Investigations
1st investigations to order
EEG
Test
Order in the initial assessment of all patients. EEG should be conducted when the patient is sleep deprived, to include 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.
The EEG may be repeated to assess treatment response in patients with childhood absence epilepsy (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]
Result
generalised 3 Hz spike-and-wave for typical absence seizures; generalised ≤2.5 Hz spike-and-wave for atypical absence seizures
Investigations to consider
MRI brain
Test
Required only if the history, clinical course, physical examination, or EEG findings do not fit with typical absence seizures or generalised epilepsy syndromes, or if clinical course is not typical (e.g., a patient with suspected CAE has not responded to first 2 treatment modalities).
Result
usually normal in childhood absence epilepsy (CAE), juvenile myoclonic epilepsy (JME) and juvenile absence epilepsy (JAE); variety of findings ranging from focal encephalomalacia or cortical dysplasia to diffuse cortical malformations may be found in epilepsies such as Lennox-Gastaut syndrome
testing for metabolic disorders (e.g., serum amino acids, urine organic acids, lactate pyruvate or specific enzymatic tests)
Test
Metabolic tests are generally indicated when the clinical and EEG findings do not fit with typical absence seizures or an epilepsy syndrome 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]
Result
variable, depending on specific test
cerebrospinal fluid glucose and serum glucose
Test
Consider 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]
Result
cerebrospinal fluid glucose low; serum glucose normal
genetic testing
Test
Genetic testing is not routinely recommended for typical absence seizures associated with a genetic generalised epilepsy, such as childhood absence epilepsy or juvenile myoclonic epilepsy, because there is presumed complex inheritance potentially involving multiple genes.
Genetic testing should be considered in a child presenting with onset of absence seizures at aged less than 4 years, as 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 SCNA 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 Lennox-Gastaut syndrome.[39]
Result
may be positive
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