Specialist Assessment Techniques Flashcards
What is an EEG?
Electroencephalography. Takes 30-40 minutes, but around an hour to complete including set up. Odd numbered electrodues are the left hemisphere, and even numbered electrodes are for the right hemisphere. Electrodes are connected to EEG machine via wires which then amplifies and records signals
How are EEGs interpreted?
Individual signals are shown in a few microvolts in amplitude. Test of electrical activity. Artefacts due to blinks or movements must be avoided or rejected, sometimes by videoing the test to check what was going on at points of activity to discount irrelevant changes
What can abnormal results on an EEG test be due to?
An abnormal structure in the brain, such as tumour. Attention problems. Cerebral infarction or haemorrhage. Drug or alcohol use. Head injury. Inflammation of the brain (encephalitis). Encephalopathies such as Creutzfeldt-Jakob disease. Seizure disorders such as epilepsy. Sleep disorders such as narcolepsy
What is a normal EEG like?
Brain activity has certain frequencies that are normal for different levels of consciousness. Brain waves are faster when you are awake than asleep, and there are normal patterns to these waves
What are the main EEG rhythms?
Delta, theta, alpha, low beta and high beta
What are delta rhythms?
9.5 to 3.5 Hz. Associated with stage III and IV sleep
What are theta rhythms?
4 to 7 Hz. Accompanies drowsiness, the transition to sleep, and REM sleep
What are alpha rhythms?
8 to 13 Hz. Present in about 75% of adults when they are calm and awake, and is replaced by faster beta rhythm when move, read or solve problems
What are low beta rhythms?
13 to 21 Hz. Appears when you focus your attention
What are high beta rhythms?
20 to 32 Hz. Appears when you experience anxiety and worry
Why is it important to keep patients calm during an EEG?
A patient that may have low frequency, may actually show normal EEG results if they are anxious
What do abnormal EEG results look like?
Generalised changes in frequency of electrical activity (abnormal waveforms detected). These are changes in all channels that show whole brain is affected. Or there can be focal areas of abnormal activity eg local defects such as infarction, trauma or tumour (non-epileptiform activity)
What is epileptiform activity?
Epileptiform transients are evidence of seizure
What are some limitations of EEG?
Sensitivity and spatial resolution in detecting focal brain lesions is poor relative to modern neuroimaging. Also cannot explore all brain areas such as deep structures like the thalamus or basal ganglia, and lesions of the temporal lobe (under surface) may not be evidenced in a surface EEG
What are seizures?
Caused by a sudden burst of excess electrical activity causing a temporary disruption in the conduction of signals. Experienced by 5% of the population
What is epilepsy?
Tendency to have recurrent seizures (1% population), and can by symptomatic (caused by something else) or idiopathic (no known cause)
What are two types of seizure?
Partial (local) or generalised. Symptoms experienced depend on where the epileptic activity begins
What are two types of partial seizure?
Simple and complex
What are simple partial seizures?
Consciousness is spared and manifestations (aura) depend on locus. Symptoms include phosphenes in contralateral visual field, visual hallucinations, hearing simple sounds or hearing difficulties, rhythmic twitching (clonus) or sustained contract (tonus), panic and dejavu
What are complex partial seizures?
Impairment of consciousness. Consciousness most likely affected due to seizure affecting wider regions of the cortex or brainstem. Symptoms are initially an aura, followed by loss of awareness and automatisms (involuntary/uncontrollable)
What are generalised seizures?
Most common type is tonic-clonic. Start with tonic phase (loss of consciousness, generalised contraction of all muscles, lasts around 15 seconds), then the clonic phase (rhythmic jerking of extremities for around 1-2 minutes)
How can EEG be used for seizures?
They cannot diagnose epilepsy but can provide useful information about type, locus or triggers. Frontal spike/abnormality may indicate irritating lesion affecting cortex. Generalised abnormalities may suggest primary epilepsies. But not all people with epilepsy have abnormal EEG so an in-depth history is essential
What are two special types of EEG?
Sleep EEG (complimentary to routine EEG or to test for sleep disorders) and ambulatory EEG (record activity through day and night over couple of days, and record activities to match EEG traces)
What is CT?
Computed tomography. CT computes absorption of x-rays at different points. Absorption of x-rays is proportional to density of the structure
How are CT scans interpreted?
In terms of density. Dense brain components appear bright and less dense elements appear dark
What would show up bright on a CT scan?
Bone, blood, and grey matter (cortex and subcortical nuclei)
What would show up as dark on a CT scan?
Cerebrospinal fluid, white matter, and infarcted tissue
How do CTs show ischaemic stroke?
Ischaemic infarctions are often not visible in first 6 hours (hyperacute stages). After that time there is an area of hypodensity. Features include wedge-shaped with linear borders, homogenous area of hypodensity (dark), and will be defined by vascular territory
How do CTs show haemorrhages?
When fresh it will be bright white (hyperdense). One week later it will be isodense. 2-3 weeks later it will be dark (hypodense)
What are possible risks of CT?
Exposure to ionising radiation, which has a very low risk of damaging brain and DNA. Contrast agents carry a small risk of causing an allergic reaction. In most cases, the benefits of identifying a neurological disease with imaging far outweigh the risks
What are MRIs?
Magnetic resonance imaging. Records the reflected electromagnetic signal from protons in water molecules. Person placed in static magnetic field and proved with pulses of radio-wave magnetic energy. When pulse stops, protons relax and give off energy in form of radio frequency magnetic energy
What are advantages of MRI?
Produces high resolution images of brain in all three slice orientations. Also has excellent spatial resolution
What are two main types of MRI?
T1 and T2
What is a T1 weighted MRI?
Shows CSF as dark. Grey matter as grey. White matter as pale grey. Infarcted tissue as dark. T1 can be useful for identifying lesions and tumours
What is a T2 weighted MRI?
CSF as light. White matter as dark. Contrast between grey and white matter is reduced but certain pathologies can be enhanced like oedema. T2 useful in diagnosis of MS (discrete ovoid lesions around ventricles, but problem with lack of specificity)
What are clinical uses of fMRI?
Functional imaging may provide valuable prognostic information for patients with disorders of consciousness. Correlation between fMRI responds to sounds/speech and likelihood of recovery from coma
What are the possible risks of MRI?
Allergic reactions are extremely rare with MRI contrast. Claustrophobic patients often find them challenging. MRI scanner acts as a powerful magnet so it has the potential to physically move objects that contain iron (not possible in some patients, eg if have pacemaker)
When are CT preferred over MRI?
Preferred for head trauma and cranial fractures, suspected intracerebral or subarachnoid haemorrhage, detection of metal foreign bodies, and as a first screening method for most lesions, especially in emergency settings. Also it is quicker, less claustrophobic, cheaper and not limited by electrical equipment or metal
When are MRIs preferred over CT?
When on basis of patient history and exam, there are suspected low-contrast lesions, brainstem or skull-base lesions or a tumour. Used as a secondary technique when lesion is suspected but not visible on CT. Preferred when multiple imaging is required due to no ionising radiation. Also gives clearer imaging of basilar structures and has higher contrast
