Ch14 Electrodiagnostics Flashcards
(39 cards)
What are PLEDs?
Periodic lateralizing epileptiform discharges - occur with any acute focal cerebral insult. If bilateral then diagnostic of HSE.
What are the features of PLEDs?
Lateralising or focal, periodic, polyphasic spikes and sharp waves. Ipsilateral background slowing/reduction of activity.
What is the diagnosis in a patient with jerks that are associates with high voltage discharges every 4-15 seconds?
Subacute sclerosing panencephalitis (measles)
What is the diagnosis in a patient with myoclonic jerks in association with bilateral sharp waves 2Hz?
CJD
What are the frequencies of alpha, beta, delta and theta waves?
alpha 8-12Hz, beta >13Hz, delta <4Hz, theta 4-8Hz. Note gamma is >25 Hz
What are the different types of evoked potentials?
Somatosensory evoked potential (SSEPs) - stimulation of peripheral nerves
Visual evoked potentials (VEPs) - flashing lights through goggles
Auditory evoked potentials (AEPs) - auditory clicks
What is a significant change in evoked potentials?
>10% increase in the latency
Drop in amplitude of >50%
How can intraoperative SSEPs be used to identify the central sulcus?
By placing a strip of the motor and sensory cortex and identifying the site of phase reversal
What causes the peaks on BAER?
P1 - Eighth nerve
P2 - Cochlear nucleus
P3 - Olivary complex (superior)
P4 - Lateral lemniscus
P5 - Inferior colliculus
Mnemonic = ECOLI
Interpret this UL SSEP
Top trace is over central sulcus (C3) - N19 - primary sensory cortex (+ so downward deflection) and P22 is motor cortex (- so upward deflection)
Middle trace is placed over the cervical region and shows activity at the root entry zone.
Bottom trace is with electrode placed over the brachial plexus (Erb’s point) and shows the stimulation passing through the brachial plexus
Interpret this LL SSEP
L5-T12 shows activity in the lumbar plexus (positive deflection P22)
Cv7 (cervical electrode) shows activity in the dorsal columns - N27
Cz - shows P40 activity at the sensory cortex
What waveform shows the occipital cortex activation with VERs?
P100
Which nerves are typically stimulated with SSEPs?
Median, ulnar and tibial nerves
How are MEPs generated?
Transcranial stimulation of the motor cortex causing distal motor responses measured with EMG
What are contraindications to MEPs?
Epilepsy
Skull defect
Implanted electronic devices
What actions should be performed if there is an intraoperative change in MEPs or SSEPs?
Vitale checklist:
1 - Technical considerations (check connections and contacts etc)
2 - Pause surgery and alert anaesthetist
3 - Anaesthetic optimization with MAP, pCO2, temperature and pH corrections. Check drugs administered (paralytics / inhalational agents)
4 - Surgical considerations - Remove retractors / last surgical step when monitoring was lost e.g. remove spinal screw etc or papaverin if vasospasm
5 - Stagnara wake up test if all else fails to normalise IONM
What is the Stagnara wake up test?
The anaesthetic is weaned intraoperatively to a point where the patient can obey commands such as moving fingers / toes.
What are the 3 phases on an EMG study?
Insertional activity
Spontaneous activity - muscle at rest
Volitional activity - during muscle activity
What are the spontaneous findings on EMG?
At rest the EMG should be silent
Sharp waves and fibrillation potentials occur after denervation (>4 weeks)
Myotonic discharges
Complex repetitive discharges (neuropathic / myopathic disorders)
Fasciculation potentials (ALS)
What is the difference between fibrillation potentials and fasciculations?
Fibrillation potentials are only found with EMG whilst fasciculations are visible.
How are motor unit action potentials measured?
During EMG volitional activity. Increased amplitude and duration suggests a LMN problem whilst lower amplitudes suggest a muscle disorder.
With minimal effort reduced recruitment suggests neuropathic process whilst increased early recruitment suggests muscle disorder.
Why do disc prolapses not affect the SNAP?
As the compression is pre-ganglionic and therefore the cell bodies in the DRG are not affected. There is no Wallerian degeneration of the distal sensory nerve.
What is the F-wave?
Stimulation of a motor nerve also causes antidromic APs which cause the anterior horn to fire and cause a delayed (much smaller amplitude) orthodromic F-wave. This may be delayed in radiculopathy but is not sensitive.
Normal F-wave <33 ms.
What is the H-wave?
Is a monosynaptic reflex with stimulation of the sensory nerve causing an orthodromic action potential stimulating the gastrocnemius. This is only useful for S1 root and gives the same information as the ankle jerk reflex.
