4.2 Activation Procedures
Key Takeaways
- Hyperventilation is typically performed for 3-5 minutes (commonly 3 minutes, extended to 5 in suspected absence epilepsy) with at least 1 minute of post-HV recording to capture delayed responses
- The normal HV response is symmetric, generalized slowing called buildup that resolves within about 60-90 seconds after stopping; a sustained or markedly asymmetric response is abnormal
- Photic stimulation is delivered with the patient about 30 cm from the lamp, using brief flash trains stepped across a low-to-high frequency range (commonly ~1-30+ Hz)
- A photoparoxysmal response (generalized spike-and-wave to flash) is the key abnormal photic finding; the technologist stops the strobe immediately when it appears to reduce seizure risk
- Hyperventilation is contraindicated in conditions such as recent stroke, significant cardiopulmonary disease, sickle cell disease/trait, severe hypertension, moyamoya, or recent intracranial hemorrhage
Purpose of Activation Procedures
Activation procedures are maneuvers performed during an EEG to provoke or accentuate abnormalities that may not appear in a resting record. They increase the diagnostic yield of a routine study and are heavily represented on the exam because the technologist controls the technique, monitors safety, and documents the response.
The four classic activations are hyperventilation, photic (intermittent photic) stimulation, sleep, and sleep deprivation. Each has a defined technique, expected normal response, and contraindications the technologist must know before starting.
Hyperventilation (HV)
Hyperventilation (HV) is deep, regular breathing that lowers blood carbon dioxide, causing cerebral vasoconstriction and a mild relative hypoxia that can provoke abnormalities, classically 3 Hz generalized spike-and-wave of absence epilepsy.
Technique:
- The patient breathes deeply and regularly for 3 minutes (often extended to 5 minutes when absence epilepsy is suspected).
- Continue recording at least 1 minute after HV stops (some protocols longer) to capture delayed responses.
- Annotate the start and stop times and describe effort and any clinical or EEG change.
Expected (normal) response: Symmetric, generalized slowing into the theta/delta range, called HV buildup, that is more prominent in children and resolves within roughly 60-90 seconds after stopping. The technologist should recognize that vigorous buildup in a child is often normal.
Abnormal responses: Focal slowing, a clearly asymmetric buildup, sustained slowing that does not resolve, or provoked epileptiform discharges or a clinical seizure (e.g., an absence with 3 Hz spike-and-wave).
Hyperventilation Contraindications
HV transiently reduces cerebral blood flow, so it is contraindicated or used with caution in patients where reduced perfusion or increased cardiopulmonary demand is dangerous.
| Contraindication Category | Examples |
|---|---|
| Cerebrovascular | Recent stroke or transient ischemic attack, recent intracranial hemorrhage, symptomatic carotid disease, moyamoya disease |
| Cardiopulmonary | Significant heart disease, severe asthma or COPD, recent myocardial infarction |
| Hematologic | Sickle cell disease or sickle cell trait (risk of provoking crisis) |
| Other | Severe uncontrolled hypertension, recent subarachnoid hemorrhage, significantly elevated intracranial pressure, inability to cooperate |
When HV is contraindicated, the technologist does not perform it and documents the reason. The exam expects you to recognize these conditions and to stop HV early if the patient develops chest pain, severe dizziness, syncope, tetany, or distress.
Photic Stimulation
Intermittent photic stimulation (IPS) uses a strobe lamp to deliver brief, repetitive flashes that can provoke a photoparoxysmal (photoconvulsive) response, especially in generalized epilepsies and photosensitive epilepsy.
Typical parameters (ACNS-style protocol):
- The flash lamp is placed about 30 cm (roughly 30 cm, ~12 inches) from the patient's nasion.
- Flashes are delivered in trains of about 10 seconds, separated by at least ~7-10 seconds of rest.
- Frequencies are stepped through a range, commonly 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 and on up to roughly 30-60 Hz, typically tested with eyes open, eyes closing, and eyes closed because eye closure can enhance the response.
- Annotate each flash train's frequency and the patient's eye state and any response.
Photoparoxysmal response (PPR): Generalized (or sometimes posterior) spike-and-wave or polyspike-and-wave discharges time-locked to or outlasting the flash. A self-limited posterior response confined to the stimulus may be a normal photic driving variant, but a generalized PPR that outlasts the train is abnormal and carries seizure risk.
Photic Stimulation Safety
The most important photic safety rule on the exam: when a generalized photoparoxysmal response appears, the technologist stops the strobe immediately, because continued stimulation can precipitate a generalized seizure. Identifying the lowest provoking frequency is clinically useful, but patient safety takes priority over completing the frequency series. The technologist documents the response, the frequency, eye state, and any clinical accompaniment, and proceeds only as protocol or the physician directs.
Sleep and Sleep Deprivation
Sleep is itself an activation procedure: drowsiness and sleep substantially increase the yield of epileptiform discharges, particularly in focal epilepsies. The technologist optimizes sleep capture by reducing stimulation and lighting and by recording all sleep transients (vertex waves, spindles, K-complexes, POSTS).
Sleep deprivation is an ordered protocol in which the patient sleeps little or not at all the night before. It increases the chance of capturing natural sleep during the study and independently raises the yield of epileptiform abnormalities. Document the degree of sleep deprivation and whether sleep was achieved during the recording.
Other Activation Methods
- Patterned/visual or specific triggering stimuli may be used for reflex (e.g., pattern-sensitive) epilepsies under physician direction.
- Auditory, startle, or eye-closure maneuvers are sometimes used to elicit specific reflex responses.
- In long-term and epilepsy monitoring, medication reduction and provocation by the supervising physician are activation strategies, but these are physician-directed, not technologist-initiated.
Photic Driving Versus Photoparoxysmal Response
A frequently tested distinction: photic driving is a normal response in which occipital activity follows the flash frequency (time-locked, confined to posterior leads, stops when the flash stops). A photoparoxysmal response (PPR) is abnormal - generalized spike-and-wave or polyspike-and-wave that is not confined to the flash rate and often outlasts the train, carrying seizure risk. Driving = normal and frequency-locked; PPR = abnormal, generalized, and self-sustaining. Mistaking normal driving for a seizure (or vice versa) is a classic trap.
Reactivity Testing as an Activation
In obtunded or comatose patients, reactivity testing functions as an activation: the technologist applies auditory (calling the name, loud sound) and noxious stimuli and documents whether the background changes (e.g., attenuation, increased frequency) or shows no reactivity. Reactivity carries prognostic weight in coma and ICU recordings and helps separate reactive metabolic patterns from fixed epileptiform ones, so it must be performed and annotated deliberately.
The exam expects the technologist to perform standard activations, to know contraindications, and to recognize when a maneuver must be modified or withheld for safety.
What is the expected normal EEG response to hyperventilation, and how should it behave after HV stops?
Each of the following is a contraindication to hyperventilation EXCEPT which one?
During photic stimulation, a generalized spike-and-wave discharge appears that outlasts the flash train. What should the technologist do first?
Approximately how far (in centimeters) from the patient's nasion is the photic stimulation lamp typically positioned?