Why is JME so often misdiagnosed?

Juvenile myoclonic epilepsy (JME) is a common epilepsy syndrome, which occurs in 7% of all people with epilepsy. If correctly diagnosed, up to 90% of people with JME become seizure-free on a regimen of medication, usually Depakote (valproate). Unfortunately, JME is often misdiagnosed, delaying the start of effective treatment. Even neurologists in excellent epilepsy centers sometimes misdiagnose JME.

Why does this happen, especially considering that people with JME typically show distinctive symptom patterns associated with their seizures and have typical EEG results? Several diagnostic pitfalls have been identified. To complicate the situation, most cases of misdiagnosis involve more than one of these pitfalls:

Inadequate history:
The patient may not report the myoclonic jerks (perhaps thinking they are insignificant), and the doctor may fail to ask about them. It often helps for the doctor to demonstrate what a myoclonic jerk looks like.

Misinterpretation of history:

Doctors may misinterpret myoclonic jerks that involve only one arm, shoulder, or leg as simple partial seizures. Sometimes they also mistake the staring spells of absence seizures for complex partial seizures.

Not the usual symptoms:

The symptoms of some patients with JME do not strictly follow the usual patterns. For instance, seizures may occur throughout the day instead of just first thing in the morning or after a nap.

Misinterpretation of the EEG:

Typically, the EEG of a patient with JME shows generalized discharges of single or multiple spikes and slow waves with a frequency faster than 3 Hz (cycles per second). Sleep and awakening from sleep typically are major triggers for this syndrome, so EEGs that do not include a sleep section may appear normal and lead to misdiagnosis.

Another EEG procedure that sometimes is omitted is photic stimulation (repeated flashing of bright light during the EEG). Photic stimulation will produce typical abnormalities in 25-30% of people with JME.

One other common reason for misdiagnosis is finding an abnormality on only one side of the brain. In this situation, the EEG is incorrectly interpreted as showing a localized abnormality and a diagnosis is made of a type of partial epilepsy rather than JME.

Confusion with other disorders:

In most patients with JME, the neurological exam (including intelligence testing) and neuroimaging (CT scan and MRI, if performed) yield normal results. If one of these tests reveals a mild abnormality, the doctor may incorrectly rule out JME.

The doctor also may incorrectly rule out JME if the patient reports past head trauma, since such injuries often are followed by other types of epilepsy.

Children with a first diagnosis of childhood absence epilepsy also create a diagnostic problem for doctors if they develop myoclonic jerks many years later. It is estimated that 15% of children with typical childhood absence epilepsy will later develop JME.

What's the key to diagnosis?
A correct and timely diagnosis of JME ultimately depends on a good history from the patient or other observers of the seizures, as well as a strong knowledge of the syndrome by the doctor. Such knowledge leads the doctor to look for and correctly interpret the myoclonic jerks that are the key element in JME diagnosis.

The EEG usually is a secondary diagnostic tool because it is often normal and in a few cases it shows only a localized abnormality.