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Question 1

what are some common causes of chronic patin

Answer 1

trauma: external injury, nerve compression, inflammation

genetic predisposition: inhereted neurodegeneration, metabolic or endocrine abnormailities.

therapeutic intervention (most common): surgery, chemotherapy, irradiation

disease process: infection/inflammation, neurotoxicity, tumor infiltration, metabolic abnormality.

Question 2

describe the process for peripheral nerve regeneration

Answer 2

when a nerve is severed, schwann cells proliferate to attempt to recover the nerve.

could regenerate and remyelinate, or it could cause problems such as neuromas or lack of long distance regeneration

Question 3

For the following types of nerve fibres, what is the information carried,

A-alpha
A-beta
A-delta
C

note that in reality, nerves are not this distinct, there is a alot of overlap between classes.

Answer 3

A-alpha: proprioception

A-beta: touch

A-delta: pain (mechanical and thermal)

C: pain (mechanical, thermal and chemical)

Question 4

describe the difference between

normal nociceptor responses

normal low-threshold mechanoreceptor responses

and central sensitization (Hyperalgesia / Allodynia) responses

Answer 4

Normal nociceptor => pain

Normal mechanoreceptor => touch.

Hyperalgesia: nociceptors have increased response i.e. more pain is experienced than normally

Allodynia: mechanoreceptors trigger nociception. i.e. pain response is triggered from stimuli that do not normally provoke pain.

Question 5

what are some first line treatments for neuropathic pain

Answer 5

Cognitive and Behavioural Modifications

Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

Transcutaneous Electrical Nerve Stimulation (TENS)

Biofeedback

OTC Pain Medications / Rehabilitative Therapy

Question 6

what are some second line treatments for neuropathic pain

Answer 6

Systemic Opioids

Nerve Blocks

Question 7

what are some third line treatments for neuropathic pain

Answer 7

Neurostimulation
Implantable Drug Pumps
Neuroablation

Question 8

what is the gate control theory of pain and describe its biological mechanism.

what is the gate control theory mechanism of action for spinal cord stimulation

Answer 8

The theory that the spinal cord contains a neurological “gate” that blocks pain signals or allows them to pass on to the brain. The “gate” is opened by the activity of pain signals traveling up small nerve fibres and is closed by activity in larger fibres or by information coming from the brain.

Activates inhibitory interneurons by stimulating myelinated a-beta nerves in the dorsal column, thus preventing the transmission of nociception to the brain. The inhibititory interneurons mediate GABA release into the neuron pool, which leads to analgesia.

Question 9

Where was the first spinal cord stimulator prototype implanted

Answer 9

Subarachnoid space (in Cerebrospinal fluid)

Question 10

Describe the placement of the spinal cord stimulator devices over time

Answer 10

Started in CSF, lead to infections

Then sub-dural space

Now in epi-dural space (safest placements)

Question 11

why does the Nevro device stimulate at high frequency (~10kHz)

Answer 11

patients don’t preceive stimulation (paraesthesia free)

At high frequencies, the stimulus current required to generate an action potential drops

but also reduces excitability over time (needs resting period to get same response)

Question 12

where are the electrodes (leads) of SCS implanted

Answer 12

Electrodes are placed in thoracic region (around T7) to stimulate lower back and lower limbs

Question 13

where is the pulse generator of SCS implanted

Answer 13

on lower back between ribs and pelvis

Question 14

what is a Dermatome map

Answer 14

Diagram of the areas of skin innervated by a single spinal nerve

Question 15

Anecdotally SCS is said to work for 65% of
people who achieve 50% or better pain relief.

what are the 3 main issues with SCS products

Answer 15

Unpredictable Benefit: trial stimulation is needed to assess benefit

Unstable Therapy: patient movement produces over-stimulation. long term electrode migration

Invasive Process: long procedure time, paraesthesia and side effects

Question 16

what are Evoked Compound Action Potentials (ECAPs)

Answer 16

Measures strength (voltage) of action potentials directly. indicates total electrical activity of neurons

ECAP = sum of all action potentials that have been evoked by a stimulus pulse

Question 17

why do Saluda devices continually measure ECAPs

Answer 17

to adjust level of stimulation to maintain threshold

Question 18

What does the strength of a-beta recruitment correlate to?

Answer 18

The degree of pain relief

Question 19

what was learned from ECAP experiments

Answer 19

as you increase the stimulus intensity, the CAP increases (recruitment of a-beta axons), and the perception of stimulus increases

amplitude growth curve of stimulus is close to linear (linear relationship between stimulation current and ECAP voltage).

ECAP amplitudes coincide with intervertebral disks.

distance from axon is not linearly correlated with ECAP amplitude because of terminating fibers along the spinal cord (80% of fibers terminate from the dorsal horn to the brain).

Question 20

what is the threshold frequency for producing multiple pulses

Answer 20

Below 1kHz the fibres correspond to the Stimuli

Above 1.5kHz and after ~ 20 pulses the same amplitude ECAP is produced independent of frequency

Question 21

What is the sub and super threshold of frequency?

Answer 21

10kHz

Question 22

what is the Saluda device general control loop (for each stimulation pulse

Answer 22

Capture ECAP –> Compare amplitude with a set point –> Calculate new stimulation current –> Generate new stimuli –> Capture another ECAP

Question 23

why would you want live ECAP data as the SCS is being implanted

Answer 23

to find the right position - this way the surgeons don’t need the patient’s feedback about whether they feel stimulated or not

Question 24

describe closed loop control in the Saluda SCS

Answer 24

periods of the feedback loop are turned off to prevent the influence of overstimulation events, such as coughing or movements

allows therapeutic benefits to increase (% pain relief) for longer periods of time (i.e. it can use higher amplitudes with less side effects)

Question 25

what is the closed loop control response time

Answer 25

about 2-3 Hz, based on a cough

which is one of the most quick responses that affect stimulation.

Question 26

what is Guthega

Answer 26

Acute Trial Device.

Belt worn take home device. Designed for double blind cross over study (30days). Implements full closed loop control. identical firmware to the IPG.

Question 27

what is an ASIC component

Answer 27

This device implements the recording and stimulation functionality required for closed loop control.

Question 28

Outcomes of Saluda device implantation trials

Answer 28

More effective dose of stimulation 24 hours per day = 74% of subjects use the device while sleeping.

Patients are less disabled = 84% of implanted subjects show a significant improvement in disability.

Pain severity and interference is improved = 88% of subjects at 3 months have shown a significant improvement in pain severity and pain interference.

All patients are satisfied or very satisfied.

Patients report no changes in stimulation intensity = no postural effects of stimulation while undergoing daily activities as reported by in-clinic assessment of positional changes and patients feel less stimulation over time.

Patients are less conscious of stimulation over time.

Patients only need minimal changes to their device.