Clinical Analysis and Most Likely Diagnosis

1. Adult-Onset Still's Disease (AOSD) - possibly complicated by
2. Hemophagocytic Lymphohistiocytosis (HLH) / Macrophage Activation Syndrome (MAS)

Feature-by-Feature Breakdown

Most Likely Unifying Diagnosis

Adult-Onset Still's Disease (AOSD) with Macrophage Activation Syndrome (MAS/HLH)

• AOSD is a systemic autoinflammatory disease characterized by quotidian fever, rash, arthritis, pharyngitis (tonsillitis), hepatosplenomegaly, and extreme hyperferritinemia.
• MAS/HLH complicating AOSD is a known, potentially fatal complication driven by uncontrolled macrophage and T-cell activation - a cytokine storm - with IL-1β, IL-6, IL-18, IFN-γ, and TNF-α as key mediators.
• Ferritin >10,000 µg/L is the hallmark and may reflect both direct liver cell damage releasing ferritin AND massive cytokine-driven ferritin production (Harrison's Principles, 22E, p. 508-509).

HLH-2004 Diagnostic Criteria to Check (5 of 8 needed)

• Fever (likely present, not explicitly stated)
• Splenomegaly (likely given abdominal distension)
• Bicytopenia - check CBC urgently (Hb <90 g/L, platelets <100×10⁹/L, neutrophils <1.0×10⁹/L)
• Hypertriglyceridemia / hypofibrinogenemia - check fasting TG and fibrinogen
• Hemophagocytosis - bone marrow biopsy (not done yet)
• Ferritin ≥500 µg/L - STRONGLY positive at 11,000
• Low/absent NK cell activity - send sCD25, NK function assay
• Soluble IL-2 receptor (sCD25) ≥2400 U/mL - send urgently

Differential Diagnoses to Consider

Urgent Workup Recommended

1. CBC with differential - look for bicytopenia
2. Fasting triglycerides + fibrinogen - HLH criteria
3. sCD25 (soluble IL-2R) - most specific HLH marker
4. NK cell function assay
5. Bone marrow biopsy - hemophagocytosis
6. LDH, uric acid - markers of cell turnover
7. Viral serology panel - HHV-6, parvovirus B19, adenovirus, EBV repeat (early)
8. CT chest/abdomen/pelvis - splenomegaly, lymphadenopathy, organ infiltration
9. HScore calculator - bedside scoring tool (publicly available at mdcalc.com) to estimate HLH probability
10. CRP, ESR, complement levels, ANA, anti-dsDNA - rule out lupus-triggered MAS
11. CK level - quantify myopathy severity
12. Genetic testing - PRF1, UNC13D, STX11, STXBP2 if primary HLH suspected

Management Implications

• High-dose corticosteroids (dexamethasone if HLH, methylprednisolone for AOSD/MAS)
• IL-1 inhibitor (anakinra - first-line biologic in AOSD, works rapidly)
• IL-6 inhibitor (tocilizumab - alternative)
• Etoposide if refractory HLH per HLH-94/2004 protocol
• IVIG in some cases
• Cyclosporine (calcineurin inhibitor to reduce cytokine production)
• Treat the underlying trigger - if infectious, address it

This is a medical emergency. Untreated HLH/MAS carries mortality rates >50%. Early recognition and treatment are life-saving.

Revised Differential - With HLH and Still's Ruled Out

Top Diagnosis: Dermatomyositis (DM) - Severe / Anti-NXP2 or Anti-MDA5 Subtype

Why Anti-NXP2 or Anti-MDA5 Subtype is Most Relevant

• Anti-NXP2 DM: Strongly associated with severe muscle involvement, profound weakness, GI vasculopathy (perforation, ileus, abdominal pain), calcinosis. GI involvement in NXP2+ patients carries high mortality.
• Anti-MDA5 DM: Associated with markedly elevated ferritin (>10,000 is described), skin ulceration, angioedema-like edema, rapidly progressive disease, and ILD. Often has little muscle inflammation on biopsy ("amyopathic" pattern) but can have profound weakness.
• Anti-SAE DM: Dysphagia predominant, systemic inflammatory features.

Second Consideration: Immune-Mediated Necrotizing Myopathy (IMNM)

• Anti-SRP or anti-HMG-CoA reductase antibodies
• Very high CK, rapidly progressive profound proximal weakness (power 1-2 is consistent)
• Less skin involvement, less GI involvement compared to DM
• Would not explain periorbital edema as well
• Still possible as a co-existing or overlap entity

Third Consideration: Overlap Connective Tissue Disease / Antisynthetase Syndrome

• Anti-Jo-1, PL-7, PL-12 antibodies
• Myopathy + lung + joint + mechanic's hands + Raynaud's
• Ferritin can be high but typically not 11,000
• Less likely to explain the full picture including periorbital edema and severe GI involvement

Urgent Workup to Confirm

1. Myositis-specific antibody (MSA) panel - anti-MDA5, anti-NXP2, anti-TIF1-γ, anti-Mi-2, anti-SAE, anti-SRP, anti-Jo-1, anti-HMGCR - this is the key test
2. Muscle biopsy - perifascicular atrophy confirms DM; necrosis without inflammation confirms IMNM
3. MRI muscles - edema pattern in proximal muscles
4. CT chest/abdomen/pelvis - look for ILD (especially in anti-MDA5), GI vasculopathy, perforation, lymphadenopathy, ascites
5. ANA, anti-dsDNA, ANCA, complement - rule out SLE/vasculitis overlap
6. EMG - myopathic pattern
7. Skin biopsy if rash present - interface dermatitis is typical
8. Echocardiogram - DM can cause myocarditis

Treatment if DM Confirmed

• High-dose IV methylprednisolone (1 g/day x 3-5 days) - urgent in severe disease
• IVIG - especially effective in GI/NXP2 DM and rapidly progressive MDA5 DM
• Rituximab or mycophenolate as steroid-sparing agents
• Tacrolimus / cyclosporine in refractory cases, especially anti-MDA5 with ILD
• For GI involvement: bowel rest, NG tube, proton pump inhibitor, surgical consult if perforation risk
• Monitor for aspiration (pharyngeal involvement), cardiac involvement, ILD

Revised Analysis: Negative Myositis Panel Does NOT Rule Out Inflammatory Myopathy

Why Seronegative Does Not Rule Out Dermatomyositis / IMNM

1. Clinical features (which this patient has in abundance)
2. Muscle biopsy - the gold standard when antibodies are negative
3. EMG - myopathic pattern
4. MRI muscles - edema and inflammation in proximal groups

Revised Working Diagnosis Framework

1. Seronegative Inflammatory Myopathy (DM or IMNM) - Still #1

• Negative antibodies occur in ~20-40% of DM and in a defined subset of IMNM
• The full clinical picture (periorbital edema/heliotrope, proximal weakness grade 1-2, elevated CK, liver enzymes from muscle leak, hypoalbuminemia, GI vasculopathy, hyperferritinemia) remains most consistent with this
• Muscle biopsy + EMG + MRI muscles are now mandatory
• Also check: anti-HMGCR specifically (often missed in standard panels)

2. Paraneoplastic Syndrome with Myopathy

• DM in adults over 40 has a ~30% association with occult malignancy (especially if seronegative or anti-TIF1-γ positive)
• Ferritin 11,000 in a malignancy context (lymphoma, GI cancer, lung, ovarian) can indicate tumor burden or tumor-driven macrophage activation
• Abdominal distension could be from hepatomegaly, ascites, or bowel wall involvement by tumor
• Tonsillitis-like presentation can be lymphoma in the pharyngeal/tonsillar region
• CT chest/abdomen/pelvis with contrast + PET scan are urgent

3. Viral Myositis with Systemic Inflammatory Response (Non-EBV/CMV)

• HHV-6, parvovirus B19, adenovirus, HTLV-1, HIV, hepatitis C - all can cause severe myopathy with high ferritin, liver involvement, periorbital edema, and systemic inflammation
• EBV and CMV negative does NOT rule out viral triggers
• Send full viral panel including HHV-6 IgM/IgG, parvovirus B19, HTLV-1, HIV, hepatitis B and C if not done

4. Drug-Induced Myopathy / IMNM

• Statins, fibrates, hydroxychloroquine, colchicine, checkpoint inhibitors
• Drug history is essential - was the patient on any of these?
• Immune checkpoint inhibitor (ICI)-associated myopathy is increasingly recognized, has high mortality (42%), can be seronegative, often has ocular involvement and lymphopenia

5. Mitochondrial Myopathy (Late-Presenting)

• Can present in middle age, proximal > distal weakness, elevated CK and liver enzymes (from mitochondrial dysfunction in hepatocytes), lactic acidosis, abdominal pain
• Ferritin can be markedly elevated
• Often seronegative
• Check: serum lactate, pyruvate, lactate:pyruvate ratio, urine organic acids, muscle biopsy with electron microscopy and NADH/COX staining

What Still Needs To Be Done (Priority Order)

Bottom Line

The muscle biopsy is now the most important investigation remaining.

Muscle Biopsy Site Selection When No Specific Muscle Is Pointed

The Core Principle

• Too severely weak (power 0-1, wasted muscle) - biopsy shows only fibrosis and fat replacement, no diagnostic tissue left
• Too mildly affected - biopsy appears normal, misses the pathology

Step 1: Do MRI of Muscles FIRST (Best Practice)

• STIR/T2 hyperintensity = active edema/inflammation = the RIGHT place to biopsy
• Avoids sampling end-stage fibrotic muscle
• Dramatically increases diagnostic yield
• In DM/IMNM, the quadriceps (especially vastus lateralis and rectus femoris) and anterior thigh compartment show the most reliable STIR signal

"Thigh muscles that are affected on MRI in DM/PM patients show a significantly higher number of inflammatory cells compared to non-affected muscles - indicating increased histological yield after MRI-guided biopsy." (Tomasova Studynkova et al.)

Step 2: Default Biopsy Sites (If MRI Not Available)

Step 3: Critical Rules to Follow

1. Avoid the side where EMG was done - EMG needle causes local inflammatory artefact that can mimic or mask disease on biopsy. If EMG was done in the right thigh, biopsy the left.
2. Do not biopsy a muscle that has been injected (e.g. IM injection site) - causes focal necrosis artefact.
3. Take biopsy from a moderately weak muscle - in this patient where lower limbs are power 1-2 (severely weak), consider the upper limb (deltoid) which may be less damaged and show more interpretable pathology.
4. Open biopsy > needle biopsy when diagnosis is uncertain - gives larger sample, better architecture, allows multiple staining including electron microscopy.
5. Fresh tissue handling - biopsy must go fresh (not in formalin) to the lab for frozen sections, enzyme histochemistry, immunofluorescence, and electron microscopy. Formalin fixation destroys enzyme stains.

Practical Recommendation for This Patient

MRI thighs (T2 STIR) → biopsy the muscle with highest STIR signal in the quadriceps group → if MRI not available → open biopsy of vastus lateralis (contralateral to any EMG done), or deltoid if upper limb is less severely weak

• H&E, Gomori trichrome
• ATPase (fiber typing)
• NADH, COX, SDH (mitochondrial stains)
• Immunofluorescence (C5b-9, MHC-I, CD4, CD8, CD68)
• Electron microscopy (if mitochondrial myopathy in differential)
• MxA staining if DM suspected (highly specific for DM even when seronegative)