How to distinguish a true Babinski from tickly feet

Switch the sound off and watch the clip above. Is this a true positive Babinksi, or simply a withdrawal response? How did you tell? And could you explain it to someone else?

But everyone has tickly feet…

If you were to stroke the soles of most people’s feet with a blunt object, they wouldn’t like it. Most people will make some movement to get away from the annoying stimulus. The question I posed to the medical students was how do you distinguish a desire to move the foot away from the stimulus from a true positive Babinksi.

The answer comes from reading Joseph Babinski’s original description of the sign. When the lateral aspect of the foot was stroked (not scraped or anything painful) he observed:

In normal people, there was flexion synergy of the thigh, knee and toes

BUT

In those with hemi or monoplegia of central origin there was flexion of the thigh and knee but upwards extension of the toes.

Why so?

Let’s go back to being a newborn baby. If there’s something annoying on your foot, you want to shorten your limb to get away from it. The spinal cord has this built in functionality out of the box, and has a reflex to deal with this situation without any input from the pyramidal tract. This involves flexing everything in the lower limb except your toes, which you extend. This is called flexion synergy.

Why is it called flexion synergy when the toes are clearly not flexing? The toes are clearly not in synergy with everything else.

True. When you think things through in limb anatomy, the naming conventions make no sense (e.g. naming movement of the thumb). What we call extending the toes makes the limb shorter, which is unusual for extension. The built in spinal cord level response is a flexion withdrawal and this involves extending the toes to shorten the limb. Maybe it was all flexion once upon an ancient evolutionary path when our toes pointed in a different direction.

So what happens as we age?

Around the time the infant starts walking, the pyramidal tract matures enough to suppress this flexion synergy response. The contraction of the tensor fascia lata of the thigh and the knee may remain, but the toes no longer respond as part of the flexion synergy.

The toes don’t respond? I thought they were down going normally?

They don’t respond as part of the flexion synergy. The reason they go downwards is to do with a local cutaneous reaction. That is why if you elicit the Babinski using an alternative method e.g. Oppenheimer’s sign, you don’t get the toes curling down but you would get the other components of the sign.

Hold on, Oppenheimer?

There are over 30 ways of eliciting the Babinksi. I have no intention of learning them all, but the Oppenheimer is a useful alternative to have in your arsenal as it doesn’t elicit a tickly response so can be helpful for sensitive soles.

I see what you did there with the last two words of that paragraph.

Thank you.

So what features help distinguish a true positive Babinski?

1. Upward movement of the toe is pathological only if caused by contraction of extensor hallicus longus muscle. Beware of labelling upwards movements of the toes due to ankle flexion as a Babinski.

2. Contraction of extensor hallicus longus muscle is pathological only if it occurs synchronously with reflex activity in other flexor muscles. The easiest one to observe is the tensor fascia lata of the thigh – see you the YouTube video.

3. A true up-going toe sign is reproducible, unlike voluntary withdrawal. I was often taught as a medical student that you can ‘tire out’ a Babinski. In reality, you are much more likely to tire out a voluntary withdrawal than a Babinski.

Go back to the video at the top that demonstrates a very clear true Babinski. See if you can identify features one and two. Note how the flexion synergy occurs at the same time as the great toe extension.

Further reading:  An excellent review by Van Gijn

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