Is numbness the absence of one or all touch sensations?

Question

I am studying the effects of tetrodotoxin and its symptoms when consumed. Numbness is one of the first sensations reported.

But I googled numbness and I couldn't find information about whether this means all touch sensations (e.g. thermal, mechano, etc) have to be absent for numbness or just one specific kind.

My Question:

Can someone

1. Define numbness
2. Explain whether numbness is the absence of one or more than one or all touch sensations.

Answer 1

The definition of numbness has been answered by yourself, and I will focus on the second part of the question.

In terms of the underlying physiological mechanism behind numbness I think it's good to narrow the question down and focus on local anesthetics, which numb a local area of the body for minor surgical procedures. Cocaine and related compounds have been shown to block not only pain, but also warmth, cold and touch perception, as well as blocking motor function. Cocain and related compounds are basically voltage-gated Na⁺ channel blockers, but also block K⁺ channels to some extent. Their affinity to various different channels differs, explaining the differential effects. E.g., pain is blocked more effectively than motor functions (Scholz, 2002).

We all know the feeling of numbness when our limbs are asleep due to restriction of blood flow. Likely all neural activity is blocked here too, as the cutting off of oxygen affects all neurons.

Given the broad range of effects of local anesthetics and restriction of blood flow, I would tentatively conclude that numbness can be interpreted as a reduction of sensitivity of all the senses.

<sup>Reference
- Scholz, Br J Anaesth (2002); 89; 52-61</sup>

Answer 2

An Initial clarification of what "loss of sensation" means:

The history in the patient with “numbness” is extremely important. First of all, as with most neurologic complaints, you must determine what the patient means by “numbness.” Some patients are describing loss of sensitivity (anesthesia or hypesthesia) or distorted sensations (paresthesia), which is often described as tingling. Actual loss of sensitivity is more likely to represent true damage to sensory pathways in the nervous system, while paresthesia has a much broader differential. Dysesthesia, that is the perception of an unpleasant (often burning) sensation or allodynia (the perception of innocuous stimuli as being painful) may result from damage to the nervous system or, more commonly, be the manifestation of an underlying painful condition.

Some properties of numbness

The pattern of numbness may help define its origin. Numbness confined to a specific nerve or nerve root distribution (see the maps of dermatomes and peripheral nerve sensory innervation) lead to consideration of peripheral nerve or nerve root damage. Loss of sensation on one side of the body is more likely to result from central nervous system damage and if the sensory loss also involves the same side of the face, you can be sure that the cause is located above the level of the pons.

When there is a clear sensory level (a line on the body, below which sensations are lost), a spinal cord lesion must be suspected. Loss of sensation over the upper limbs or upper part of the trunk bilaterally, with preservation over the lower limbs and buttocks, suggests an expanding intraspinal mass. This so-called “sacral sparing” is seen with intraspinal tumors or syrinxes. Numbness over one side of the face, while being a relatively common presentation of psychiatric disease, can also be caused by damage to the lateral part of the brain stem, or upper spinal cord (spinal tract of the trigeminal). Lateral brain stem damage can also produce a picture of sensory decrease on one side of the head and on the opposite side of the body.

But the article concludes

In summary, the evaluation of numbness can be quite frustrating due to its subjective nature.

So it is in essence a term that can have specific meanings uncertain contexts, but one must be careful to clarify exactly how the term is being used since there isn't one single meaning.

This is all from this link here https://www.dartmouth.edu/~dons/part_2/chapter_13.html