Which of the following relates to theta rhythms?
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp produced by the firing of neurons within the brain.
In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20-40 minutes, as recorded from multiple electrodes placed on the scalp.
It is mainly used as a test to rule out organic conditions. Research has showed it to be of use in differentiating dementia from disorders such as metabolic encephalopathies, CJD, herpes encephalitis, and non-convulsive status epilepticus. EEG may also help to distinguish dementia from pseudo-dementia. Similarly EEG has a role in distinguishing possible psychotic episodes and acute confusional states from non-convulsive status epilepticus.
Not all abnormal EEG's represent an underlying condition, a study of trainee pilots found 0.5% (n=69) to have epileptiform discharges and only 1 of them went on to develop epilepsy. EEG's are affected by psychotropics, this generally manifests as generalised slowing but spike activity is also seen.
Occasionally EEG abnormalities are triggered purposely by the means of activation procedures. These procedures include; hyperventilation, photic stimulation, certain drugs, and sleep deprivation.
Quantitative EEG is essentially an EEG used with a sophisticated computer that is able to do a more detailed analysis of the trace.
Specific wave forms are seen in an EEG and you need to be familiar with these.
| Type | Frequency | Normally found | Normally seen in |
|---|---|---|---|
| Delta | 1-4Hz | Frontally in adults and posteriorly in children | Slow wave sleep and in babies. Should not be present when awake, when present if awake this strongly suggests pathology |
| Theta | 4-8Hz | Generalised | Young children, drowsy and sleeping adults, with certain medications, meditation. Small amount seen in awake adults, excessive amount when awake may indicate pathology |
| Alpha | 8-12Hz | Posteriorly | When relaxed and when the eyes are closed (whilst awake) |
| Beta | 12-30Hz | Frontally | When busy or concentrating |
| Sigma | 12-14Hz | Frontal and central regions | (aka sleep spindles) Bursts of oscillatory activity that occur in stage 2 sleep. Along with k-complexes they are the defining characteristic of stage 2 sleep |
| Gamma | 30-100Hz | No specific areas | Meditation |
Certain conditions are associated with specific EEG changes (see below)
| Condition | EEG findings |
|---|---|
| CJD (sporadic only, does not apply to variant) | Early on there is non specific slowing, later periodic biphasic and triphasic synchronous sharp wave complexes superimposed on a slow background rhythm |
| Huntington's | Low voltage EEG, in particular no alpha (flattening) |
| Delirium | Diffuse slowing, decreased alpha, increased theta and delta |
| Delirium tremens | Hyperactive trace, fast |
| Alzheimer's | Reduced alpha and beta, increased delta and theta |
| Petit mal epilepsy (absence seizure) | Generalised, bilateral, synchronous, 3Hz (3 waves per second) spike and wave pattern |
| Generalised epilepsy | Sharp spikes, 25-30Hz |
| Partial epilepsy | Focal spikes |
| Myoclonic epilepsy | Generalised spike and wave activity |
| Encephalopathy | Diffuse slowing |
| Normal aging | Diffuse slowing, which can be focal or diffuse, if focal most commonly seen in the left temporal region |
Medications can have important effects on the EEG and you must be aware of these.
| Drug class | Effect on EEG |
|---|---|
| Antipsychotics (typical) | Decreased beta with Increased alpha, and delta, haloperidol least effect |
| Antipsychotics (atypical) | Varied effect, clozapine most significant effect |
| Antidepressants | Reduce beta, increase all others |
| Anticonvulsants | No effect |
| Lithium | Slowing |
| Benzodiazepines | Increase beta, decrease alpha |
| Barbiturates | Increase beta |
| Drug of abuse | Effect on EEG |
|---|---|
| Stimulants (cocaine, nicotine) | Increase alpha |
| Depressants (alcohol, opioids) | Decrease alpha |
| Cannabis | Increase alpha |
