Approach

The critical components to the diagnosis of an absence epilepsy syndrome are a detailed description of the patient's unusual episodes and the EEG characteristics.

History

A detailed description of the unusual episode is essential, preferably from witnesses as well as the patient. A video recording of an episode can be very useful. Important features to assess include:[9][29][30]​​​​​

  • Patient's activity at onset: behavioural arrest or staring; interruption of otherwise normal activity

  • Simple or complex automatisms: any associated movements of eyes, face, and hands[31]

  • The duration of an event: typically lasting 5 to 10 seconds (range from 3 to 20 seconds)

  • The frequency of events: several per day, but may be under-recognised

  • No aura and minimal to no postictal state should occur

  • Age of onset; childhood absence epilepsy (CAE) has a typical age of onset between 4 and 10 years of age is pathognomonic, and juvenile myoclonic epilepsy (JME) has a typical age of onset between 9 and 13 years.

  • Birth and developmental history, including specifically any history of learning disabilities or behavioural problems such as attention deficit hyperactivity disorder (ADHD), as well as any history of seizures of any type, are also significant in considering the specific classification of the electroclinical syndrome.

Family history should be ascertained. Key risk factors for the development of absence seizure include family history of CAE or JME.[9]

Physical Examination

For CAE, juvenile absence epilepsy (JAE), and JME, a patient should typically have an entirely normal physical examination. However, hyperventilation is an easily performed manoeuvre that often will trigger absence seizures and can be diagnostic on clinical grounds.[9][30]​​

If the patient has evidence of cognitive impairment or abnormalities in muscle tone or tendon reflexes, an epilepsy syndrome associated with an identifiable aetiology (e.g., genetic, structural, metabolic), such as Lennox-Gastaut syndrome (LGS), is probable. Abnormal physical or cognitive findings indicate the need for further diagnostic work-up, such as brain MRI or metabolic and genetic testing.

EEG

An EEG should be ordered in the initial assessment of all patients. The EEG should be conducted when the patient is sleep deprived, to include those periods of time when the patient is alternately awake and asleep.[32][33][34]​​​​​ It is essential to ask the patient to hyperventilate for 3 minutes in order to induce an absence seizure, a diagnostic finding.

The EEG may be repeated to assess treatment response in patients with CAE.[34]​ There is some suggestion that normalisation of EEG correlates with greater likelihood of resolution of CAE. Patients with longer seizures at baseline may have more favourable initial treatment response, but are at greater risk for inattention.[35]

For typical absence seizures, a classic 3 Hz generalised spike-and-wave pattern, often activated by hyperventilation, is considered the most specific and sensitive test confirming a diagnosis of absence seizures.[34]​ [Figure caption and citation for the preceding image starts]: 3 Hz generalised spike-and-wave pattern on EEG pathognomonic for typical absence seizures and childhood absence epilepsyFrom the personal collection of Dr M. Wong; used with permission [Citation ends].com.bmj.content.model.Caption@15b70757​ For atypical absence seizures, a slow (<2.5 Hz) generalised spike-and-wave pattern is characteristic.[14][34]​​​​ [Figure caption and citation for the preceding image starts]: Slow (<2.5 Hz) generalised spike-and-wave on EEG associated with atypical absence seizures and Lennox-Gastaut syndromeFrom the personal collection of Dr M. Wong; used with permission [Citation ends].com.bmj.content.model.Caption@1ff5ca01

Magnetic resonance imaging (MRI) brain

A MRI scan is required only if the history, clinical course, physical examination. or EEG findings do not fit with typical absence seizures or generalised epilepsy syndromes.

Metabolic testing

Metabolic tests are generally indicated when the clinical and EEG findings do not fit with typical absence seizures or typical epilepsy syndromes, but suggest a symptomatic aetiology. There are a broad variety of metabolic tests that can be performed and these need to be tailored to the individual patient. Possible metabolic disorders causing atypical absence seizures include aminoacidurias, organic acidurias, mitochondrial disorders, and lysosomal storage diseases.[36]

Testing of cerebrospinal fluid glucose and serum glucose should be considered for patients with typical absence seizures that began before age 4 years or with intractable absence epilepsy, to evaluate for glucose transporter type 1 deficiency syndrome (GLUT1-DS).[26]​​

Genetic testing

For typical absence seizures associated with a genetic generalised epilepsy such as CAE or JME, genetic testing is not routinely recommended because there is presumed complex inheritance potentially involving multiple genes. In CAE, pathogenic variants have been infrequently reported in several genes including GABRG2, GABRA1, and SCN1A.[9][37]​​​ Genetic testing may be considered in certain situations, such as in patients with developmental delays, intellectual disability or autistic spectrum disorder, those with drug-resistant seizures, or in patients with a family history of epilepsy. Patients in whom onset of absence seizures occurs before age 4 years should be tested for GLUT1 deficiency, because 10% of these patients will have pathogenic variants in SCL2A1 resulting in GLUT1-DS.[9][26][38] ​​In instances with a characteristic family history of other generalised epilepsies (generalised epilepsy with febrile seizures plus [GEFS+]), commercial testing for SCN1A gene mutations may be indicated. As more genes are identified for these syndromes, this may become more common. Genetic testing should also be considered in patients with developmental epileptic encephalopathies and neurodevelopmental disabilities, including syndromes involving atypical absence seizures such as LGS.[14][39]​​​​​

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