For the specialties that use neurophysiological services, the numbers we produce in reports
can be daunting and data interpretation is often perceived as a dark art.
However, whilst technical expertise is necessary in these tests , interpretation is usually
straightforward, if you follow a few simple rules.
My name is Simon and I'm a Consultant in Clinical Neurophysiology.
With this video I will kick start a series where I'll take you through some hypothetical
cases and show you how the numbers translate into a diagnosis.
I hope you’ll find it as interesting and fascinating as I do, and enhance your knowledge
and understanding of our specialty.
I have to begin with a caveat emptor, regarding the whole concept of 'Normative data'.
The definition of 'Normal' is hypothetical because it is difficult to provide look up
tables to account for all the variables we know can affect the numbers such
as age, height and limb circumference.
Additionally, all patients are individuals, and what may be normal for one patient may
not be normal for another.
So unless you have pre-existing data for a given patient, you’ll need to be able to
rely on your neurophysiologist to have a good idea of what numbers they were chasing.
Thus, second guessing a conclusion is not a good practice if you
weren’t there at the time.
Having said that, here is an easy way to remember some important numbers which work.
Whilst they are not absolutes, these could easily be applied for many scenario's and
revolve around the number 5.
These may not directly compare with some of the numbers in the literature, but these are
for illustrative purposes only and its better to remember some ballpark figures than none
at all.
So here are some minimum numbers for an average sized person at the age of 40.
As a good rule of thumb, amplitude halves as age doubles.
Lets start with sensory values.
In the hand, we routinely measure the ulnar, median and radial
sensory responses.
These nerves should be, in ascending order, 5, 10 and 15 microvolts in amplitude.
A microvolt is tiny, it's a millionth of a volt.
At the age of 20, these should be double that, so the ulnar, median and radial responses
should be at least around 10, 20 and 30uV, respectively.
By the age of 80, these could be halved to 2.5, 5 and 7.5 uV respectively.
Velocities should be at least 55m/s at the age of 20, 50 at the age of 45 and 40 by the
age of 80.
In the feet we tend routinely measure the sural and superficial peroneal sensory nerves.
Minimum amplitudes at the age of 40 are 10 and 5uV respectively.
At the age of 20, these should be double that, so 20 and 10uV and age 80, you could halve
these to 5 and 2.5uV, respectively.
Because the feet are quite distal, temperatures are lower than in the hand and so the velocities
are always slower than in the hands.
This almost never really matters, as long as they are above 40m/s at 40 and 35m/s at
80.
So to recap, for an average 40 year old we would have the following sensory responses:
(show a slide).
These would cover the absolute numbers.
We also have a relative rule which is that the amplitudes of a nerve response from one
side should not be more than 50% different to the corresponding other side.
Let’s go through the standard motor responses we record.
The first number we record is the distal motor latency or dml for short.
Unlike the sensory equivalent which is a velocity, this is much harder to calculate, as it accounts
for multiple processes.
It represents the time taken for the electrical impulse to travel from the point of stimulation
to the motor point, then the time it takes for Acetylycholine to be released across the
synaptic cleft and then for the muscle membrane to depolarize.
Because any of these can vary, we stick to the absolute time.
This is measured in msec which are thousandths of a second.
However, we can more easily quantify the conduction velocities of the intermediate and more proximal
nerve segments if we subtract the distal segment latency.
As a rule, the more proximal we test, the wider the calibre the nerve becomes and the
warmer it becomes.
Hence, the conduction velocities should always increase the more proximal we test.
In the forearm, these should again be over 50 at the age of 40 and can decrease a little
with age to around 45 at the age of 80.
Motor amplitudes, are measured in millivolts which are thousandths of a volt and so are
1000 times larger than the sensory responses.
These often reflect more than the muscle fibre activation of the muscle being tested.
This is particularly the case for the Ulnar-ADM and Tibial-AH muscles where there is an antenna
effect.
Those studies can capture responses from all the ulnar or tibial innervated muscles respectively
and so the amplitudes are disproportionally large to the actual size of the muscle in
question.
In addition, patients with small hands or feet will have smaller overall amplitudes
but again its easier to remember some numbers than none at all.
So here are some latency values for a typical 40 year old and these are 3.5, 4.5, 5.5 and
6.5 msec for the ulnar-ADM, median-APB, peroneal-EDB and Tibial-AH motor responses.
Unlike sensory values, its much harder to provide minimal amplitudes.
Personally, I would recommend that we consider the average size responses which could be
5mV for the median-APB, 10mV for the ulnar-ADM, 2.5mV for the peroneal-EDB and 5mV for the
Tibial-AH.
By 80, these could all easily halve but at the age 20 could be around 1.5x times the
size.
Any significant deviation from these or asymmetries could indicate a problem.
The final numbers to mention are the F-wave responses.
In the upper limb for a person of average height these should be less than 33msec and
in the lower limbs these should be less than 55msecs.
So to recap, here is some summary data, and whilst these cannot be perfect, they are roughly
in the ball park of what we could expect at varying ages.
In the next video, I'll explain how we interpret the data and then I'll commence a series of
videos with some worked examples.
Thank you for watching and I hope this has been useful.
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Thank you for your support and see you in the next video.