IgG4-Related Disease — Written Clinical Reasoning

SAQ 1 — Integrated Diagnostic Approach to the Patient with Multi-Organ IgG4-Related Disease (20 marks, 30 minutes)

Prompt: Outline your integrated diagnostic approach to Mr Chen's presentation, addressing: (a) the unifying diagnosis and the clinicopathological reasoning; (b) the role and limits of the serum IgG4; (c) the exclusion of pancreatobiliary malignancy and Sjogren syndrome; (d) the histopathological confirmation and the 2019 ACR and EULAR classification criteria; (e) the immediate and definitive management; and (f) the common exam trap [1] [3].

Model Answer

(a) The unifying diagnosis and the clinicopathological reasoning (3 marks): [1]

Mr Chen has IgG4-related disease with multi-organ involvement — the pancreatobiliary presentation (painless jaundice, a diffusely enlarged capsule-rimmed pancreas and a common bile duct stricture) is type 1 autoimmune pancreatitis with IgG4-related sclerosing cholangitis, the submandibular gland enlargement is Mikulicz syndrome, and the renal cortical lesions with the periaortic rind represent IgG4-related kidney disease with early retroperitoneal fibrosis [1]. The "one disease, many organs" concept is the organising principle: the simultaneous involvement of the pancreas, biliary tree, salivary glands and kidneys in an older man with atopy is pathognomonic in pattern, and the markedly elevated serum IgG4 and the characteristic imaging consolidate it. The flank discomfort and the creatinine rise from bilateral ureteric obstruction by the periaortic rind add an urgent element.

(b) The role and limits of the serum IgG4 (3 marks): [1]

The serum IgG4 of 3.2 g per litre (more than twice the upper limit of 1.35) is strongly supportive, because a markedly elevated IgG4 with a normal CA19-9 favours IgG4-related disease over malignancy [1]. Two limits must be stated. First, IgG4 is supportive, not diagnostic — it is falsely elevated in 10 to 15 per cent of pancreatobiliary cancers and cholangiocarcinoma, in primary sclerosing cholangitis, and in asthma and atopy (relevant here). Second, a normal IgG4 does not exclude the disease, because 20 to 30 per cent of biopsy-proven type 1 autoimmune pancreatitis are seronegative [1]. The diagnosis therefore rests on the clinicopathological pattern with histopathology as the gold standard, not on the serology.

(c) The exclusion of pancreatobiliary malignancy and Sjogren syndrome (4 marks): [1] [3]

The critical exclusion is pancreatic adenocarcinoma and cholangiocarcinoma, both of which cause a pancreatic head mass with painless jaundice and can elevate IgG4 mildly [1]. The features favouring IgG4-related disease over cancer are the diffusely enlarged sausage-shaped pancreas rather than a focal mass, the absence of vascular encasement, the normal CA19-9, the multi-organ involvement and the marked IgG4 elevation. Despite this, adequate tissue is required before immunosuppression — endoscopic ultrasound-guided core biopsy of the pancreas, with sufficient tissue for histology, immunostaining and, where needed, flow cytometry. A diagnostic steroid trial is unsafe when malignancy has not been excluded, because lymphoma and even cancer can transiently shrink on steroids [1][3].

The salivary gland enlargement is distinguished from Sjogren syndrome by the male sex, the older onset, the prominent gland enlargement rather than sicca, the negative anti-Ro and anti-La, and the elevated IgG4 [1]. Sjogren is female-predominant, sicca-dominant and seropositive. Lymphoma (marginal zone or MALT) must be excluded with flow cytometry on the biopsy when a gland is asymmetric or rapidly growing.

(d) The histopathological confirmation and the 2019 ACR and EULAR classification criteria (4 marks): [2]

The histopathology is the gold standard. The diagnostic triad is a dense lymphoplasmacytic infiltrate rich in IgG4-positive plasma cells, storiform (cartwheel-like) fibrosis, and obliterative phlebitis (veins occluded by the infiltrate while arteries are spared), with an IgG4 to total IgG plasma cell ratio above 40 per cent [1][2]. The IgG4 cell count threshold is organ-specific (for example, above 30 per high-power field for pancreas), and the count is not diagnostic in isolation — it must be interpreted with the triad and the clinical context.

The 2019 ACR and EULAR classification criteria formalise the diagnosis as a three-step process: an entry criterion (characteristic organ involvement), the application of 32 exclusion criteria (any one excludes the diagnosis), and a weighted inclusion score across eight domains, with a total of 20 points or more classifying the case as IgG4-related disease [2]. The pathological domain carries the heaviest weight. Mr Chen meets the entry criterion, has no exclusion criterion (normal CA19-9, no cachexia, no granulomas), and would score well above 20 on the multi-organ pattern plus the marked IgG4 elevation plus the diagnostic histology.

(e) The immediate and definitive management (4 marks): [3] [4]

The immediate priority is the renal impairment from bilateral ureteric obstruction — decompress with ureteric stents or nephrostomy first, because renal recovery depends on the duration of obstruction [3]. In parallel, address the obstructive jaundice with endoscopic biliary drainage if the stricture is causing cholestasis or cholangitis risk. Once the obstruction is relieved and the diagnosis is histologically secure, commence prednisolone 30 to 40 mg daily for two to four weeks for the active inflammatory disease, then taper over three to six months [3]. Give bone protection (calcium, vitamin D, a bisphosphonate where indicated) and consider pneumocystis prophylaxis. Plan a maintenance strategy from the outset — for multi-organ disease, this is usually a planned course of rituximab (375 mg per square metre weekly for four doses, or 1 g on days 1 and 15), having first screened for hepatitis B and tuberculosis, because relapse occurs in 40 to 50 per cent and most rituximab responders relapse within a year [3][4].

(f) The common exam trap (2 marks): [1] [3]

The trap is to treat elevated serum IgG4 as diagnostic and to commit to a diagnostic steroid trial in a pancreatic mass without adequate tissue [1][3]. A radiological response to steroids can occur with lymphoma and even transiently with cancer, so tissue is mandatory before immunosuppression when malignancy is plausible. The corollary trap is to assume a normal IgG4 excludes the disease — 20 to 30 per cent of biopsy-proven type 1 autoimmune pancreatitis are seronegative, and the diagnosis is clinicopathological [1].

SAQ 2 — Interpretation of a Relapsing Course and the Role of Rituximab (20 marks, 30 minutes)

Prompt: Eight weeks after starting prednisolone 35 mg daily, Mr Chen's jaundice and gland enlargement resolved and his serum IgG4 normalised. As the prednisolone was tapered below 10 mg daily, his jaundice, submandibular gland enlargement and renal cortical lesions recurred, with the serum IgG4 rising to 2.9 g per litre. Outline: (a) the interpretation of this course; (b) the most appropriate next therapy and its mechanism; (c) the pre-treatment safety checks; (d) the monitoring plan; and (e) the communication with the patient [3] [4].

Model Answer

(a) The interpretation of this course (3 marks): [3]

Mr Chen has a steroid-dependent relapse during the taper, which is the typical natural history of multi-organ IgG4-related disease — roughly 40 to 50 per cent of patients relapse during or after the steroid taper [3]. The recurrence of the original lesions with the rising IgG4 confirms active inflammatory disease rather than a burnt-out fibrotic phase, so immunosuppression remains appropriate. A single relapse during a taper is an indication to escalate to a steroid-sparing maintenance strategy rather than to chase the disease with repeated steroid courses.

(b) The most appropriate next therapy and its mechanism (5 marks): [1] [4]

The treatment of choice is rituximab (anti-CD20 B-cell depletion) — 375 mg per square metre weekly for four doses, or 1 g on days 1 and 15 — which is highly effective for relapsing, multi-organ and steroid-dependent IgG4-related disease [4]. The mechanism is elegant: rituximab depletes the B-cell compartment that provides antigen presentation and co-stimulation to the effector cytotoxic T cells (SLAMF7-positive CD4-positive and CD8-positive cells) that drive both the inflammation and the fibrosis [1]. As the T-cell-driven inflammation subsides, the IgG4-positive plasma cells (which are CD20-negative and not directly depleted) decline in parallel, and the serum IgG4 falls. This T-cell-centric model explains why B-cell depletion works without IgG4 being directly pathogenic. Rituximab is increasingly used as a planned maintenance strategy rather than only as rescue therapy, because most responders relapse within a year [4].

(c) The pre-treatment safety checks (4 marks): [3] [4]

Before the first dose, screen for hepatitis B (HBsAg, anti-HBc and HBV DNA) and tuberculosis, because rituximab reactivates hepatitis B and can cause fatal fulminant hepatitis — give antiviral prophylaxis (entecavir or tenofovir) where indicated [3]. Check baseline immunoglobulins and the full blood count, because repeat courses can cause hypogammaglobulinaemia and late-onset neutropenia. Update vaccinations (influenza, pneumococcal, recombinant zoster, hepatitis B if non-immune) before the first dose, because live vaccines are avoided during and after B-cell depletion and the vaccine response is blunted once B cells are depleted [3]. Screen for latent tuberculosis with an IGRA where risk factors exist. Premedicate with paracetamol, an antihistamine and an intravenous steroid to reduce infusion reactions [4].

(d) The monitoring plan (4 marks): [3] [4]

Monitor for infusion reactions during the infusion. After treatment, track the clinical response (resolution of jaundice and gland enlargement, improvement in renal function and imaging), the serum IgG4 trend (as a marker of activity, not a diagnostic test), the peripheral B-cell count (CD19-positive B cells fall to zero within weeks; repopulation often heralds relapse), and the immunoglobulins for hypogammaglobulinaemia [4]. Reassess the renal and hepatic function and repeat the cross-sectional imaging at three and six months. Plan the next rituximab course or a maintenance schedule (for example, 1 g every four to six months) based on the disease activity and the B-cell reconstitution, and provide bone protection if any steroid exposure continues [3].

(e) The communication with the patient (4 marks): [3]

Frame the relapse as the expected natural history of a treatable but relapsing multisystem disease, and explain that rituximab is the steroid-sparing strategy that controls the disease without the cumulative burden of repeated steroids [3]. Discuss the infusion-reaction risk (usually mild and confined to the first dose), the infection risk (including the need for vaccination before treatment and the avoidance of live vaccines afterwards), the hepatitis B safety check, and the expectation of a planned maintenance course rather than a one-off cure. Set up a structured surveillance plan and name the lead clinician coordinating rheumatology, gastroenterology, nephrology and ophthalmology, so the patient knows whom to contact. Address the practical and psychological impact of a chronic relapsing illness on retirement, family and travel [3].