What are the clinical manifestations, recommended evaluation, characteristic immunohistochemical markers, and key differential diagnoses of Erdheim‑Chester disease?

Erdheim-Chester Disease: Clinical Manifestations, Evaluation, and Diagnosis

Clinical Manifestations

Erdheim-Chester disease (ECD) is a rare multi-organ histiocytic neoplasm that most commonly presents with bone pain (50% of cases) and demonstrates pathognomonic bilateral symmetric osteosclerosis of the long bone metadiaphyses, particularly around the knees 1, 2.

Key Signs and Symptoms by Organ System:

Skeletal (95%):

• Lower extremity bone pain as the most common presenting symptom
• Bilateral symmetric involvement of femur, tibia, and fibula metadiaphyses
• Less commonly: skull, axial skeleton, and small bones of feet 1

Cardiovascular (37-62%):

• Right atrial pseudotumor
• Periaortic "coated aorta" appearance (62%)
• Pericardial involvement (8%)
• Atrioventricular infiltration 1, 2

Renal/Retroperitoneal (65%):

• Perinephric infiltrates creating "hairy kidney" sign
• Potential ureteral obstruction causing renal insufficiency 1, 3

Central Nervous System (38-40%):

• Cognitive impairment, ataxia, peripheral neuropathy (40-50%)
• Headaches (20%)
• Posterior fossa predilection with white matter T2 hyperintensities
• Dural, pituitary stalk, or orbital masses 1, 2

Endocrine (47-60%):

• Diabetes insipidus (47%)
• Hypogonadism (60%)
• Requires annual endocrine evaluation even during treatment 1

Pulmonary (52%):

• Pleural-based nodules or interstitial fibrosis
• Pleural thickening/effusion
• Widened lobular septa and bronchial vascular bundle thickening 1, 4

Dermatologic (25-33%):

• Xanthelasma (33%)
• Papular rash or subcutaneous nodules 1, 2

Orbital (27%):

• Retro-orbital infiltration with exophthalmos 1

Recommended Evaluation

For all newly diagnosed patients, perform comprehensive baseline imaging regardless of symptoms 1:

Mandatory Imaging Studies:

1. FDG PET-CT (skull-to-toes including brain and distal extremities)

   • Most sensitive for detecting bone and soft tissue involvement
   • Guides biopsy site selection (target most FDG-avid accessible lesions)
   • Superior to 99mTc bone scintigraphy 1, 2

2. Brain MRI with gadolinium

   • Essential for detecting CNS involvement even in asymptomatic patients
   • Identifies white matter changes, dural masses, pituitary involvement 1, 2

3. Cardiac MRI

   • Detects right atrial pseudotumor and myocardial infiltration 1

4. Dedicated chest/abdomen/pelvis CT

   • Demonstrates pulmonary, periaortic, and perinephric infiltrates
   • May be unnecessary if performed with PET-CT 1

Laboratory Evaluation:

• Complete blood count
• Renal and hepatic function tests
• Endocrine assessment (pituitary function, gonadal hormones)
• Inflammatory markers
• Immunological assessment 1

Tissue Biopsy and Histopathologic Assessment

Tissue biopsy is mandatory in all cases for diagnosis confirmation and mutation identification 1.

Biopsy Strategy:

• Target the most FDG-avid accessible site (preferably skin, bone, or perinephric soft tissue)
• Obtain multiple core biopsies due to low tumor cellularity and lesion heterogeneity
• Percutaneous CT-guided approach typically used 1

Histopathologic Features:

Classic findings include:

• Foamy histiocytes with small nuclei
• Surrounding fibrosis
• Multinucleated giant cells and Touton giant cells
• Reactive lymphocytes and plasma cells 1, 5, 6

Spectrum of appearances:

• Xanthogranulomatous infiltrates
• Lymphohistiocytic infiltrates
• Fibrohistiocytic patterns with scattered foamy histiocytes
• May have florid lymphohistiocytic infiltrates or fibrotic lamellae with rare Touton cells 1, 7

Immunohistochemical Markers

The characteristic IHC profile that distinguishes ECD from other histiocytic disorders 1, 2, 7:

Positive Markers:

• CD68: Positive (macrophage marker)
• CD163: Positive (macrophage marker)
• Factor XIIIa: Positive
• Fascin: Positive

Negative Markers:

• CD1a: Negative (distinguishes from Langerhans cell histiocytosis)
• CD207 (Langerin): Negative (distinguishes from Langerhans cell histiocytosis)

Variable Markers:

• S100: Negative to focally/weakly positive (20-30% of cases) - This is a critical distinction, as classic teaching suggested ECD was S100-negative, but focal positivity is now recognized 1, 2, 7

Molecular Markers:

• BRAF V600E mutation: Present in approximately 50-51% of cases 1, 8
• Other MAPK pathway mutations (ARAF, KRAS, MAP2K1) in BRAF-negative cases 1, 9
• Cyclin D1 IHC may aid diagnosis 7

Important caveat: BRAF V600E immunohistochemistry may have insufficient sensitivity; molecular testing methods (PCR, next-generation sequencing) are recommended to definitively exclude mutations 2, 7.

Key Differential Diagnoses

Primary Differentials:

1. Langerhans Cell Histiocytosis (LCH):

• CD1a: Positive (vs. negative in ECD)
• Langerin: Positive (vs. negative in ECD)
• S100: Strongly positive (vs. negative/focal in ECD)
• Bone lesions are typically lytic/punched-out (vs. sclerotic in ECD)
• Axial skeleton more commonly involved
• May coexist with ECD in 10-20% of cases (mixed/overlap histiocytosis) 1, 2, 5

2. Rosai-Dorfman Disease (RDD):

• S100: Strongly positive (vs. negative/focal in ECD)
• CD1a: Negative (similar to ECD)
• Langerin: Negative (similar to ECD)
• Presents with isolated or generalized lymphadenopathy (30-50%)
• Emperipolesis (lymphocytes within histiocytes) on histology
• Rarely involves bones as primary site
• May coexist with ECD 2, 3

3. Reactive Inflammatory Processes:

• Lack characteristic bilateral symmetric long bone osteosclerosis
• Absence of "coated aorta" or "hairy kidney" signs
• No MAPK pathway mutations
• Clinical correlation essential 7

4. Xanthomatous Conditions:

• Lack systemic involvement pattern
• Different IHC profile
• No MAPK mutations 10

5. Sarcoidosis:

• Non-caseating granulomas (vs. foamy histiocytes)
• Different organ distribution
• Negative for CD163 typically 1

6. Metastatic Disease:

• Imaging shows asymmetric involvement
• Histology shows malignant cytologic features
• Different IHC profile based on primary tumor 4

Critical Diagnostic Algorithm

When ECD is suspected:

1. Obtain FDG PET-CT (skull-to-toes), brain MRI with gadolinium, and cardiac MRI
2. Identify bilateral symmetric long bone osteosclerosis - if present, highly suggestive
3. Look for characteristic signs: "coated aorta," "hairy kidney," right atrial mass
4. Perform biopsy of most FDG-avid accessible site with multiple cores
5. Confirm histology: foamy histiocytes, Touton giant cells, fibrosis
6. Verify IHC profile: CD68+/CD163+/Factor XIIIa+/CD1a-/Langerin-/S100 negative or focal
7. Obtain molecular testing for BRAF V600E and other MAPK pathway mutations (not just IHC)
8. Correlate all findings - diagnosis requires integration of clinical, radiologic, histologic, and molecular data 1, 7

Common Pitfalls to Avoid

• Do not rely on S100 negativity alone - 20-30% of ECD cases show focal S100 positivity 1, 7
• Do not accept BRAF V600E IHC alone - molecular testing is more sensitive and required 2, 7
• Do not miss overlap syndromes - ECD can coexist with LCH or RDD; examine tissue carefully for mixed populations 2, 5
• Do not skip brain/cardiac imaging - CNS and cardiac involvement significantly impacts prognosis and may be asymptomatic initially 1, 2
• Do not obtain inadequate tissue - multiple core biopsies needed due to heterogeneity and low cellularity 1
• Do not delay endocrine evaluation - diabetes insipidus and hypogonadism are common and require ongoing monitoring 1

The diagnosis of ECD requires high clinical suspicion given its rarity, with mean diagnostic delay of 4.2 years from symptom onset 8. The combination of bilateral symmetric long bone osteosclerosis on imaging, characteristic organ involvement pattern, appropriate IHC profile (CD68+/CD163+/CD1a-/Langerin-), and MAPK pathway mutations establishes the diagnosis 1, 7.