Clinical Immunology

Systemic Diseases

Neurosarcoidosis & Systemic Autoimmune Diseases

What Do You Need to Know?

  • Neurosarcoidosis: Non-caseating granulomas; cranial neuropathies (facial nerve #1) and hypothalamic–pituitary dysfunction are the hallmark presentations; biopsy is the gold standard — serum ACE has only ~60% sensitivity
  • Neuro-Behçet: Oral + genital ulcers with CNS involvement — parenchymal (brainstem predilection) vs. non-parenchymal (cerebral venous sinus thrombosis); pathergy test and HLA-B51 support diagnosis
  • Neuropsychiatric SLE: 19 ACR-defined syndromes; distinguish inflammatory (immunosuppression) vs. thrombotic (anticoagulation for antiphospholipid syndrome); anti-ribosomal P antibodies correlate with psychosis
  • Sjögren syndrome: Peripheral neuropathy (sensory ganglionopathy most characteristic) > CNS involvement; anti-SSA/Ro and anti-SSB/La; Schirmer test + lip biopsy for diagnosis
  • IgG4-related disease: Hypertrophic pachymeningitis, orbital pseudotumor, hypophysitis; storiform fibrosis + IgG4+ plasma cells on tissue biopsy; rituximab is highly effective
  • Susac syndrome: Triad of encephalopathy + branch retinal artery occlusion (BRAO) + sensorineural hearing loss; central callosal “snowball” lesions on MRI (vs. MS peripheral callosal lesions)
  • Key board principle: Each systemic autoimmune disease has a characteristic neurological signature — matching the pattern to the disease is the fastest path to the correct answer
Neurosarcoidosis

Overview & Pathology

  • Definition: CNS/PNS involvement by sarcoidosis — a multisystem granulomatous disease of unknown etiology
  • Pathology: Non-caseating (non-necrotizing) granulomas composed of epithelioid histiocytes, multinucleated giant cells, and surrounding lymphocytes
  • Epidemiology: ~5–15% of sarcoidosis patients develop neurological involvement; may be the presenting feature in up to 50% of neurosarcoidosis cases
  • Predilection: African Americans, women, age 25–50 years

Clinical Manifestations

Manifestation Frequency Key Features
Cranial neuropathies 50–70% CN VII (facial nerve) #1 — may be bilateral; CN II (optic nerve) #2 — optic neuritis, papilledema; any CN can be affected
Leptomeningeal disease 10–20% Basal leptomeningeal enhancement on MRI; chronic meningitis with CSF lymphocytic pleocytosis
Hypothalamic–pituitary 10–15% Diabetes insipidus (most common endocrine manifestation); hyperprolactinemia; panhypopituitarism
Myelopathy 5–10% Dorsal subpial enhancement (“trident sign” on axial MRI); longitudinally extensive; mimics NMOSD
Peripheral neuropathy 15–20% Small fiber neuropathy most common; polyradiculopathy; mononeuritis multiplex
Parenchymal mass 5–10% Ring-enhancing or solid lesion; mimics tumor or abscess
Hydrocephalus 5–10% Communicating (leptomeningeal) or obstructive (mass lesion)

Heerfordt Syndrome (Uveoparotid Fever)

  • Classic tetrad: Parotid gland enlargement + anterior uveitis + facial nerve palsy + fever
  • Pathognomonic for sarcoidosis — highly specific clinical presentation
  • May be the initial presentation of systemic sarcoidosis
💎 Board Pearl
  • Bilateral facial palsy + uveitis = sarcoidosis until proven otherwise. The differential for bilateral CN VII palsy is narrow: sarcoidosis, Lyme disease, GBS, HIV, leukemia
  • Diabetes insipidus + leptomeningeal enhancement on MRI = think neurosarcoidosis (also consider lymphoma, TB, and carcinomatous meningitis)
  • Trident sign on axial spinal MRI = dorsal subpial gadolinium enhancement — characteristic of neurosarcoidosis myelopathy

Diagnostic Workup

Test Findings Pearls
Serum ACE Elevated in ~60% of systemic sarcoidosis Low sensitivity (~60%) and poor specificity; normal ACE does NOT exclude diagnosis; false positives with diabetes, hyperthyroidism, lymphoma
Chest CT Bilateral hilar lymphadenopathy (BHL) in ~90% of sarcoidosis BHL is the most common thoracic finding; absence significantly lowers probability but does not exclude
Gallium-67 / PET scan “Panda sign” (lacrimal + parotid uptake); “Lambda sign” (bilateral hilar + right paratracheal uptake) FDG-PET largely replacing gallium; useful for identifying biopsy sites
CSF Lymphocytic pleocytosis (50–70%), elevated protein (50–70%), low glucose (10–20%), elevated CSF ACE CSF ACE has ~55% sensitivity and ~95% specificity for neurosarcoidosis; more useful than serum ACE for CNS disease
MRI brain/spine Leptomeningeal enhancement (basal predominance), parenchymal lesions, hypothalamic/infundibular thickening, cranial nerve enhancement Leptomeningeal pattern is the most suggestive imaging feature
Biopsy Non-caseating granulomas = gold standard Target most accessible tissue: lymph node > lung > skin > meninges/brain; must exclude TB, fungal infection, foreign body

Zajicek Diagnostic Criteria

Category Criteria
Definite Compatible clinical presentation + positive nervous system biopsy (non-caseating granulomas) + exclusion of other causes
Probable Compatible clinical presentation + evidence of CNS inflammation (CSF/MRI) + positive systemic biopsy (non-CNS tissue) + exclusion of other causes
Possible Compatible clinical presentation + exclusion of other causes + no tissue confirmation (supportive labs: ACE, chest imaging, etc.)
💎 Board Pearl
  • Serum ACE is a poor screening test — sensitivity only ~60%, and many false positives; never rely on a normal ACE to rule out neurosarcoidosis
  • Always get a chest CT — bilateral hilar lymphadenopathy is present in ~90% and provides a safer biopsy target than CNS tissue
  • “Probable” neurosarcoidosis is the most common working diagnosis — CNS biopsy is invasive and often avoided if systemic disease is confirmed

Treatment

Line Agent(s) Details
First-line Corticosteroids IV methylprednisolone 1 g/day × 3–5 days for acute/severe → oral prednisone taper over months; most patients require prolonged therapy
Steroid-sparing Methotrexate, azathioprine, mycophenolate mofetil Methotrexate is the most commonly used steroid-sparing agent; typically added within 2–3 months to reduce steroid burden
Refractory Infliximab, adalimumab (anti-TNF-α) Infliximab most studied; effective for CNS and PNS disease; monitor for infection, demyelination
  • Prognosis: ~30% monophasic (self-limited); ~30% relapsing–remitting; ~30% chronic progressive despite therapy
  • Mortality: 5–10% in neurosarcoidosis; worse with parenchymal disease, hydrocephalus, chronic meningitis
Neuro-Behçet Disease

Overview

  • Behçet disease: Chronic relapsing systemic vasculitis of unknown etiology — affects all vessel sizes
  • Classic triad: Recurrent oral ulcers + genital ulcers + uveitis
  • Epidemiology: “Silk Road disease” — highest prevalence in Turkey, Iran, Japan, Korea; HLA-B51 positive in 50–70%
  • Neuro-Behçet: CNS involvement in 5–10% of Behçet patients; may be the presenting feature

Parenchymal vs. Non-Parenchymal Disease

Feature Parenchymal (~80%) Non-Parenchymal (~20%)
Pathology Meningoencephalitis with perivascular inflammation Venous thrombosis / intracranial hypertension
Location Brainstem > diencephalon > basal ganglia; subcortical white matter; spinal cord Cerebral venous sinus thrombosis (CVST); dural sinus thrombosis; rarely arterial
Presentation Brainstem syndrome, hemiparesis, behavioral changes, encephalopathy Headache, papilledema, elevated ICP, cranial nerve palsies
MRI T2/FLAIR hyperintensity in brainstem/diencephalon; may enhance Venous thrombosis on MRV; empty delta sign
CSF Pleocytosis (neutrophilic early, lymphocytic late), elevated protein Elevated opening pressure; CSF may be normal
Prognosis Worse — progressive brain atrophy and disability Better — typically responds to anticoagulation

Diagnostic Clues

  • Pathergy test: Hyperreactivity to skin prick → papule/pustule at 24–48 hours; ~60–70% positive in endemic populations; less reliable in Western patients
  • HLA-B51: Strongest genetic association; supports diagnosis but not specific
  • No pathognomonic lab test — diagnosis is clinical (International Study Group criteria)
💎 Board Pearl
  • Oral + genital ulcers + brainstem lesion on MRI = Neuro-Behçet — the combination of mucocutaneous ulcers with brainstem predilection is the classic board scenario
  • Parenchymal Neuro-Behçet loves the brainstem — a brainstem–diencephalic T2 lesion in a young patient from the Silk Road region should trigger this diagnosis
  • CVST in a young patient with oral/genital ulcers = non-parenchymal Neuro-Behçet
  • Pathergy test is SPECIFIC but not sensitive — a positive test strongly supports, but a negative test does not exclude

Treatment

  • Parenchymal: High-dose IV corticosteroids → oral taper + azathioprine (first-line steroid-sparing); anti-TNF agents (infliximab, adalimumab) for refractory disease
  • Non-parenchymal (CVST): Anticoagulation + corticosteroids; avoid anticoagulation alone without immunosuppression
  • Maintenance: Azathioprine is the most studied maintenance agent; treat for ≥2 years minimum
Neuropsychiatric Systemic Lupus Erythematosus (NPSLE)

ACR Classification: 19 Neuropsychiatric Syndromes

CNS Syndromes (12) PNS Syndromes (7)
Headache, seizures, cerebrovascular disease, cognitive dysfunction, psychosis, acute confusional state, anxiety, mood disorder, demyelinating syndrome, movement disorder (chorea), myelopathy, aseptic meningitis Polyneuropathy, mononeuropathy, cranial neuropathy, autonomic neuropathy, myasthenia gravis, plexopathy, Guillain-Barré syndrome

Most Common Manifestations

  • Headache: Most frequent (>50%) but least specific; not always attributable to SLE
  • Cognitive dysfunction: 20–80% depending on testing rigor; subtle memory and attention deficits
  • Seizures: 7–20%; may be focal or generalized; associated with antiphospholipid antibodies and active disease
  • Cerebrovascular disease: 5–15%; ischemic stroke > hemorrhagic; strongly linked to antiphospholipid syndrome
  • Psychosis: 2–5%; associated with anti-ribosomal P antibodies

Key Antibody Associations

Antibody Neuropsychiatric Association Clinical Significance
Antiphospholipid (aPL) Stroke, CVST, chorea, seizures, cognitive dysfunction Present in 30–40% of SLE; drives thrombotic manifestations → anticoagulation, not immunosuppression
Anti-ribosomal P Lupus psychosis Most specific antibody for NPSLE psychosis; 90% specificity
Anti-dsDNA Active SLE flare (general marker) High titers correlate with disease activity; not specific for neuropsychiatric involvement
Anti-NMDA-R (NR2) Cognitive dysfunction, psychosis Distinct from anti-NMDA-R encephalitis antibodies; cross-reactive with anti-dsDNA

Inflammatory vs. Thrombotic: The Critical Distinction

Feature Inflammatory NPSLE Thrombotic NPSLE (Antiphospholipid-Related)
Mechanism Autoimmune inflammation, complement activation, vasculitis Thrombosis of cerebral vessels (arterial or venous)
Manifestations Psychosis, myelitis, optic neuritis, aseptic meningitis, acute confusional state Stroke, TIA, CVST, multi-infarct cognitive decline, chorea
MRI White matter lesions (MS-like), diffuse changes, meningeal enhancement Infarcts (territorial or lacunar), cortical atrophy
Labs Low complement (C3/C4), high anti-dsDNA, active urinalysis Positive aPL (lupus anticoagulant, anticardiolipin, anti-β2-glycoprotein I)
Treatment Immunosuppression: corticosteroids, cyclophosphamide, rituximab Anticoagulation: warfarin (INR 2–3, or 3–4 for recurrent events); DOACs controversial
💎 Board Pearl
  • Anti-ribosomal P + psychosis = lupus psychosis — the most specific antibody–syndrome pairing in NPSLE
  • Stroke in young woman with SLE = check antiphospholipid antibodies → treatment is anticoagulation, NOT immunosuppression
  • Chorea in SLE = antiphospholipid syndrome — the most common movement disorder in SLE; mechanism is thrombotic/ischemic to basal ganglia
  • Complement levels distinguish: Low C3/C4 = inflammatory flare (immunosuppress); normal complement + positive aPL = thrombotic (anticoagulate)
  • Lupus myelitis may mimic NMOSD — longitudinally extensive lesion; test for AQP4-IgG
Clinical Pearl
  • Not all neuropsychiatric symptoms in a lupus patient are attributable to SLE — infection, metabolic derangement, medication side effects, and primary psychiatric disease must be excluded. The 2019 EULAR recommendations emphasize attribution as a critical step: symptoms within 6 months of SLE diagnosis or during active disease are more likely SLE-related.
Sjögren Syndrome

Overview

  • Definition: Chronic autoimmune disease targeting exocrine glands (salivary + lacrimal) → sicca syndrome
  • Sicca symptoms: Dry eyes (keratoconjunctivitis sicca) + dry mouth (xerostomia)
  • Demographics: 9:1 female predominance; peak onset 40–60 years
  • Antibodies: Anti-SSA/Ro (70%) and anti-SSB/La (40%); ANA positive in ~80%
  • Neurological involvement: ~20% of Sjögren patients; PNS far more common than CNS

Peripheral Nervous System Manifestations

PNS Manifestation Frequency Key Features
Small fiber neuropathy Most common Burning pain, allodynia; normal NCS/EMG; diagnosed by skin biopsy (reduced IENFD) or QSART
Sensory ataxic neuropathy / ganglionopathy Characteristic Dorsal root ganglion attack (ganglionopathy) → asymmetric, non-length-dependent sensory loss + sensory ataxia; most characteristic PNS manifestation
Sensorimotor polyneuropathy Common Length-dependent; axonal > demyelinating
Trigeminal neuropathy Uncommon but classic Pure sensory involvement of CN V; may be bilateral; highly suggestive of Sjögren when isolated
Mononeuritis multiplex Uncommon Vasculitic mechanism
Autonomic neuropathy Underrecognized Adie pupil, orthostatic hypotension, anhidrosis

CNS Manifestations (Rare but Board-Relevant)

  • Optic neuritis: May mimic MS or NMOSD
  • Transverse myelitis: Longitudinally extensive → must test for AQP4-IgG (Sjögren + NMOSD overlap)
  • MS-like disease: White matter lesions, relapsing course; controversial whether this is true Sjögren CNS disease or comorbid MS
  • Aseptic meningitis
  • Cognitive dysfunction

Diagnostic Workup

Test Purpose
Anti-SSA/Ro, anti-SSB/La Serological hallmark; SSA more sensitive, SSB more specific
Schirmer test <5 mm wetting in 5 min = abnormal; objective measure of tear production
Minor salivary gland (lip) biopsy Focus score ≥1 (aggregate of ≥50 lymphocytes per 4 mm2); most specific test
Skin biopsy (IENFD) Reduced intraepidermal nerve fiber density confirms small fiber neuropathy
💎 Board Pearl
  • Sensory ataxia + non-length-dependent sensory loss + dry eyes/mouth = Sjögren ganglionopathy — the most characteristic neurological manifestation
  • Isolated trigeminal sensory neuropathy (especially bilateral) should prompt evaluation for Sjögren syndrome
  • Sjögren + longitudinally extensive myelitis → test for AQP4-IgG — up to 10–20% of NMOSD patients have comorbid Sjögren; AQP4 antibody may be the unifying pathology
  • Neuropathy may precede sicca symptoms by years — consider Sjögren in any unexplained sensory ganglionopathy or small fiber neuropathy
IgG4-Related Disease (IgG4-RD)

Overview

  • Definition: Chronic fibro-inflammatory condition characterized by tumefactive lesions, storiform fibrosis, and dense IgG4+ plasma cell infiltrate
  • Systemic manifestations: Autoimmune pancreatitis (#1 associated), retroperitoneal fibrosis, sclerosing cholangitis, sialadenitis, thyroiditis, interstitial nephritis
  • Demographics: Middle-aged to older men predominate

Neurological Manifestations

Manifestation Key Features
Hypertrophic pachymeningitis Most common CNS manifestation; thickened, enhancing dura on MRI; headache, cranial neuropathies; IgG4-RD is the #1 cause of idiopathic hypertrophic pachymeningitis
Orbital pseudotumor Orbital mass/swelling; proptosis, diplopia; IgG4-RD is the most common cause of orbital inflammatory pseudotumor
Hypophysitis Pituitary enlargement → hypopituitarism, diabetes insipidus; infundibular thickening
Cranial neuropathies Secondary to pachymeningitis or direct nerve infiltration; any cranial nerve
Perineural disease Trigeminal and infraorbital nerve involvement

Diagnostic Criteria

  • Serum IgG4: Elevated (>135 mg/dL) in ~60–70%; not sensitive or specific alone — can be normal in tissue-confirmed cases and elevated in other conditions
  • Tissue biopsy (gold standard):
    • Storiform fibrosis (swirling, cartwheel-like pattern)
    • Dense lymphoplasmacytic infiltrate with ≥10 IgG4+ plasma cells/HPF (or IgG4/IgG ratio >40%)
    • Obliterative phlebitis

Treatment

  • First-line: Corticosteroids — dramatic initial response in most patients; rapid improvement is itself a diagnostic clue
  • Steroid-sparing / refractory: Rituximab is the most effective steroid-sparing agent; depletes IgG4-producing B cells
  • Other: Azathioprine, mycophenolate for maintenance
  • Relapse rate: High (~30–50%) after steroid discontinuation → long-term maintenance often needed
💎 Board Pearl
  • Hypertrophic pachymeningitis + elevated serum IgG4 = IgG4-RD until proven otherwise — the #1 cause of idiopathic hypertrophic pachymeningitis
  • Storiform fibrosis on biopsy is the histopathological hallmark — cartwheel/swirling pattern of fibrosis with IgG4+ plasma cells
  • Orbital mass + pachymeningitis + autoimmune pancreatitis = classic IgG4-RD multisystem pattern
  • Rituximab > conventional immunosuppressants for steroid-sparing therapy in IgG4-RD
Susac Syndrome

Overview

  • Definition: Autoimmune endotheliopathy affecting arterioles of the brain, retina, and cochlea
  • Classic triad: Encephalopathy + branch retinal artery occlusion (BRAO) + sensorineural hearing loss
  • Pathophysiology: Autoimmune attack on endothelial cells → arteriolar occlusion; anti-endothelial cell antibodies proposed but not validated as diagnostic
  • Demographics: Young women (20–40 years); 3:1 female predominance
  • Important: Complete triad present at onset in only ~15% — components may emerge over weeks to months

Clinical Features

Component Features Evaluation
Encephalopathy Subacute cognitive decline, confusion, personality change, paranoia; headache common; may mimic psychiatric disease MRI brain (see below)
BRAO Visual field deficits; may be subclinical; often bilateral and multifocal Fluorescein angiography (FA) is the key diagnostic test → arteriolar wall hyperfluorescence + branch occlusions; OCT shows retinal thinning
Sensorineural hearing loss Low- and mid-frequency loss (cochlear apical involvement); may fluctuate; tinnitus common Audiometry; may progress to bilateral deafness

MRI Findings: Susac vs. MS

Feature Susac Syndrome Multiple Sclerosis
Callosal lesions Central callosal “snowball” lesions — round, involve the central fibers of the corpus callosum Peripheral callosal lesions — ovoid, perpendicular to the ventricle (“Dawson fingers”), extend from the calloseptal interface
Deep gray matter Internal capsule, thalamus, basal ganglia involvement common Less common; cortical and juxtacortical lesions more typical
Leptomeningeal enhancement May occur Uncommon (though leptomeningeal enhancement is increasingly recognized)
Spinal cord Typically spared Commonly involved
💎 Board Pearl
  • Central callosal “snowball” lesions = Susac; peripheral callosal “Dawson fingers” = MS — the single most important MRI distinction on boards
  • Young woman + encephalopathy + hearing loss + visual loss = Susac triad — even if incomplete at presentation
  • Fluorescein angiography is essential — BRAO may be subclinical; FA reveals arteriolar wall hyperfluorescence not seen on fundoscopy alone
  • Low-frequency hearing loss distinguishes Susac from most other causes (noise-induced, presbycusis = high-frequency)

Treatment

  • Aggressive early immunosuppression is critical — the disease is self-limited (2–4 years) but cumulative damage (hearing loss, cognitive impairment, visual loss) can be devastating
  • Induction: IV methylprednisolone + IVIg; PLEX for severe/refractory cases
  • Maintenance: Mycophenolate mofetil or azathioprine + low-dose aspirin (antiplatelet for endothelial component)
  • Anticoagulation: Not standard; low-dose aspirin is recommended for the vascular component
  • Monitoring: Serial audiometry and fluorescein angiography to detect subclinical relapses
Comprehensive Comparison Table

Systemic Autoimmune Diseases with Neurological Manifestations

Disease Hallmark Neurological Feature Key Antibody / Lab Classic MRI Finding First-Line Treatment
Neurosarcoidosis Cranial neuropathy (CN VII > II), diabetes insipidus, leptomeningeal disease ACE (~60% sensitivity); non-caseating granulomas on biopsy Basal leptomeningeal enhancement; infundibular thickening; trident sign (spinal) Corticosteroids → MTX/AZA → infliximab
Neuro-Behçet Brainstem meningoencephalitis; CVST HLA-B51; pathergy test Brainstem–diencephalic T2 lesions; venous sinus thrombosis Corticosteroids + azathioprine; anti-TNF for refractory
NPSLE Stroke (aPL), psychosis (anti-ribosomal P), seizures, cognitive dysfunction Anti-dsDNA, aPL, anti-ribosomal P; low complement White matter lesions; territorial infarcts Inflammatory: immunosuppression; Thrombotic: anticoagulation
Sjögren Sensory ganglionopathy; small fiber neuropathy; trigeminal neuropathy Anti-SSA/Ro, anti-SSB/La Rarely abnormal (white matter lesions if CNS involved) Corticosteroids + IVIg; rituximab for severe
IgG4-RD Hypertrophic pachymeningitis; orbital pseudotumor; hypophysitis Serum IgG4; storiform fibrosis + IgG4+ plasma cells on biopsy Diffuse dural thickening/enhancement; orbital mass Corticosteroids → rituximab
Susac syndrome Encephalopathy + BRAO + SNHL No specific antibody; fluorescein angiography diagnostic Central callosal “snowball” lesions; deep gray matter involvement IV steroids + IVIg + aspirin; mycophenolate maintenance

Differentiating by Meningeal Pattern

Meningeal Pattern Diseases to Consider
Leptomeningeal enhancement (pia-arachnoid) Neurosarcoidosis, carcinomatous meningitis, TB meningitis, neurosyphilis
Pachymeningeal enhancement (dura) IgG4-RD (#1 cause of idiopathic), granulomatosis with polyangiitis (GPA), intracranial hypotension, meningioma, metastases

Differentiating by Neuropathy Pattern

Neuropathy Type Disease Association
Sensory ganglionopathy (non-length-dependent) Sjögren (#1 autoimmune cause), paraneoplastic (ANNA-1/Hu), cisplatin, vitamin B6 toxicity
Small fiber neuropathy Sjögren, sarcoidosis, SLE, celiac disease, diabetes
Mononeuritis multiplex Vasculitis (PAN, GPA, EGPA), Sjögren, SLE, sarcoidosis, hepatitis B/C
Cranial neuropathies (multiple) Sarcoidosis, IgG4-RD, GPA, lymphoma, carcinomatous meningitis, Lyme
💎 Board Pearl
  • Leptomeningeal = sarcoid; pachymeningeal = IgG4-RD — the meningeal enhancement pattern immediately narrows the differential
  • Non-length-dependent sensory neuropathy + sicca = Sjögren ganglionopathy
  • Central callosal = Susac; peripheral callosal = MS
  • Brainstem lesion + oral/genital ulcers = Neuro-Behçet; brainstem lesion + BHL = neurosarcoidosis
  • Thrombotic complications in SLE = anticoagulate; inflammatory complications = immunosuppress — the most important management distinction in NPSLE
Clinical Pearl
  • When facing a board question about systemic autoimmune disease with neurological symptoms, use a two-step approach: (1) identify the systemic disease by its extraneurological features (ulcers → Behçet; sicca → Sjögren; rash + arthritis → SLE; BHL → sarcoidosis), then (2) match the neurological pattern to that disease’s known CNS/PNS signature.

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