis
• Amyloidosis (AA); Achilles enthesitis
• Atlanto-axial subluxation; osteoporosis with spinal fracture
• IgA nephropathy; inflammatory bowel disease

Investigations:

• raised ESR and CRP (may be normal)
• HLA-B27 (supportive, not diagnostic — it is common in the healthy population)
• X-ray sacroiliac joints showing sclerosis, erosion, pseudo-widening then fusion
• spine showing squaring of vertebrae, syndesmophytes and the "bamboo spine"
• MRI with STIR sequences, which detects bone marrow oedema and active sacroiliitis years before radiographic change — the key advance permitting early diagnosis.

Management:

• Exercise and physiotherapy are the cornerstone — daily spinal extension exercises, postural training, swimming; this determines long-term function more than any drug.
• NSAIDs — first-line pharmacological treatment, effective for pain and stiffness, and possibly slowing radiographic progression when used continuously.
• Biologics — TNF-α inhibitors and IL-17 inhibitors (secukinumab) for active disease despite NSAIDs; these transformed outcomes in axial disease.
• Conventional DMARDs (sulfasalazine, methotrexate) work only for peripheral arthritis, not for axial disease — an important examination point.
• Smoking cessation; osteoporosis assessment; ophthalmology referral for uveitis; surgery for severe hip disease or spinal deformity.

Reactive Arthritis

Definition: a sterile inflammatory arthritis occurring 1–4 weeks after a distant infection, most often gastrointestinal or genitourinary. The organism is not recoverable from the joint.

Triggers:

• Post-dysenteric: *Shigella, Salmonella, Campylobacter, Yersinia*
• Post-venereal: *Chlamydia trachomatis*, *Ureaplasma*

Features:

• Classic triad (Reiter's syndrome): "can't see, can't pee, can't climb a tree" — conjunctivitis, urethritis and arthritis. The full triad occurs in only about a third.
• Asymmetrical oligoarthritis of the lower limbs — knees, ankles, feet; dactylitis ("sausage digit"); enthesitis with heel pain.
• Mucocutaneous: keratoderma blennorrhagica (pustular hyperkeratotic lesions on the soles, resembling pustular psoriasis), circinate balanitis, painless oral ulcers, nail dystrophy.
• Ocular: conjunctivitis early; anterior uveitis in recurrent disease.
• Sacroiliitis in chronic or HLA-B27-positive cases.

Investigations:

• raised ESR and CRP
• HLA-B27 positive in 50–80%
• stool and urethral cultures, chlamydia NAAT on first-void urine
• joint aspiration to exclude septic and crystal arthritis — synovial fluid is inflammatory but sterile
• rheumatoid factor and ANA negative.

Management:

• NSAIDs first-line
• intra-articular or systemic corticosteroids
• antibiotics for the triggering genitourinary infection and for sexual contacts (they do not alter established arthritis, and do not help post-dysenteric disease)
• sulfasalazine or methotrexate for persistent disease
• TNF inhibitors in refractory cases. Most cases resolve within 3–12 months, but a proportion relapse or become chronic.