Clinical Neurophysiology Flashcards
What does neurophysiology involve?
- Nerve conduction studies / electromyography (EMG)
- Electro-encephalopathy (EEG)
- Evoked potentials
What is electromyography used for?
To investigate problems of peripheral nerve and muscle
What is electroencephalopathy used for?
- Looks at electrical activity within the brain
* Used to investigate epilepsy and states of altered consciousness
What are evoked potentials used for?
Visual and somatosensory - investigates problems in central pathways
Response in occipital cortex from flashing lights, so looks at pathway all the way from the retina
What is the purpose of nerve conduction studies?
- Conduction velocity
* Amplitude
How do nerve conduction studies work?
Recording electrode is placed onto your skin (usually on your hand, arm or leg) and then another electrode is used to stimulate the skin.
The stimulator produces small electrical pulses which cause muscle contraction.
Look at amplitude of response and delay of where the reponse comes in at.
What are sensory studies?
Similar technique of nerve conduction studies where you stimulate the sensory nerve fibres in the fingers allowing measurement of sensory and motor function separately
Record directly from a nerve, rather than a muscle - allows you to look at sensory fibres rather than motor fibres
What is ulnar neuropathy?
A result of damage to your peripheral nerves, often causes weakness, numbness and pain, usually in your hands and feet
Where does ulnar neuropathy usually occur and what does it cause?
Most often gets trapped at the elbow, occasionally at the wrist
Causing ulnar distribution numbness and wasting of small muscles
What is radiculopathy?
Referred to as pinched nerve, refers to a set of conditions in which one or more nerves are affected and do not work properly (a neuropathy). This can result in pain (radicular pain), weakness, numbness, or difficulty controlling specific muscles.
When carrying out nerve studies, what are you looking for?
Conduction block / slowing due to demyelination
Why does conduction block occur in neuropathy?
- Pressure on nerve –> lose myelin coating of nerve (responsible for saltatory conduction) which causes conduction block so AP cannot propagate passively so AP fades (acute)
- Moves ion channels into demyelinated segment after a few days that can propagate AP, but transmission is very slow in comparison to myelination
Describe electrophysiology in ulnar neuropathy?
Focal demyelination and conduction block in areas of compression
Therefore, nerve studies show the smallest action potentials in that area
Describe the electrophysiology in ulnar neuropathy
- Slowing across elbow
- Evidence of conduction block at elbow
- Small sensory response from ulnar nerve
Therefor, ulnar neuropathy at the elbow
Describe different electrophysiology of radiculopathy
- Motor response are uniformly small, but conduction velocity is the same in all areas
- Sensory response is normal
Why are motor responses small but the same speed, and sensory responses the same in radiculopathy?
Depends where compression is along nerve pathway
If in ramus:
Sensory nerve dies, as it is cut from the cell body, so no nutrients = no response
If cuts through sensory nerve and motor nerve outside of spinal cord:
Sensory cell body lives outside of the spinal cord, the peripheral sensory remains alive, but part of nerve in root is dead
What can neurophysiology be used for?
Investigate nerve function - combination with anatomy allows diagnosis of nerve entrapment and guides subsequent surgery
What is myaesthenia gravis?
A chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles
What are the features of myaesthenia gravis?
- Produces antibodies that bind and block the post-synaptic ACh receptor
- Decreased effectiveness of released ACh
- Present with weakness, fatigue, normal sensation
What are signs of myaesthenia gravis?
Diplopia and ptosis
How is myaesthenia gravis diagnosed?
- Detect antibodies
* Neurophysiology - repetitive stimulation and single fibre EMG
What is EMG?
Routine EMG looks at action potentials from whole motor units
Looks at electrical activity produced by skeletal muscles by placing needle electrode in muscle and records passively when patient contracts muscle voluntarily
What is single fibre EMG?
Uses filter, sensitivity and timebase settings to isolate the action potentials from individual muscle fibres within one motor unit
Why is EMG useful?
Needle can be positioned to record from two fibres within the same motor unit.
Normally little variation in time of firing between two such fibres.
What is jitter on an EMG?
In NM junction disease that tight relationship between the two is lost
The second wave comes in a bit early or late
Why does jitter occur on an EMG?
- Endplate synaptic potential fire AP when reaches threshold
- Usually has a reserve – more ACh released than required for triggering AP, so endplate usually greater then threshold
- In myaesthenia, postsnaptic receptors are blocked by antibodies, so endplate potential is not as big, to this causes AP to fire later
Why does block occur on an EMG in worse myaesthenia?
Strength in muscle still normal as don’t mind that AP firing a little bit late or small
But when EPSP is blocked to such a degree that it doesn’t reach threshold potential –> block –> patient experiences weakness
What can you see with EEGs?
- Epileptic activity
- States of consciousness: sleep, stages of sleep
- Encephalopathy
What can EEGs diagnose?
Look at brain function to allow diagnosis of:
• Epilepsy
• Altered states of consciousness
• Non-epileptic attacks
• Facilitate medical or surgical treatment for epilepsy
