Rasmussen syndrome is a rare inflammatory brain disorder in which the immune system attacks one hemisphere of the brain, causing it to slowly deteriorate. It almost exclusively affects children, typically beginning between ages 1 and 14, and leads to seizures that worsen over time, progressive weakness on one side of the body, and cognitive decline. With an estimated 2.4 new cases per 10 million children each year, it is one of the rarest neurological conditions in pediatric medicine.
What Happens in the Brain
Rasmussen syndrome is driven by an immune system attack on brain tissue, but only on one side. A specific type of immune cell, called a cytotoxic T cell, infiltrates the brain’s cortex and begins destroying neurons. These T cells release enzymes that trigger a chain reaction of inflammation: surrounding support cells called microglia become overactivated, and protective cells called astrocytes are depleted. The combined effect disrupts normal brain function and causes the affected hemisphere to physically shrink over time.
What initially triggers this immune attack remains unknown. Researchers believe some kind of external trigger, possibly a viral infection or another environmental factor, sets off the T cells in genetically susceptible children. Once activated, these immune cells take up residence in brain tissue and sustain the damage long after any initial trigger has passed. This is what makes the disease progressive rather than a one-time event.
The Three Stages of Progression
Rasmussen syndrome unfolds in three recognizable stages, though the pace varies from child to child.
In the prodromal stage, a child experiences occasional focal seizures, meaning seizures that start on one side of the body. These may be infrequent enough that the condition isn’t immediately recognized as Rasmussen syndrome. Neurological function often appears normal between episodes.
The acute stage marks a turning point. Seizures become more frequent and harder to control. The affected brain hemisphere begins visibly shrinking on imaging scans. Children develop weakness on the opposite side of the body, and depending on which hemisphere is involved, may experience vision changes, speech difficulties, or problems with memory and thinking. This stage is where the most rapid neurological decline occurs. Weakness and other problems typically appear one to three years after seizures first begin.
In the residual stage, seizure frequency may decrease somewhat, but the accumulated brain damage has already caused significant, often permanent, neurological deficits. The affected hemisphere has lost substantial volume, and children are left with lasting impairments in movement, cognition, or both.
How It’s Diagnosed
Diagnosing Rasmussen syndrome takes time because the early symptoms, particularly occasional seizures, overlap with many other conditions. Brain imaging with MRI is required for diagnosis. In the early prodromal stage, MRI may appear normal, which makes the condition especially tricky to identify. As the disease progresses into the acute stage, MRI reveals characteristic inflammation and shrinkage confined to one hemisphere.
The International League Against Epilepsy considers the diagnosis confirmed when a child shows drug-resistant seizures originating from one hemisphere, progressive neurological decline, and the expected MRI changes. Children who fit the clinical picture but still have a normal MRI can be suspected of having the syndrome “in evolution,” but the imaging findings are ultimately required for a definitive diagnosis.
Several other conditions can mimic Rasmussen syndrome and need to be ruled out. These include focal cortical dysplasia (a structural brain abnormality present from birth), mitochondrial disorders that cause stroke-like episodes, other autoimmune brain conditions that tend to affect both hemispheres, and hemiconvulsion-hemiplegia-epilepsy syndrome.
Why Standard Seizure Medications Don’t Work
One of the hallmark features of Rasmussen syndrome is that seizures resist conventional epilepsy medications. Because the underlying problem is ongoing immune-mediated destruction of brain tissue rather than a fixed electrical abnormality, simply suppressing seizure activity with standard drugs doesn’t address the root cause. The seizures keep returning as the disease progresses and new areas of the brain become involved in the inflammatory process.
This drug resistance is often one of the earliest clinical clues that something beyond typical epilepsy is occurring, particularly when seizures consistently originate from the same hemisphere and gradually increase in frequency.
Immunotherapy and Medical Treatment
Since the disease is immune-driven, treatments that suppress or modulate the immune system can slow progression and reduce seizure frequency. Maintenance steroids and intravenous immunoglobulin (IVIG) are most often considered first-line options. These don’t cure the condition, but they can buy time by dampening the inflammatory attack on the brain.
When first-line treatments aren’t enough, several second-line immunotherapies have shown varying degrees of effectiveness. In pooled data from case series, azathioprine showed seizure response in about 70% of patients, rituximab in about 60%, and adalimumab in about 50%. The challenge is that responses vary widely between individuals, and no single immunotherapy has proven consistently effective across all patients.
Surgery: Hemispherectomy
The most definitive treatment for Rasmussen syndrome is hemispherectomy, a surgical procedure that disconnects or removes the affected hemisphere. This is a major decision because the surgery itself causes permanent loss of function on the opposite side of the body, including weakness or paralysis on one side and loss of peripheral vision in one eye. For many families, the surgery becomes necessary when seizures are so frequent and severe that they pose a greater threat to quality of life and development than the surgical consequences.
Outcomes data from UCLA’s experience with 44 patients illustrates both the benefits and limitations. At one year after surgery, 68% of patients were seizure-free. By five years, that number dropped to 48%, and by ten years, only 22% remained seizure-free. Among those whose seizures returned, the median time to the first post-surgical seizure was about 39 weeks. The researchers described hemispheric surgery as palliative rather than curative, meaning it significantly reduces seizure burden but doesn’t necessarily eliminate seizures permanently.
A separate study found more encouraging long-term numbers: 68% seizure-free at last follow-up, with 70% classified as having good seizure outcomes. Functionally, 68% of patients could walk after surgery, and 84% could speak. Cognitive function remained stable in 64% of patients postoperatively, gross motor function was unchanged in 54%, and 74% retained functional use of their hand.
These numbers reflect an important reality. Children’s brains have remarkable plasticity, and the younger the child at the time of surgery, the better the remaining hemisphere can compensate for lost functions. Language, in particular, can often reorganize to the healthy hemisphere, especially in children who have surgery before age 5 or 6.
Living With Rasmussen Syndrome
Whether or not a child undergoes surgery, Rasmussen syndrome requires long-term neurological care and rehabilitation. Physical therapy helps maximize mobility on the weakened side. Occupational therapy supports daily living skills, and speech therapy may be needed if the language-dominant hemisphere is affected. Many children also benefit from neuropsychological support to address learning challenges and cognitive changes as they grow.
The trajectory of the disease is highly individual. Some children stabilize after the acute phase with manageable deficits, while others experience more profound impairment. Early recognition and treatment, whether through immunotherapy, surgery, or both, offer the best chance of preserving neurological function during the critical years of brain development.