Subacute Sclerosing Panencephalitis
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Direct answer
Subacute sclerosing panencephalitis is a rare, progressive and fatal encephalitis caused by persistent, defective measles virus in the brain, appearing years after a measles infection. It produces behavioural and cognitive decline, myoclonus and rigidity, with periodic high-amplitude EEG complexes and raised measles antibodies in the CSF. Measles vaccination is the only reliable prevention.
What you must remember
- Cause: persistent, defective measles virus in the brain, often with matrix protein defects that allow it to evade immune clearance.
- Latency: onset typically two to ten years after measles, most often in children and young adults who had measles early in life.
- Early features: subtle behaviour change, falling school performance and memory loss, followed by myoclonic jerks.
- Later features: dystonia, rigidity, ataxia, visual disturbance and akinetic mutism, progressing to coma.
- EEG: periodic, generalised, high-amplitude slow-wave complexes recurring every few seconds, the most characteristic finding.
- CSF: raised measles-specific IgG and an intrathecal antibody pattern, with a usually normal cell count.
- Diagnosis: the clinical course, the characteristic EEG and CSF measles antibodies are considered together, and brain biopsy shows intranuclear inclusions when needed.
- Prevention and treatment: two doses of measles-containing vaccine on schedule are the key measure, while inosine pranobex and interferon have only uncertain benefit.
An approach to a suspected case
The approach relies on three linked findings, each of which supports the others. None is diagnostic alone, so the history, EEG and CSF are read together.
- Take the history: Ask about measles in early childhood, and look for an insidious fall in school performance or a change in behaviour over several months.
- Examine for myoclonus: Look for startle or action myoclonus, along with ataxia, rigidity and visual symptoms, which suggest progression.
- Order the EEG: Look for periodic, generalised high-amplitude complexes recurring every few seconds, the characteristic pattern of the disease.
- Test the CSF: Measure measles-specific antibodies, and compare CSF with serum levels, noting that the cell count is usually normal.
- Exclude mimics: Creutzfeldt-Jakob disease is the key mimic, with periodic sharp wave complexes at about one per second and a rapidly progressive course.
- Counsel the family: There is no proven cure, so nursing support, palliation and prevention in siblings are central to the plan.
Worked example: a 9-year-old boy with a measles episode at age three has a year of falling marks and new jerks of the arms. The EEG shows periodic complexes every few seconds, and CSF measles antibodies are raised, so the diagnosis is subacute sclerosing panencephalitis.
Where the examiner tests the disease
Questions usually describe a child with a remote measles history and progressive decline, then ask for the test or the complication.
- Trap: Treating a recent measles infection as the diagnosis. Correct: SSPE follows measles by years, so the typical patient has a remote measles history and a slowly progressive course.
- Trap: Calling the periodic EEG complexes simple epileptic discharges. Correct: Periodic generalised complexes are the characteristic EEG of SSPE, pointing to measles rather than epilepsy alone.
- Trap: Expecting antiviral therapy to reverse the disease. Correct: Inosine pranobex and interferon have only uncertain benefit, so prevention and supportive care dominate management.
- Trap: Overlooking the immunisation record. Correct: The preventive lesson is that timely measles vaccination, given under the Universal Immunization Programme, is the main protection.
Frequently asked questions
What causes subacute sclerosing panencephalitis?
Subacute sclerosing panencephalitis is caused by persistent, defective measles virus in the brain, usually acquired from a measles infection earlier in life. The defective virus evades immune clearance and persists for years. The disease therefore occurs as a late complication of measles rather than during the acute infection.
Which EEG pattern is characteristic of subacute sclerosing panencephalitis?
The characteristic EEG in subacute sclerosing panencephalitis shows periodic, generalised, high-amplitude slow-wave complexes that recur every few seconds. These complexes reflect diffuse cortical involvement from persistent measles infection. Their presence, together with raised measles antibodies in the CSF, supports the diagnosis.
Why is CSF measles antibody testing important in subacute sclerosing panencephalitis?
CSF measles antibody testing is important in subacute sclerosing panencephalitis because it demonstrates intrathecal measles antibody production, the immunological basis of the disease. The antibody level is raised in the CSF relative to serum, while the cell count is usually normal. This result is read alongside the EEG and the clinical course.
How does measles vaccination prevent subacute sclerosing panencephalitis?
Measles vaccination prevents subacute sclerosing panencephalitis because the disease follows natural measles infection, so preventing measles also prevents this late complication. Timely doses of measles-containing vaccine under the Universal Immunization Programme give the strongest protection. Routine immunisation is therefore the most effective preventive measure.