Hypertrophic Pachymeningitis (HP)

Definition

• Bradley and Daroff's Neurology in Clinical Practice, p. 2586
• Adams and Victor's Principles of Neurology, 12th Ed., p. 648

1. Etiology

Immune-Mediated / Autoimmune (most common non-infectious group)

Infectious

• Tuberculous meningitis
• Syphilis (historically the most common cause - pachymeningitis cervicalis hypertrophica described by Charcot and Joffroy)
• Fungal infections (Aspergillus, Cryptococcus)
• Lyme disease
• HTLV-1
• Pseudomonas (malignant external otitis)
• Cysticercosis

Neoplastic

• Dural carcinomatosis
• Lymphoma
• Meningioma (mimicker)
• Metastatic disease from adjacent skull bone

Other

• Trauma
• Intracranial hypotension (spontaneous or post-lumbar puncture) - produces venous engorgement mimicking dural enhancement

Idiopathic HP remains a diagnosis of exclusion after thorough evaluation.

2. Pathophysiology

General Mechanism

IgG4-RD Mechanism

• Activated CD4+ T follicular helper cells and plasmablasts drive the production of IgG4 antibodies
• Characteristic histology: dense lymphoplasmacytic infiltration with IgG4+ plasma cells (>10-40 per HPF), storiform fibrosis, and obliterative phlebitis
• The IgG4+/IgG+ plasma cell ratio is ≥30%
• Fibrotic phenotype leads to dural thickening; process also affects salivary glands, pancreas, kidneys, lungs

ANCA-Associated Mechanism

• Neutrophil and monocyte activation by ANCA (anti-MPO or anti-PR3 antibodies)
• Granulomatous inflammation with fibrous dural thickening
• Notably, classic GPA histological features (necrotizing granulomatous vasculitis) may not always be present in dural biopsy

Consequences of Dural Thickening

• Cranial nerve compression as nerves traverse thickened dura
• Venous sinus compression → hydrocephalus, raised intracranial pressure
• Spinal cord / nerve root compression → myelopathy, radiculopathy
• Cerebral venous thrombosis (increasingly recognized, especially IgG4-HP)

3. Clinical Features

Common Symptoms

Cranial HP Specifics

• Headache is typically persistent and pressure-like
• Orbital involvement: dysaesthesia around orbit
• Peri-orbital/cavernous sinus involvement: acute bilateral ophthalmoplegia

IgG4-HP Specific Profile

• Male predominance (~2-3:1), age 40-70 years
• 38.5% misdiagnosis rate (commonly mistaken for meningioma)
• Extraneurological manifestations in >50% of cases (pancreas, salivary glands, kidneys, lungs)

ANCA-HP Specific Profile

• ENT manifestations: otitis media, sinusitis, mastoiditis
• Sudden visual loss is a warning feature
• HP may be the initial clinical episode of AAV in 3-4% of patients

CSF Findings

• High protein (common)
• Lymphocytic pleocytosis
• Sterile (no organisms)
• Plasma cells not prominent (contrast with neurosyphilis/Listeria)

4. Differential Diagnosis

Key Differentials (Causes of Thickened Enhancing Dura on MRI)

GPA vs. IgG4-RD - A Common Diagnostic Dilemma

5. Radiological Features

MRI - the Gold Standard Investigation

MRI Characteristics

• Diffuse or localized irregular dural thickening with enhancement
• Can be smooth/thin (like intracranial hypotension) or nodular/irregular (more suggestive of inflammatory/neoplastic cause)
• "Sandwich" pattern (peripheral enhancement of thickened dura) - classic for IgG4-HP
• Tentorial, falcine, convexity, skull base involvement

• Thickened dura is often hypointense on T2 (fibrous tissue)
• IgG4-HP: may show T2 hypointensity due to dense fibrosis

• DWI - diffusion restriction in lymphoma
• MR spectroscopy - for neoplasm vs. inflammation
• Whole-body CT/PET - essential to detect extraneurological IgG4-RD or malignancy

6. Diagnostic Workup

1. MRI brain and spine with gadolinium (mandatory)
2. Blood tests: CBC, ESR, CRP, ANA, ANCA (MPO, PR3), RF, anti-CCP, ACE, serum IgG subclasses (IgG4), VDRL/TPHA, HIV, Lyme serology, fungal serology, serum calcium
3. CSF analysis: cell count, protein, glucose, culture, cytology, oligoclonal bands, ACE, VDRL
4. Chest CT / Whole body CT or PET-CT - to identify systemic disease
5. Dural biopsy - gold standard for definitive tissue diagnosis

IgG4-HP Diagnostic Criteria (histopathological gold standard):

• Dense lymphoplasmacytic infiltration
• Storiform fibrosis
• IgG4+ plasma cells >10-40 per HPF
• IgG4+/IgG+ ratio ≥30%
• Serum IgG4 significantly elevated (normal <1350 μg/mL; diagnostic cut-off varies)

7. Treatment

First-Line: Corticosteroids

• Prednisolone/Prednisone is the cornerstone of treatment for all immune-mediated HP
• Starting dose typically 0.5-1 mg/kg/day (IHP and IgG4-HP) or 1 mg/kg/day (ANCA-HP)
• Gradual taper over months; relapse is common during tapering
• Favorable response in ~96% of IgG4-HP treated cases (Liu et al. 2026)

Steroid-Sparing Immunosuppressants (for maintenance or relapse)

Rituximab (anti-CD20)

• Effective for refractory HP across all immune-mediated causes
• Particularly valuable in refractory ANCA-HP (GPA) and refractory IgG4-HP
• Increasingly used as induction therapy in severe ANCA-HP
• A 2023 case report [PMID 37292126] reported successful use of intrathecal rituximab for IgG4-RHP refractory to systemic therapy

Surgical

• Surgical decompression for mass effect or hydrocephalus
• Biopsy is both diagnostic and may relieve local compression
• Subtotal resection combined with steroids for IgG4-HP with significant mass effect (Liu et al. 2026)

Treatment by Etiology

8. Recent Updates (2023-2026)

1. ANCA-HP as a CNS-Limited AAV Subtype (2023)

2. MRI Patterns for Immune-Mediated HP (2023)

3. IgG4-HP - High Misdiagnosis Rate and Atypical Manifestations (2026)

4. Cerebral Venous Thrombosis as a Complication (2026)

5. IgG4-HP with Multi-System Involvement (2025)

6. Intrathecal Rituximab (2023)

7. Neurological Involvement Prevalence in IgG4-RD

Summary Table

• Bradley and Daroff's Neurology in Clinical Practice, 8th Ed. - pp. 2586-2590
• Adams and Victor's Principles of Neurology, 12th Ed. - pp. 648-649
• Shimojima Y & Sekijima Y. Autoimmun Rev 2023 [PMID 37062439]
• Matias TB et al. Acad Radiol 2023 [PMID 36882352]
• Liu XM et al. Diagnostics 2026 [PMID 41827958]
• Wang Y et al. BMC Neurol 2026 [PMID 41808071]
• Balaban DT et al. Front Neurol 2023 [PMID 37292126]
• Yang L et al. Pract Neurol 2025; 25:217-227

1. Etiology - IgG4-RD, ANCA/GPA, sarcoid, RA, TB, syphilis, fungi, idiopathic
2. Pathophysiology - fibro-inflammatory dural thickening, lymphoplasmacytic infiltration, storiform fibrosis, nerve/venous sinus compression
3. Clinical Features - headache (most common), cranial neuropathies, ataxia, seizures, myelopathy
4. Differential Diagnosis - meningioma, GPA vs. IgG4-RD, TB, lymphoma, intracranial hypotension
5. Radiological Features - gadolinium MRI patterns (with actual MRI images), "sandwich" sign, T2 hypointensity, whole-body CT/PET
6. Treatment - corticosteroids (first-line), azathioprine/MTX/MMF, cyclophosphamide, rituximab (refractory), surgical decompression
7. Recent Updates (2023-2026) - IgG4-HP misdiagnosis rate 38.5%, CVT as complication, intrathecal rituximab, ANCA-HP as CNS-limited AAV

HYPERTROPHIC PACHYMENINGITIS (HP) - Complete Review

DEFINITION

• Adams and Victor's Principles of Neurology, 12th Ed., p. 648

1. ETIOLOGY

A. Immune-Mediated / Autoimmune

B. Infectious

• Syphilis - historically the first described cause (Charcot and Joffroy described pachymeningitis cervicalis hypertrophica)
• Tuberculosis - chronic granulomatous dural thickening
• Fungal infections (Aspergillus, Cryptococcus)
• Lyme disease
• HTLV-1
• Pseudomonas aeruginosa (malignant external necrotizing otitis)
• Cysticercosis
• HIV-related

C. Neoplastic

• Dural carcinomatosis
• Lymphoma (primary CNS or systemic)
• Meningioma (actually a mimicker, not a true HP cause)
• Metastatic disease from adjacent skull bone

D. Other / Miscellaneous

• Trauma / post-surgical dural reaction
• Intracranial hypotension (spontaneous or post-lumbar puncture) - venous engorgement mimics dural enhancement
• Mucopolysaccharidosis (fibroblast-mediated dural thickening)

E. Idiopathic

• Diagnosis of exclusion
• Biopsy shows small mature lymphocytes, plasma cells, and epithelioid histiocytes without identifiable cause

2. PATHOPHYSIOLOGY

General Mechanism

IgG4-RD Mechanism

• CD4+ T follicular helper (Tfh) cells and regulatory T cells are pathologically activated
• Tfh cells drive plasmablast and plasma cell differentiation, producing IgG4 antibodies
• Characteristic histological triad:
  1. Dense lymphoplasmacytic infiltration with IgG4+ plasma cells (>10-40 per HPF)
  2. Storiform (cartwheel) fibrosis - swirling pattern of dense collagen deposition
  3. Obliterative phlebitis - inflammatory occlusion of small veins
• IgG4+/IgG+ plasma cell ratio ≥30%
• Activated fibroblasts and myofibroblasts perpetuate fibrosis
• The same process affects pancreas, salivary glands, kidneys, retroperitoneum, lungs

ANCA-Associated Mechanism (GPA)

• ANCAs (anti-MPO or anti-PR3) activate neutrophils and monocytes
• Neutrophil degranulation releases proteases and reactive oxygen species
• Granulomatous inflammation with giant cells forms in dura
• Importantly, classic GPA histology (necrotizing granulomatous vasculitis) is not always present in dural biopsies
• T-cell mediated fibrosis follows the acute inflammatory phase

Consequences of Dural Thickening

• Cranial nerve compression as nerves traverse thickened basal dura (most common mechanism of deficits)
• Venous sinus compression/thrombosis - raised intracranial pressure, hydrocephalus
• Cerebral venous thrombosis - increasingly recognized complication, especially in IgG4-HP
• Spinal cord compression - myelopathy
• Nerve root compression - radiculopathy, amyotrophy

3. CLINICAL FEATURES

Cranial HP

Spinal HP

• Cervical region most commonly affected (historically called pachymeningitis cervicalis hypertrophica)
• Myelopathy: progressive sensorimotor deficits below level of compression
• Radiculopathy: root pain, paresthesia, sensory loss
• Amyotrophy of upper limbs (cervical cord/root involvement)
• Paraparesis: cord compression at any level

IgG4-HP Specific Profile

• Male predominance (~2-3:1), peak age 40-70 years
• 38.5% misdiagnosis rate - most commonly mistaken for meningioma
• Extraneurological manifestations in >50%: pancreas, salivary glands, kidneys, lacrimal glands, lungs, retroperitoneum
• Subacute onset, may relapse during or after steroid taper

ANCA-HP (GPA) Specific Profile

• ENT manifestations: otitis media, sinusitis, mastoiditis - important clues
• Sudden visual loss is a warning feature
• HP may be the initial clinical episode of AAV in 3-4% of patients
• CNS-only ANCA-HP may not meet classification criteria for GPA

CSF Findings

• Elevated protein (common)
• Lymphocytic pleocytosis (mild to moderate)
• Sterile cultures
• Glucose - normal or mildly low
• Plasma cells not prominent in IgG4-HP (contrast with neurosyphilis/Listeria meningitis)
• Elevated IgG fraction may be seen in IgG4-HP

4. DIFFERENTIAL DIAGNOSIS

Full Differential of Thickened Enhancing Dura on MRI

• Idiopathic cranial or spinal pachymeningitis

• Spontaneous intracranial hypotension
• Post-lumbar puncture (smooth, diffuse enhancement - usually distinguishable)

• Lyme disease
• Syphilis / Neurosyphilis
• Mycobacterium tuberculosis
• Fungal infections (Aspergillus, Cryptococcus)
• Cysticercosis
• HTLV-1
• Malignant external otitis (Pseudomonas)

• IgG4-Related Disease
• Granulomatosis with Polyangiitis (ANCA-AAV)
• Rheumatoid Arthritis
• Sarcoidosis
• Behçet Disease
• Sjögren Syndrome
• Giant Cell Arteritis
• SLE
• Isolated CNS angiitis
• Vogt-Koyanagi-Harada syndrome

• Dural carcinomatosis (breast, prostate, lung are common primaries)
• Lymphoma (primary CNS or systemic)
• Meningioma (most common mimicker)
• Metastatic disease from adjacent skull bone

GPA vs. IgG4-RD - The Critical Diagnostic Dilemma

5. RADIOLOGICAL FEATURES

MRI Brain and Spine with Gadolinium - Gold Standard

T1 Post-Contrast (Gadolinium) - Primary Sequence

• Diffuse or localized dural thickening with enhancement - the hallmark
• Enhancement can be:
  • Smooth and thin (suggests intracranial hypotension - venous engorgement)
  • Irregular, nodular, or localized (more suggestive of inflammatory or neoplastic cause)
  • Mass-like (may mimic meningioma - especially IgG4-HP)
• "Sandwich" pattern - peripheral enhancement of thickened dura - described in IgG4-HP
• Tentorium, falx, skull base, and convexity dura can all be involved

T2 / FLAIR

• Thickened dura is often hypointense on T2 due to dense fibrous tissue - a key distinguishing feature from leptomeningeal disease
• Marked T2 hypointensity favors IgG4-HP (dense storiform fibrosis)
• Adjacent brain parenchyma: look for edema, hydrocephalus

DWI (Diffusion Weighted Imaging)

• Diffusion restriction suggests lymphoma or abscess
• No restriction in typical inflammatory HP

Advanced MRI Patterns by Etiology

CT Findings

• Dural thickening (hyperdense)
• Sinus/mastoid opacification (GPA clue)
• Skull destruction (atypical IgG4-HP, metastasis)
• Calcification (TB, old sarcoid)

Whole-Body Imaging

• CT chest/abdomen/pelvis - mandatory to detect systemic IgG4-RD (pancreatic enlargement, retroperitoneal fibrosis, lymphadenopathy) or malignancy
• PET-CT - identifies hypermetabolic extraneurological lesions to guide biopsy

MR Venography / CT Venography

• Indicated when venous sinus thrombosis is suspected (headache + papilledema)
• Increasingly recommended in all HP cases given recent recognition of CVT as a complication

6. DIAGNOSTIC WORKUP (Stepwise)

• MRI brain + spine with gadolinium (mandatory)
• CT brain if MRI contraindicated

• CBC, ESR, CRP
• ANA, ANCA (MPO, PR3), RF, anti-CCP
• ACE (angiotensin-converting enzyme)
• Serum IgG subclasses (IgG1-IgG4), total IgG
• VDRL/TPHA (syphilis serology)
• HIV serology
• Lyme serology
• Fungal serology (Cryptococcus antigen, Aspergillus galactomannan)
• Serum calcium, LDH

• Cell count and differential
• Protein and glucose
• Culture (bacterial, TB, fungal)
• Cytology
• Oligoclonal bands
• ACE level
• VDRL
• Flow cytometry (if lymphoma suspected)

• Chest CT (sarcoid, TB, malignancy)
• CT abdomen/pelvis (IgG4-RD organs)
• Whole-body PET-CT

• Required for definitive diagnosis
• Histology: H&E staining, IgG/IgG4 immunostaining, cultures
• IgG4-HP criteria: >10-40 IgG4+ plasma cells/HPF, IgG4+/IgG+ ratio ≥30%, storiform fibrosis, obliterative phlebitis
• Serum IgG4 >1350 μg/mL supports IgG4-HP diagnosis

7. TREATMENT

A. Corticosteroids - First-Line for All Immune-Mediated HP

• Prednisolone / Prednisone
  • Starting dose: 0.5-1 mg/kg/day (IHP and IgG4-HP)
  • Starting dose: 1 mg/kg/day (ANCA-HP / GPA)
  • Gradual taper over several months
• Favorable response seen in ~96% of IgG4-HP cases
• Relapse is common during or after taper - monitor closely with repeat MRI
• Pulse IV methylprednisolone (1g/day x 3 days) for severe/acute presentations

B. Steroid-Sparing Immunosuppressants

C. Rituximab (Anti-CD20 Monoclonal Antibody)

• First-choice for refractory HP across all immune-mediated subtypes
• Mechanism: depletes B cells and plasmablasts
• Particularly effective in:
  • Refractory ANCA-HP (GPA) - now used as induction therapy in some centers
  • Refractory IgG4-HP
• Dose: typically 375 mg/m² weekly x 4, or 1g x 2 doses separated by 2 weeks
• Intrathecal rituximab explored for CNS-refractory IgG4-RHP (novel approach, 2023)

D. Surgical Management

• Surgical decompression for mass effect, spinal cord compression, or hydrocephalus
• Biopsy - both diagnostic and may relieve local compression
• Subtotal/gross resection combined with steroid therapy in IgG4-HP with significant mass effect
• VP shunting for obstructive hydrocephalus

E. Treatment by Specific Etiology

F. Monitoring

• Repeat MRI at 3-6 months to assess response
• Serum IgG4 levels for monitoring IgG4-HP
• ANCA titers for monitoring ANCA-HP
• Watch for relapse during taper (dose adjustment required)
• Long-term follow-up is mandatory due to relapse risk

8. RECENT UPDATES (2023-2026)

1. ANCA-HP as a CNS-Limited AAV Subtype (Shimojima & Sekijima, Autoimmun Rev 2023)

2. MRI Patterns for Immune-Mediated HP (Matias et al., Acad Radiol 2023)

3. IgG4-HP - High Misdiagnosis Rate and Atypical Manifestations (Liu et al., Diagnostics 2026)

• 73.5% male predominance, mean age 48.6 years
• Headache in 58.8% of cases
• 38.5% misdiagnosis rate - most commonly mistaken for meningioma
• Skull destruction with subcutaneous mass formation - an atypical but recognized presentation
• Histopathology + serum IgG4 = gold standard for diagnosis
• Glucocorticoids alone or combined with immunosuppressants achieved favorable outcomes in 96% of cases
• Rituximab effective for refractory cases [PMID 41827958]

4. Cerebral Venous Thrombosis as a Complication (Wang et al., BMC Neurol 2026)

5. IgG4-HP with Multi-System Involvement (Frontiers Immunology 2025)

6. Intrathecal Rituximab for Refractory IgG4-RHP (Balaban et al., Front Neurol 2023)

7. Neurological Involvement in IgG4-RD More Common Than Previously Thought (Practical Neurology 2025)

8. Pachymeningitis-ANCA Dual Positivity (Gautier et al., Autoimmun Rev 2023)

SUMMARY TABLE

REFERENCES

1. Bradley and Daroff's Neurology in Clinical Practice, 8th Ed. - pp. 2586-2590
2. Adams and Victor's Principles of Neurology, 12th Ed. - pp. 648-649
3. Shimojima Y, Sekijima Y. Hypertrophic pachymeningitis in ANCA-associated vasculitis. Autoimmun Rev 2023 [PMID 37062439]
4. Matias TB et al. Immune-Mediated Hypertrophic Pachymeningitis and its Mimickers: MRI Findings. Acad Radiol 2023 [PMID 36882352]
5. Liu XM et al. IgG4-Related Disease Manifested as Hypertrophic Pachymeningitis. Diagnostics 2026 [PMID 41827958]
6. Wang Y et al. Recurrent CVT associated with IgG4-related HP. BMC Neurol 2026 [PMID 41808071]
7. Balaban DT et al. Treatment of IgG4-RHP with intrathecal rituximab. Front Neurol 2023 [PMID 37292126]
8. Gautier F et al. Pachymeningitis associated with IgG4-RD and ANCA positivity. Autoimmun Rev 2023 [PMID 36738953]
9. Yang L et al. IgG4-related disease in the nervous system. Pract Neurol 2025;25:217-227