Rasmussen Encephalitis and Epilepsy

How a rare immune-mediated disorder can progressively damage one cerebral hemisphere and produce severe focal epilepsy

Rasmussen encephalitis, increasingly also called Rasmussen syndrome, is a rare neurological disorder characterised by inflammation affecting predominantly one cerebral hemisphere, progressive neurological loss and focal seizures that often become drug-resistant.

It usually begins in childhood, although adolescent- and adult-onset cases are recognised. The condition is unusual because the epilepsy, inflammation and progressive loss of function are intertwined: seizures are not simply occurring in an otherwise stable structural lesion.

What happens in Rasmussen syndrome?

The disorder typically affects one side of the brain much more than the other. Over time, the affected hemisphere can show progressive cortical atrophy on MRI while the person develops worsening focal seizures and neurological deficits.

Clinical progression can include increasingly frequent focal seizures, epilepsia partialis continua, progressive weakness on the opposite side of the body, loss of fine motor control, visual-field impairment, language difficulty when the language-dominant hemisphere is affected, and cognitive or behavioural change.

The exact course varies. Some people deteriorate rapidly; others have a slower or more fluctuating course.

Epilepsia partialis continua

One particularly characteristic feature is epilepsia partialis continua (EPC): repeated or near-continuous focal motor seizures affecting part of the body, sometimes persisting for hours, days or longer.

EPC can involve rhythmic or irregular twitching of the face, hand, arm or another body region while awareness may remain preserved.

It is strongly associated with Rasmussen syndrome but is not unique to it. Other structural, metabolic, inflammatory and vascular disorders can also cause EPC.

Why does one hemisphere become inflamed?

The precise initiating cause remains uncertain.

Current evidence strongly supports an immune-mediated process. Neuropathological studies have demonstrated prominent involvement of cytotoxic CD8-positive T cells, activated microglia, inflammatory signalling, neuronal injury and gliosis.

No single virus, antibody or other trigger has been established as the universal cause.

This distinction matters. Rasmussen syndrome is often discussed alongside autoimmune epilepsy because immune mechanisms are central to the disease, but it does not behave like a typical antibody-mediated autoimmune encephalitis with one consistently identifiable antibody target.

Is it an autoimmune epilepsy?

Rasmussen syndrome is immune-mediated and is frequently considered within the broader spectrum of inflammatory and autoimmune neurological disease.

However, it has a distinctive clinical pattern: predominantly unilateral cortical inflammation, progressive hemispheric atrophy, focal epilepsy and progressive neurological loss.

That pattern is sufficiently specific that Rasmussen syndrome deserves its own page rather than being treated as simply another example of Autoimmune Epilepsy.

How does it begin?

Some people initially present with ordinary-looking focal seizures before obvious weakness or hemispheric atrophy has developed.

This early phase can make diagnosis difficult. MRI may show only subtle changes, EEG abnormalities may not yet be strikingly unilateral, and the neurological examination can be relatively preserved.

As the disease evolves, the combination of seizure progression, unilateral EEG abnormalities, progressive focal neurological deficits and serial MRI change can make the diagnosis clearer.

Diagnosis is longitudinal

Rasmussen syndrome is rarely diagnosed from one test alone.

The 2026 international consensus recommends integrating detailed seizure and neurological history, serial neurological examinations, EEG, serial MRI, blood investigations, cerebrospinal fluid studies where appropriate, neuropsychological assessment, and investigation for competing autoimmune, infectious, genetic, metabolic or structural diagnoses. Brain biopsy is reserved for selected cases when diagnostic uncertainty remains.

The need for serial assessment is important because progression itself can be diagnostically informative.

What does MRI show?

MRI can show progressive focal or hemispheric cortical atrophy, signal abnormalities and changes involving the cortex and subcortical structures of the affected hemisphere.

Early disease may be subtle. Serial imaging can therefore be more informative than a single scan.

Progressive unilateral atrophy is a major clue, but MRI findings still need to be interpreted with the clinical and EEG picture because other disorders can also produce unilateral brain abnormalities.

What does EEG show?

EEG commonly demonstrates abnormalities over the affected hemisphere, including focal or hemispheric slowing and epileptiform discharges.

As the disease progresses, the EEG background can deteriorate and seizure activity may become increasingly frequent.

However, EEG findings are not perfectly confined to one side in every patient. Independent contralateral abnormalities can occur, particularly later in the disease, and do not necessarily mean that both hemispheres are undergoing the same destructive process.

This is another reason that EEG must be interpreted alongside serial MRI, examination and seizure semiology.

Why seizures become drug-resistant

Antiseizure medications can reduce some seizure types, but they do not stop the underlying inflammatory disease.

As progressive cortical injury and network reorganisation occur, seizures frequently become resistant to medication. EPC can be especially difficult to suppress completely.

Medication is still important for reducing seizure burden and treating acute exacerbations, but repeated medication changes alone do not address the central disease process.

Immunotherapy

Because Rasmussen syndrome is immune-mediated, treatments that suppress or modify immune activity are an important part of management.

The 2026 international consensus recommends intravenous corticosteroids as first-line acute immunotherapy for seizure exacerbations and status epilepticus, with or without intravenous immunoglobulin depending on the clinical situation.

Longer-term immunotherapies can include corticosteroid strategies, intravenous immunoglobulin and other steroid-sparing immune treatments. Comparative evidence remains limited, so there is no single maintenance immunotherapy proven superior for every patient.

Immunotherapy may slow neurological deterioration or reduce inflammatory activity in some people, particularly earlier in the disease, but it does not reliably produce lasting seizure freedom.

Why immunotherapy and surgery have different goals

Immunotherapy aims to suppress the inflammatory process and preserve neurological function for as long as possible.

Hemispheric surgery aims primarily to stop seizures by disconnecting the diseased hemisphere from the rest of the brain.

These goals can conflict. Delaying surgery may preserve existing motor or language function for longer, but ongoing seizures and inflammation may continue to damage the hemisphere and wider development.

For this reason, modern management is not simply “medicine first, surgery last”. Timing is individualised according to age, neurological function, seizure burden, disease progression, dominant-hemisphere involvement and the values and priorities of the person and family.

Hemispherotomy and functional hemispherectomy

For severe drug-resistant Rasmussen syndrome, hemispheric disconnection remains the most effective treatment for seizures.

Modern procedures usually aim to disconnect the affected hemisphere rather than physically remove the entire hemisphere. Techniques include functional hemispherectomy and hemispherotomy.

The 2026 consensus identifies hemispheric disconnection as the most effective seizure treatment. An international practice survey published in 2024 likewise found that about 90% of respondents regarded functional hemispherectomy or hemispherotomy as the surgical standard of care.

The history of how these procedures developed is covered separately in Hemispherectomy and Hemispherotomy: From Removing a Hemisphere to Disconnecting It.

Why would disconnecting half the brain ever be considered?

It sounds extreme because it is major neurosurgery.

But in classic Rasmussen syndrome, one hemisphere may already be progressively losing function while generating disabling seizures that cannot be controlled medically.

If that hemisphere remains connected, epileptic activity can continue to disrupt the functioning of the healthier hemisphere and the person’s overall development.

The operation accepts predictable neurological consequences in exchange for the possibility of major seizure control and protection of the remaining hemisphere from continuing epileptic disruption.

What neurological effects follow hemispheric disconnection?

People undergoing hemispheric disconnection usually have, or are expected to develop, significant weakness on the opposite side of the body and a contralateral visual-field deficit.

Fine hand function on the affected side is generally particularly limited.

Language risk depends heavily on which hemisphere is affected, age at onset, age at surgery and how successfully language functions have reorganised before surgery.

This is why detailed neuropsychological, language and functional assessment is central to surgical planning.

Neuroplasticity

The developing brain can reorganise functions to a remarkable degree, particularly in children.

Language and other cognitive functions can sometimes reorganise to the remaining hemisphere, but outcomes are variable and depend on preoperative function, age, disease burden and timing.

Neuroplasticity is not unlimited. Surgery does not make the remaining hemisphere equivalent to two healthy hemispheres, and some deficits can be permanent.

Dominant-hemisphere Rasmussen syndrome

Disease affecting the hemisphere that supports language creates especially difficult decisions.

Clinicians may try to preserve language function through immunotherapy and careful timing while monitoring whether language is reorganising to the opposite hemisphere.

Eventually, uncontrolled epilepsy or continuing neurological decline may make surgery necessary despite the language risks.

These decisions require specialist epilepsy surgery, neuropsychology, neuroradiology and rehabilitation expertise.

Adult-onset Rasmussen syndrome

Although most cases begin in childhood, adult-onset Rasmussen syndrome is recognised.

Adults may have a slower or less stereotyped course, and the differential diagnosis can be broader because focal epilepsy, autoimmune encephalitis, tumours, vascular disease and other disorders become competing possibilities.

The lower degree of developmental neuroplasticity in adulthood can also affect surgical decision-making.

Why brain biopsy is not always necessary

Brain biopsy can show characteristic inflammatory changes, including T-cell-dominant encephalitis, microglial activation, neuronal loss and gliosis.

But inflammation can be patchy, and a biopsy can miss diagnostic tissue. Histology is also not unique enough to replace the clinical, EEG and imaging picture.

The 2026 consensus therefore places biopsy within the diagnostic pathway for selected cases rather than making it mandatory for everyone who otherwise meets criteria.

What else can look like Rasmussen syndrome?

The differential diagnosis can include focal cortical dysplasia, brain tumours, vascular lesions, mitochondrial and other genetic disorders, chronic infections, antibody-mediated autoimmune encephalitis and other causes of progressive focal epilepsy.

This matters particularly early in the disease, when unilateral atrophy has not yet become obvious.

A diagnosis of Rasmussen syndrome should therefore not be based on drug-resistant focal seizures alone.

Rehabilitation is part of treatment

Management does not end with seizure treatment.

Physiotherapy, occupational therapy, speech and language therapy, neuropsychology, educational support, visual rehabilitation and mental-health care may all be required.

These needs can change over time, especially after hemispheric surgery when the person must adapt to permanent motor and visual deficits while the remaining hemisphere reorganises function.

The key distinction

Rasmussen syndrome is not simply a focal epilepsy with an abnormal MRI.

It is a progressive immune-mediated neurological disease in which inflammation, seizure activity, hemispheric atrophy and neurological decline interact.

Antiseizure medication treats seizures but does not stop the disease. Immunotherapy can modify inflammatory activity but often cannot eliminate the epilepsy. Hemispheric disconnection can offer the strongest seizure control but carries major permanent functional consequences.

The central challenge is therefore not merely choosing a treatment. It is deciding when preserving a progressively diseased hemisphere becomes more harmful than disconnecting it.

Sources and further reading

Evidence checked: September 2026.

This page is for general education and does not replace individual assessment by a specialist epilepsy, neuroimmunology or neurosurgical team.