Neuropathic pain: Physical examination sensory tests

Hi, I'm Dr Michael Vagg and in this short video I'm going to be demonstrating some of the sensory findings that can help with the diagnosis of neuropathic pain.

The patient who we're going to be examining today is called Matthew.

He's a 45 year old man in good general health who's developed some burning pain down in the left leg into the foot and some back pain as well and that's been there for a few weeks now and seems to be getting a bit worse.

The possible diagnosis of radicular pain is what we're going to test by doing some sensory examination.

Okay so Matt's on the bed here and we're going to be examining his lower limbs from a sensory point of view.

It's always important to remember to test one dermatome against another and one side against the other.

So I'm going to focus on the L4, L5 and S1 dermatomes on each side, which is represented by the medial part of the calf, the dorsum of the foot and the lateral part of the ankle respectively.

We’re going to begin with Light Touch. I like to use the tip of the brush to test light touch, although, if you don’t have a brush, something like a tissue which can be rolled up is fine.

So Matt, can you feel that?

[Matt] Yes

Can you feel me touch you there?

[Matt] Yep

And there?

[Matt] Yep

What about on here?

[Matt] Yeah

And there?

[Matt] Yep

And there?

[Matt] That’s a bit less there

Okay, so that’s a bit less?

[Matt] Yeah

And then

[Matt] That’s more

There compared to there

[Matt] The same

Okay

So it looks as if there may be loss of light touch sensation consistent with an L5 dermatome change.

So I’m going to move on now to testing pinprick sensation.

I like to use a toothpick for this because many patients who may have neuropathic pain will be very sensitive if they have hyperalgesia so you don’t want to use anything too sharp because some of these patients may have a very heightened sensitivity to pain.

I find that using a toothpick creates a sensation of sharpness with very little risk of causing significant harm or distress to the patient.

It’s very important to make sure your testing is consistent, to hold the toothpick so that it slips through your fingers each time that you touch the skin and then that means you’re using a consistent amount of pressure each time.

So we’ll go again through our dermatomes with the toothpick.

Matt, can you tell me, do you feel a sensation of sharpness?

[Matt] Yes

And there?

[Matt] Yes

And here?

[Matt] Yep

And here?

[Matt] A bit less there

That’s a bit reduced?

[Matt] Yeah.

And here?

[Matt] Yep.

That’s normal?

[Matt] That’s normal, yeah.

[Matt] Yeah, that’s normal too

Okay. So again we have a reduction to pinprick sensation consistent with the L5 dermatome on the left.

The last modality I’m going to test for is one of the most important ones when you’re trying to make a diagnosis of neuropathic pain.

If it’s present, it’s a very sensitive sign that there is again a function change in the nerve and it’s highly correlated with the presence of neuropathic pain.

To test we use for this is to run the brush dynamically across the skin.

So rather than just using the end of the brush, one strokes the skin, just with enough force to bend the bristles of the brush.

This is referred to as a test for dynamic mechanical allodynia.

So, through the dermatomes again.

Okay Matt, can you feel me

[Matt] Yeah

touching you there? Does that feel normal?

[Matt] That feels normal, yeah

And on that side?

[Matt] That’s normal

On this side?

[Matt] Normal

That’s L5

[Matt] Yeah

And there?

[Matt] Yeah

[Matt] Oo-yuh!

Okay, so that, can you describe that sensation as I brush there?

[Matt] It’s pain there

So that’s painful?

[Matt] Painful

And right there to the end?

[Matt] Yeah

And then on the outer part?

[Matt] Oh, that’s okay

That’s okay, that feels normal on both sides?

That’s the same on both sides?

[Matt] Yeah

Okay, so we’ve demonstrated in Matt the findings of reduced sensation to light touch, or hypaesthesia to light touch, as well as reduced sensation to pinprick.

And increased sensation to dynamic stimulation with the brush, otherwise known as brush allodynia.

These findings support the diagnosis of nerve dysfunction in the L5 dermatome because they’re anatomically consistent and will then provide you with support for your decision as to whether or not this is neuropathic pain. 

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