Not All EEGs Are the Same: Matching the Test to the Question

BLOG SERIES: "What Your Child's EEG Really Means" — Post 4 of 6

Not All EEGs Are the Same: Matching the Test to the Question

An abnormal EEG in a child who has never had a seizure is one of the most common — and most misunderstood — findings in pediatric neurology. For children with autism, ADHD, or cerebral palsy, EEG abnormalities are far more common than most families realize, and far less predictive of epilepsy than most assume.

EEGs vary significantly in length and intensity, and the right test depends on the clinical question being asked. A routine recording is the appropriate starting point for most children. Extended multi-day studies serve a specific purpose — identifying seizures that are suspected but not yet documented — and are not a general screening tool. Applying them indiscriminately adds cost, burden, and interpretive complexity without improving care.

The Yield Curve: Why Longer Isn't Always Better

The incremental diagnostic yield of ambulatory EEG follows a well-established pattern of sharply diminishing returns:

Recording Duration	Cumulative Yield	Incremental Gain
First 8 hours (incl. first sleep)	~55%	—
24 hours	~83%	+28%
48 hours	~91%	+8%
72 hours	~95%	+4%

The single highest-yield period in any ambulatory EEG recording is the first night of sleep. NREM sleep powerfully activates epileptiform discharges — which is why a well-executed 24-hour study that captures one full sleep cycle answers the clinical question in the vast majority of outpatient neurodevelopmental evaluations.

When Is a 72-Hour Study Genuinely Indicated?

Extended ambulatory recordings are a valuable tool in specific, well-defined clinical situations:

•       Seizure frequency is estimated at less than once per 24–48 hours, making capture in a shorter window unlikely

•       A prior 24-hour study was non-diagnostic and clinical suspicion of seizures remains high

•       Quantifying spike-wave index across multiple nights for ESES/CSWS evaluation (see Post 6)

•       Pre-surgical epilepsy workup requiring extended characterization of seizure onset

A longer EEG is the right answer when you have a specific, unresolved clinical question that a shorter study cannot address. It is not a default — and not a substitute for clinical reasoning.

When a 72-Hour Study Is Difficult to Justify

Routine ordering of multi-day ambulatory EEG as a first-line screen — without a prior standard study, and without a specific clinical hypothesis — is not supported by the evidence base and creates real burdens for families:

•       Longer recordings generate more biological noise and low-amplitude transients of uncertain significance — increasing equivocal results, not diagnostic clarity

•       Significant physical and behavioral burden for the child (addressed fully in Post 5)

Questions Families Can Ask

Before agreeing to a prolonged EEG study, families are entitled to ask:

•       What specific clinical question is this study designed to answer?

•       Has my child had a standard or 24-hour EEG first?

•       What symptom or clinical finding makes you suspect seizures that haven't been seen yet?

•       What will we do differently based on the result?

These are not adversarial questions. They are the questions that good clinical practice should be able to answer before any test is ordered.

References

1.     Faulkner HJ, et al. Continuous EEG monitoring: indications and practical considerations. Seizure. 2012. https://pubmed.ncbi.nlm.nih.gov/22483864/

2.     American Clinical Neurophysiology Society. Guideline 8A: Ambulatory EEG. 2008. https://www.acns.org/practice/guidelines

3.     Canitano R, et al. EEG abnormalities and ASD: a qualitative review. PMC 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC10779511/

4.     Tomas Vila M, et al. Epilepsy and interictal epileptiform activity in ASD. Epilepsy Behav. 2019. https://pubmed.ncbi.nlm.nih.gov/30611007/

➡ Up next: Post 5 addresses something rarely discussed with families before they agree to a study: what 72 hours of electrodes actually feels like for a child with sensory sensitivities.