Connective-Tissue Disease Overview

Mixed CTD

Definition, immunopathogenesis and epidemiology

Mixed connective-tissue disease (MCTD) is an overlap autoimmune syndrome defined by high-titre anti-U1 ribonucleoprotein antibodies and a characteristic constellation of features resembling systemic lupus erythematosus, systemic sclerosis, inflammatory myopathy and rheumatoid arthritis. It is not merely “undifferentiated CTD”: the presence of anti-U1-RNP is central, and the clinical phenotype is dominated by Raynaud phenomenon, swollen hands/puffy digits, inflammatory arthritis, myositis and oesophageal dysmotility, with pulmonary arterial hypertension (PAH) being the major late determinant of mortality.

The anti-U1-RNP response targets proteins complexed with U1 small nuclear RNA involved in pre-mRNA splicing. Pathogenesis includes loss of tolerance to spliceosomal antigens, type I interferon activation, B-cell hyperactivity, immune-complex formation and vascular endothelial injury. Genetic associations include HLA-DR4 and DR2 in some cohorts. MCTD is uncommon, with estimated prevalence approximately 3–4 per 100,000, female predominance around 9:1, and typical onset in the second to fourth decade. The clinical phenotype may evolve over years; some patients later fulfil criteria for a defined CTD, particularly systemic sclerosis or SLE.

Clinical phenotype and examination priorities

Raynaud phenomenon is present in >80–90% and may precede other manifestations by years. “Puffy hands” due to tenosynovitis and oedema are highly characteristic early. Arthritis is usually symmetric and inflammatory; deforming non-erosive arthropathy may occur, but erosions suggest rheumatoid overlap. Myositis manifests with proximal weakness and raised creatine kinase, often milder than idiopathic inflammatory myopathy. Scleroderma-spectrum features include sclerodactyly, telangiectasia, calcinosis and oesophageal hypomotility, but diffuse skin thickening and renal crisis are less typical than in systemic sclerosis.

• Pulmonary disease: PAH and interstitial lung disease (ILD) are key. PAH may be clinically silent until advanced; exertional dyspnoea, loud P2, right ventricular heave and reduced DLCO are exam-relevant clues. ILD is usually NSIP-pattern on HRCT.
• Cardiac disease: pericarditis, myocarditis, conduction disease and right-heart failure from PAH.
• Renal disease: clinically important nephritis is less frequent than in SLE; heavy proteinuria, active urinary sediment or low complement should prompt reconsideration of lupus overlap.
• Gastrointestinal disease: reflux and dysphagia from oesophageal dysmotility; aspiration risk may compound ILD.

Classification and diagnostic approach

No universally accepted diagnostic criteria exist; classification sets are used pragmatically. High-titre anti-U1-RNP is essential in most definitions. ANA is usually positive with a coarse speckled pattern. Anti-dsDNA and anti-Sm are typically absent or low-level; anti-centromere or anti-topoisomerase I suggest systemic sclerosis phenotype rather than classical MCTD. ESR is often raised, whereas CRP may be only modestly elevated unless serositis, infection or inflammatory arthritis is prominent.

Criteria set	Core serology	Clinical requirements	Exam relevance
Alarcón-Segovia	Anti-U1-RNP titre typically ≥1:1600	At least 3 of: hand oedema, synovitis, myositis, Raynaud phenomenon, acrosclerosis; must include synovitis or myositis	High specificity; commonly cited in postgraduate exams
Kasukawa	Anti-U1-RNP positive	Common features plus mixed manifestations from SLE-like, systemic sclerosis-like and polymyositis-like domains	More sensitive for overlap presentations
Sharp	High anti-ENA/anti-RNP with absent anti-Sm	Major/minor clinical overlap criteria	Historically important; less often used clinically

Baseline assessment should define organ involvement rather than merely confirm serology: FBC, U&E, LFT, CK, ESR/CRP, urinalysis with protein:creatinine ratio, complements, ENA profile, antiphospholipid antibodies if thrombosis or pregnancy morbidity, ECG, echocardiography, pulmonary function tests including DLCO, HRCT if respiratory symptoms or abnormal PFTs, and nailfold capillaroscopy. Annual screening for PAH with echocardiography and PFTs is widely practised; in systemic sclerosis-spectrum disease, a DLCO <60% predicted or falling DLCO with preserved volumes should heighten suspicion. Right-heart catheterisation is required to diagnose PAH: mean pulmonary arterial pressure >20 mmHg, pulmonary artery wedge pressure ≤15 mmHg and pulmonary vascular resistance >2 Wood units.

Management

Treatment is organ-directed and resembles management of the dominant overlap phenotype. General measures include smoking cessation, vaccination before immunosuppression, cardiovascular risk control, pregnancy counselling and aggressive reflux management.

Manifestation	Typical treatment	Key numerical points
Raynaud phenomenon	Cold avoidance; dihydropyridine calcium-channel blocker such as nifedipine MR; consider phosphodiesterase-5 inhibitor for refractory disease	Nifedipine MR 30–60 mg once daily; sildenafil commonly 20 mg three times daily
Arthritis/serositis	Hydroxychloroquine, NSAID if renal/GI risk acceptable, low-dose glucocorticoid	Hydroxychloroquine ≤5 mg/kg/day actual body weight; retinal screening baseline then annually after 5 years, earlier if high risk
Myositis	Prednisolone with steroid-sparing immunosuppression	Prednisolone 0.5–1 mg/kg/day initially; methotrexate 15–25 mg weekly or azathioprine 1–2 mg/kg/day
ILD	Mycophenolate mofetil; cyclophosphamide for severe/progressive disease; rituximab considered in refractory overlap CTD-ILD	Mycophenolate 1–1.5 g twice daily; IV cyclophosphamide often 500–750 mg/m² monthly for 6 months
PAH	Specialist PAH therapy: endothelin receptor antagonists, PDE-5 inhibitors, prostacyclin analogues; anticoagulation not routine unless another indication	Risk-stratified combination therapy; confirm by right-heart catheterisation before PAH-specific drugs

Glucocorticoids should be used cautiously: although scleroderma renal crisis is less common in MCTD than systemic sclerosis, high-dose steroids are a recognised risk in scleroderma-spectrum disease. Blood pressure and urinalysis monitoring are therefore mandatory when prednisolone doses exceed approximately 15–20 mg/day for sustained periods.

Prognosis and examination pitfalls

Overall 10-year survival is commonly reported around 80–90%, but outcomes are heterogeneous. PAH, ILD, myocarditis and severe myositis worsen prognosis; PAH is the leading cause of disease-related death. A frequent MRCP pitfall is assuming MCTD is benign because renal disease is less common than in SLE. Another is diagnosing MCTD solely from a positive anti-RNP: low-titre anti-RNP occurs in SLE and other CTDs. The diagnosis requires compatible clinical overlap, high-titre anti-U1-RNP and longitudinal reassessment as the phenotype evolves.

SLE

Systemic lupus erythematosus is a relapsing–remitting, multisystem autoimmune disease characterised by loss of tolerance to nuclear antigens, immune-complex formation and complement activation. It predominantly affects women of childbearing age, with higher prevalence and severity in Afro-Caribbean, South Asian and Hispanic populations. Pathogenesis integrates genetic susceptibility, epigenetic dysregulation, ultraviolet exposure, oestrogenic effects, impaired apoptotic debris clearance, type I interferon activation and B-cell hyperactivity. Immune complexes drive small-vessel inflammation, particularly in skin, kidney, joints, serosa and CNS; hypocomplementaemia and rising anti-dsDNA titres often parallel immune-complex activity, especially lupus nephritis.

Classification and diagnostic approach

SLE remains a clinical diagnosis; classification criteria support standardisation rather than replace judgement. The 2019 EULAR/ACR criteria require a positive ANA at least once as an entry criterion, followed by weighted clinical and immunological domains. A total score of ≥10, with at least one clinical criterion and no better alternative diagnosis, classifies SLE. In validation cohorts, sensitivity was approximately 96% and specificity approximately 93%.

Domain	High-yield MRCP points
ANA	Entry criterion: ANA ≥1:80 on HEp-2 immunofluorescence or equivalent. Sensitive but non-specific; a persistently negative ANA makes SLE unlikely.
Specific antibodies	Anti-dsDNA: high specificity, correlates with nephritis/activity. Anti-Sm: highly specific but insensitive. Anti-Ro/SSA: photosensitive rash, subacute cutaneous lupus, neonatal lupus/congenital heart block.
Complement	Low C3/C4 indicates classical pathway consumption. Typical adult ranges: C3 about 0.9–1.8 g/L, C4 about 0.1–0.4 g/L, assay-dependent.
Antiphospholipid antibodies	Lupus anticoagulant, anticardiolipin IgG/IgM, anti-β2-glycoprotein I; clinically relevant if persistent on repeat testing ≥12 weeks apart.

Clinical patterns and organ-threatening disease

Common manifestations include fatigue, fever, non-erosive inflammatory arthritis, malar rash sparing nasolabial folds, photosensitivity, oral/nasal ulcers, alopecia, Raynaud phenomenon, serositis and cytopenias. Haematological disease includes autoimmune haemolytic anaemia, lymphopenia, leucopenia and immune thrombocytopenia. Neuropsychiatric SLE is heterogeneous: seizures and psychosis are more classically attributable than headache or mood symptoms. Serositis may cause pleuritic pain, pericarditis or effusions.

Lupus nephritis is the key prognostic complication. Suspect with proteinuria, active urinary sediment, hypertension, rising creatinine or falling complement/rising anti-dsDNA. Kidney biopsy is generally indicated with proteinuria ≥0.5 g/day or urine protein:creatinine ratio approximately ≥50 mg/mmol, especially with haematuria or casts. ISN/RPS classes: I minimal mesangial, II mesangial proliferative, III focal, IV diffuse, V membranous, VI advanced sclerosing; proliferative class III/IV disease requires induction immunosuppression.

Monitoring and scoring

Disease activity and damage should be distinguished. SLEDAI-2K is a weighted activity index; a fall of ≥4 points is commonly used in trials as clinically meaningful. BILAG grades organ activity from A to E and is useful for flare categorisation. SLICC/ACR Damage Index records irreversible damage present for ≥6 months and predicts mortality. Routine monitoring includes FBC, U&E/eGFR, LFTs, ESR/CRP, urinalysis, urine protein quantification, C3/C4 and anti-dsDNA; CRP is often normal in flare unless serositis, infection or synovitis is prominent, so a markedly high CRP should prompt infection assessment.

Treatment principles

All patients without contraindication should receive hydroxychloroquine, which reduces flares, thrombosis and damage accrual. Recommended dose is ≤5 mg/kg/day actual body weight; half-life is long, approximately 40–50 days. Retinal toxicity risk rises with high dose, renal impairment and duration >5 years; baseline ophthalmic assessment and annual screening after 5 years, or earlier if high-risk, are standard.

Clinical scenario	Typical treatment	Exam-relevant cautions
Mild mucocutaneous/articular disease	Hydroxychloroquine; topical steroids/calcineurin inhibitors; NSAIDs if renal function permits; short prednisolone course, ideally <7.5 mg/day maintenance or stopped.	Avoid chronic glucocorticoids where possible: osteoporosis, infection, diabetes and accelerated atherosclerosis.
Steroid-sparing systemic disease	Methotrexate 15–25 mg weekly, azathioprine 1–2 mg/kg/day, or mycophenolate mofetil 1–1.5 g twice daily.	Check TPMT/NUDT15 where relevant before azathioprine; mycophenolate and methotrexate are teratogenic.
Class III/IV lupus nephritis induction	Mycophenolate mofetil target 2–3 g/day plus glucocorticoids, or cyclophosphamide: Euro-Lupus 500 mg IV every 2 weeks ×6; NIH regimen 0.5–1 g/m² monthly ×6.	Use ACE inhibitor/ARB for proteinuria and BP control if not pregnant. Maintenance: mycophenolate 1–2 g/day or azathioprine.
Refractory/active autoantibody-positive disease	Belimumab 10 mg/kg IV at weeks 0, 2, 4 then 4-weekly, or 200 mg SC weekly; anifrolumab 300 mg IV 4-weekly for non-renal moderate–severe disease.	BLISS trials supported belimumab; BLISS-LN showed improved renal response when added to standard therapy. Avoid live vaccines during significant immunosuppression.

Pregnancy should be planned during quiescent disease for at least 6 months. Continue hydroxychloroquine; azathioprine and low-dose prednisolone are acceptable. Anti-Ro/La positivity requires fetal surveillance for congenital heart block, particularly between 16–26 weeks. Antiphospholipid syndrome requires risk-stratified aspirin and heparin. Major causes of death are infection, active renal/CNS disease and premature cardiovascular disease; aggressive risk-factor modification is therefore integral to SLE care.

Sjögren Syndrome

Sjögren syndrome is a chronic autoimmune epithelitis characterised by lymphocytic infiltration and dysfunction of exocrine glands, producing sicca symptoms and systemic disease. It is classified as primary when occurring alone and secondary/associated when accompanying another autoimmune rheumatic disease, particularly rheumatoid arthritis, SLE or systemic sclerosis. Epidemiologically it is strongly female-predominant, typically presenting at 40–60 years; the major examination point is that morbidity is driven not only by dryness but by extraglandular disease and B-cell lymphoproliferation.

Pathogenesis and immunology

The initiating lesion is epithelial-cell activation in salivary and lacrimal glands, with type I interferon signalling, aberrant antigen presentation, BAFF overexpression and progressive T- and B-cell infiltration. Germinal-centre-like structures may form within glands, supporting autoantibody production and clonal B-cell expansion. Key autoantibodies are anti-Ro/SSA and anti-La/SSB; ANA and rheumatoid factor are common, and hypergammaglobulinaemia is frequent. Complement consumption, cryoglobulinaemia and monoclonal gammopathy are adverse prognostic markers and suggest systemic vasculitic or lymphomatous biology.

Feature	Exam relevance
Anti-Ro/SSA	Most important serological marker; included in classification criteria; associated with earlier onset, systemic disease, congenital heart block risk in pregnancy.
Anti-La/SSB	More specific in context but not sufficient alone in modern criteria; often coexists with anti-Ro.
RF, ANA, polyclonal IgG	Common supportive findings; RF positivity may mislead toward rheumatoid arthritis.
Low C4, cryoglobulins, palpable purpura	Predict lymphoma and systemic vasculitis; should prompt careful assessment.

Clinical phenotype

Glandular disease causes keratoconjunctivitis sicca, xerostomia, dysphagia for dry foods, dental caries, oral candidiasis, parotid enlargement and fatigue. Ocular symptoms include grittiness, burning and photophobia; untreated severe disease may cause corneal ulceration. Systemic manifestations include non-erosive arthralgia/arthritis, Raynaud phenomenon, interstitial lung disease, distal renal tubular acidosis from tubulointerstitial nephritis, peripheral neuropathy, small-fibre neuropathy, CNS disease, autoimmune liver disease, cytopenias and cutaneous vasculitis. Renal tubular acidosis classically produces normal anion-gap metabolic acidosis, hypokalaemia and nephrocalcinosis.

Classification, scoring and diagnosis

The 2016 ACR/EULAR classification criteria require compatible symptoms or systemic suspicion, no exclusion condition, and a weighted score ≥4. These are classification rather than diagnostic criteria but are frequently tested.

Item	Score
Labial salivary gland biopsy showing focal lymphocytic sialadenitis with focus score ≥1 focus/4 mm2	3
Anti-Ro/SSA positivity	3
Ocular staining score ≥5, or van Bijsterveld score ≥4 in at least one eye	1
Schirmer test ≤5 mm/5 min in at least one eye	1
Unstimulated whole salivary flow rate ≤0.1 mL/min	1

Exclusion conditions include previous head and neck radiotherapy, active hepatitis C infection, AIDS, sarcoidosis, amyloidosis, graft-versus-host disease and IgG4-related disease. Salivary-gland ultrasound is increasingly used, demonstrating hypoechoic areas and glandular inhomogeneity, but is not yet a core item in the 2016 criteria. Biopsy is particularly useful in seronegative disease, but interpretation requires adequate tissue and expert pathology.

Systemic activity is quantified using the EULAR Sjögren’s Syndrome Disease Activity Index (ESSDAI), covering domains such as constitutional, glandular, articular, pulmonary, renal, peripheral nervous system, haematological and biological disease. Patient symptoms are captured by ESSPRI, a 0–10 scale averaging dryness, fatigue and pain; a change of about ≥1 point or ≥15% is often considered clinically meaningful. In trials, systemic response has also used composite indices, but no single biologic has become standard for routine primary Sjögren’s disease modification.

Management

Management is stratified into symptomatic glandular treatment, prevention of complications and systemic immunomodulation. General measures include smoking cessation, meticulous dental surveillance, fluoride toothpaste or varnish, avoidance of anticholinergic drugs, humidification and management of candidiasis.

Problem	Treatment	Key dose or caution
Dry eyes	Preservative-free artificial tears; ocular lubricants; topical ciclosporin or lifitegrast in inflammatory keratoconjunctivitis; punctal plugs in refractory cases	Ciclosporin ophthalmic 0.05% commonly twice daily; ophthalmology input for severe staining or corneal disease.
Dry mouth	Water sips, sugar-free gum, saliva substitutes; muscarinic agonists	Pilocarpine 5 mg orally three to four times daily; cevimeline 30 mg orally three times daily where available. Avoid/caution in uncontrolled asthma, significant cardiac conduction disease and narrow-angle glaucoma.
Arthralgia, fatigue, mild inflammatory arthritis	Hydroxychloroquine may be tried, though benefit for dryness/fatigue is limited	Hydroxychloroquine ≤5 mg/kg/day actual body weight; retinal screening per ophthalmology guidance.
Organ-threatening disease	Glucocorticoids plus steroid-sparing immunosuppression tailored to organ involvement	Options include methotrexate, azathioprine, mycophenolate mofetil, cyclophosphamide or rituximab, depending on phenotype.

Hydroxychloroquine was evaluated in the randomised JOQUER trial and did not significantly improve dryness, pain or fatigue at 24 weeks compared with placebo, explaining why it should not be presented as a universal sicca therapy. Rituximab has biological plausibility and may be considered for selected severe systemic manifestations, particularly cryoglobulinaemic vasculitis or lymphoma-associated features, but randomised trials have not shown consistent benefit for uncomplicated sicca symptoms.

Lymphoma and pregnancy

The lifetime risk of non-Hodgkin lymphoma, usually marginal zone/MALT lymphoma, is approximately 5–10%, markedly higher than the general population. Red flags include persistent unilateral parotid enlargement, lymphadenopathy, splenomegaly, palpable purpura, low C4, cryoglobulinaemia, monoclonal gammopathy and unexplained constitutional symptoms. Anti-Ro/SSA and anti-La/SSB positivity in pregnancy confers risk of neonatal lupus and congenital heart block; congenital heart block occurs in roughly 1–2% of first affected antibody-positive pregnancies, rising substantially after a previously affected child. Hydroxychloroquine is commonly continued in pregnancy and may reduce recurrence risk in high-risk anti-Ro pregnancies.

Systemic Sclerosis

Systemic sclerosis is a multisystem autoimmune fibrosing vasculopathy characterised by endothelial injury, obliterative microangiopathy and immune-driven fibroblast activation. Pathogenesis involves early vascular apoptosis with reduced nitric oxide/prostacyclin and increased endothelin-1, followed by innate/adaptive immune activation and profibrotic signalling through TGF-β, IL-6, platelet-derived growth factor and connective-tissue growth factor. The result is excessive extracellular matrix deposition in skin, lung, gastrointestinal tract, heart and kidney. Mortality is driven principally by interstitial lung disease, pulmonary arterial hypertension and cardiac involvement.

Classification, antibodies and prognostic phenotypes

Phenotype/antibody	Typical associations	Exam-relevant prognostic implication
Limited cutaneous SSc	Skin distal to elbows/knees, face; long Raynaud prodrome; CREST features	Late pulmonary arterial hypertension; anti-centromere common
Diffuse cutaneous SSc	Truncal/proximal limb skin disease; early internal organ involvement	Highest risk in first 3–5 years for ILD, renal crisis, myocardial disease
Anti-centromere	Limited disease, digital ischaemia, telangiectasia	Increased PAH risk; relatively lower ILD risk
Anti-topoisomerase I / Scl-70	Diffuse or limited disease with ILD	Strong predictor of progressive fibrotic ILD
Anti-RNA polymerase III	Rapid diffuse skin thickening, tendon friction rubs	Renal crisis risk; malignancy temporal association around disease onset

The 2013 ACR/EULAR classification criteria assign points for skin thickening, fingertip lesions, telangiectasia, abnormal nailfold capillaries, PAH/ILD, Raynaud phenomenon and SSc-specific antibodies; a total score ≥9 classifies SSc. Skin thickening of both hands extending proximal to MCP joints alone scores 9. The modified Rodnan skin score grades 17 sites from 0–3, total 0–51; rapid early rise predicts visceral involvement, whereas later fall may reflect either treatment response or natural disease evolution.

Clinical assessment and organ screening

Raynaud phenomenon is usually the earliest manifestation and may progress to digital pitting, ulcers, calcinosis and critical ischaemia. Nailfold videocapillaroscopy shows giant capillaries, haemorrhages and capillary dropout, helping distinguish primary Raynaud from SSc-spectrum disease. Gastrointestinal disease includes reflux from oesophageal dysmotility, strictures, gastric antral vascular ectasia, small intestinal bacterial overgrowth and pseudo-obstruction. Cardiac disease includes myocardial fibrosis, arrhythmia, conduction disease and pericardial disease; troponin/BNP elevation or unexplained dyspnoea should prompt echocardiography and cardiac MRI consideration.

All patients require baseline and serial respiratory assessment with HRCT and pulmonary function tests. A restrictive pattern, falling FVC or reduced transfer factor supports ILD; disproportionate DLCO reduction suggests PAH. Annual PAH screening is standard in SSc, using echocardiography, NT-proBNP and algorithms such as DETECT in higher-risk patients. Definitive PAH diagnosis requires right-heart catheterisation: mean pulmonary arterial pressure >20 mmHg, pulmonary arterial wedge pressure ≤15 mmHg and pulmonary vascular resistance >2 Wood units.

Major complications and management

Problem	Treatment principles and key doses
Raynaud/digital ulcers	Smoking cessation, warmth, avoid vasoconstrictors. Nifedipine MR 30–60 mg daily first line; PDE-5 inhibitors e.g. sildenafil 20 mg three times daily; IV iloprost 0.5–2 ng/kg/min for severe ischaemia. Bosentan 62.5 mg twice daily for 4 weeks then 125 mg twice daily reduces new digital ulcers but does not heal existing ulcers.
SSc-ILD	Mycophenolate mofetil 1–1.5 g twice daily is commonly first-line. Cyclophosphamide may be used for severe progressive disease but toxicity limits duration. Nintedanib 150 mg twice daily slows FVC decline, especially in progressive fibrosing ILD. Tocilizumab may preserve FVC in early inflammatory diffuse disease.
PAH	Manage in specialist centres with endothelin-receptor antagonists, PDE-5 inhibitors, soluble guanylate cyclase stimulators or prostacyclin analogues according to risk. Anticoagulation is not routine in SSc-PAH because bleeding and GI telangiectasia risk may outweigh benefit.
Renal crisis	Acute severe hypertension, rapidly rising creatinine, microangiopathic haemolysis. Treat immediately with ACE inhibition, classically captopril titrated aggressively. Continue ACE inhibitor even if dialysis is required, as renal recovery can occur months later. Avoid high-dose glucocorticoids; prednisolone >15 mg/day increases risk.
GI disease	High-dose proton-pump inhibitor therapy, prokinetics when appropriate, antibiotics for bacterial overgrowth, nutritional support; avoid blind immunosuppression for mechanical dysmotility.

Evidence to know: Scleroderma Lung Study I showed oral cyclophosphamide produced a modest FVC benefit that waned after cessation; SLS II found 24 months of mycophenolate comparable to cyclophosphamide with better tolerability. SENSCIS showed nintedanib reduced annual FVC decline by approximately 41 mL/year versus placebo in SSc-ILD. RAPIDS trials showed bosentan reduced formation of new digital ulcers, particularly in patients with multiple baseline ulcers. Autologous haematopoietic stem-cell transplantation improves event-free survival in selected severe early diffuse disease in ASTIS/SCOT-type populations, but carries early treatment-related mortality and requires expert selection.

For MRCP, high-yield discriminators are: anti-centromere with limited disease and late PAH; anti-Scl-70 with ILD; anti-RNA polymerase III with renal crisis; ACE inhibitors treat but do not prophylactically prevent renal crisis; corticosteroids are hazardous; and isolated PAH requires right-heart catheter confirmation rather than echocardiographic diagnosis alone.