Rassmussens Encephalitis

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Rasmussen's Encephalitis (Rasmussen Syndrome)

Rasmussen's encephalitis (RE) is a rare, chronic, progressive inflammatory brain disease predominantly affecting one cerebral hemisphere. It is classically associated with intractable focal epilepsy, progressive neurological deficits, and unihemispheric atrophy.

Epidemiology

Incidence: ~2.4 per 10 million per year
Onset typically in childhood (median age ~6–8 years), though adult-onset cases (~10%) are recognized
No sex predilection
No geographic or ethnic clustering

Pathophysiology

The etiology remains incompletely understood but is considered autoimmune in nature, with T-cell-mediated cytotoxic destruction of neurons and astrocytes:

CD8+ cytotoxic T lymphocytes are the primary effectors, found in direct apposition to neurons and astrocytes in affected cortex
Microglial nodules and perivascular lymphocytic cuffing are hallmark histological findings
Anti-GluR3 (GluA3) antibodies were historically implicated, but their pathogenic role is debated; anti-GluR3 is not consistently present
Anti-NMDA receptor antibodies and other neuronal antibodies have been found in subsets
A viral trigger (CMV, EBV, enterovirus) has been hypothesized but never proven
The process is self-limited to one hemisphere, which remains unexplained

Clinical Stages (Bien et al. 2002)

RE progresses through three recognized stages:

Stage	Name	Features
1	Prodromal	Low seizure frequency, mild or no deficits; may last months to years
2	Acute	Frequent focal seizures, epilepsia partialis continua (EPC), hemiparesis, cognitive decline
3	Residual	Permanent hemiplegia, fixed neurological deficits, reduced seizure frequency

Clinical Features

Seizures

Focal motor seizures are the hallmark, often simple partial (conscious patient)
Epilepsia Partialis Continua (EPC): continuous focal clonic jerking of one body part for hours to days — a near-pathognomonic feature (~50–70% of patients)
Seizures are typically medically refractory
Secondary generalization may occur

Neurological Deficits (progressive)

Contralateral hemiparesis (most common)
Hemianopia (contralateral visual field loss)
Aphasia (if dominant hemisphere affected)
Cognitive decline / dementia
Hemisensory deficits

Diagnosis

Diagnosis is clinical + radiological + histopathological. The European Consensus Statement (2005) provides diagnostic criteria with two pathways:

Part A (Clinical + MRI — no biopsy needed if all present)

Focal seizures (with or without EPC) and unilateral cortical deficit
EEG with unihemispheric slowing or epileptic activity, contralateral to the clinical deficit
MRI showing unihemispheric focal cortical atrophy with T2/FLAIR hyperintensity in gray or white matter, or T2 signal in ipsilateral caudate head

Part B (Less typical presentations — requires biopsy confirmation)

Focal seizures or EPC with or without unilateral cortical deficit
MRI showing progressive unihemispheric focal cortical atrophy
Histopathology: T-cell-dominated encephalitis with activated microglial cells ± reactive astrogliosis; multiple macrophages, perivascular lymphocytes

MRI Findings

Rasmussen's encephalitis MRI — progressive unihemispheric atrophy

T2-weighted coronal MRI showing characteristic unihemispheric cortical atrophy in Rasmussen's encephalitis: cortical thinning, sulcal widening, T2 hyperintensity in cortex and subcortical white matter, and ex-vacuo dilatation of the ipsilateral lateral ventricle.

Sequential MRI findings (evolving over months–years):

Early: focal cortical swelling or T2 hyperintensity, ipsilateral caudate T2 signal
Progressive: unihemispheric cortical atrophy, white matter atrophy
Late: severe ipsilateral hemispheric atrophy with compensatory contralateral hypertrophy
FDG-PET: unihemispheric hypometabolism (useful in early/atypical cases)

Differential Diagnosis

Condition	Distinguishing Features
Focal cortical dysplasia	Static lesion on MRI; no progressive atrophy
FIRES / febrile infection-related epilepsy	Bilateral involvement, acute onset post-fever
Anti-NMDAR encephalitis	Bilateral, psychiatric prodrome, specific antibody
Hemimegalencephaly	Enlarged (not atrophic) hemisphere
Sturge-Weber syndrome	Port-wine stain, leptomeningeal angioma
MELAS	Stroke-like episodes, mitochondrial pattern
Lafora disease	Progressive myoclonic epilepsy, bilateral

Treatment

RE is divided into two treatment domains: disease-modifying immunotherapy and surgical intervention.

Immunotherapy (Slows Progression, Rarely Curative)

Agent	Mechanism / Route	Evidence Level
IV Methylprednisolone	High-dose pulse steroids	First-line; may reduce seizures and slow progression
Long-term corticosteroids	Oral prednisolone	Maintenance; limited by side effects
IVIg	Immunomodulation	Short-term benefit; needs repeated dosing
Plasma exchange	Antibody removal	Adjunct, especially if antibodies present
Tacrolimus / cyclosporine	Calcineurin inhibitors (T-cell suppression)	Some benefit in case series
Rituximab	Anti-CD20, B-cell depletion	Case reports; may help in early/active disease
Natalizumab	Anti-α4-integrin; reduces lymphocyte trafficking	Emerging evidence; adult cases
Adalimumab	Anti-TNF	Limited case reports

Immunotherapy can slow progression but rarely stops it completely and does not cure the disease.

As noted in Harrison's Principles of Internal Medicine (21st ed., p. 2817), patients with autoimmune/paraneoplastic encephalitis "usually respond to treatment of the tumor, if found, and immunotherapy (e.g., glucocorticoids, IVIg, plasma exchange, rituximab, or cyclophosphamide)" — principles that overlap with RE management.

Surgical Treatment (Definitive)

Hemispheric surgery is the only treatment that can render patients seizure-free:

Procedure	Description	Seizure Freedom Rate
Anatomical hemispherectomy	Complete removal of affected hemisphere	~70–80%
Functional hemispherectomy	Disconnection with partial resection (less blood loss)	~65–75%
Hemispherotomy	Pure disconnection; minimal tissue removal	Similar to functional; preferred modern approach

Timing of surgery is a critical decision:

Earlier surgery → better seizure outcomes, but greater neurological deficit from the procedure
Later surgery → more established pre-existing deficits, but hemisphere already compromised
Dominant hemisphere involvement adds significant risk (aphasia, language transfer possible if < ~6 years old)

Corpus callosotomy is a palliative option when hemispheric surgery is deferred.

Prognosis

Without surgery: ~70% of patients develop significant permanent hemiplegia, aphasia (if dominant hemisphere), and cognitive impairment
With early hemispheric surgery: seizure freedom in ~65–80%; neurological deficits are present but stable
The contralateral hemisphere can compensate for motor and language functions, especially in younger children (neuroplasticity)
Adult-onset RE generally progresses more slowly but may be harder to treat

Key Points Summary

RE is a rare, chronic, progressive, unihemispheric autoimmune encephalitis
Hallmark: medically refractory focal epilepsy + epilepsia partialis continua + progressive hemiplegia
Pathology: CD8+ T-cell-mediated neuronal destruction
MRI: progressive unihemispheric cortical atrophy with T2 hyperintensity
Immunotherapy slows progression; hemispheric surgery is the only curative option
Early surgical intervention in appropriate candidates is recommended before severe deficits establish

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