Brief Answer:
Neuropathy is the term you seek...along with its cause.

Detailed Answer:
What you are describing in neurological terms can be termed a form of NEUROPATHY. This is when nerves in the body malfunction due to a problem and instead of conveying normal signals cause the body (or the brain) to perceive something unusual, bizarre, or completely incomprehensible to the owner of that bad nerve or set of nerves. The sensation you have of cooling numbness in the arm radiating downward is an example of nerves or nerve roots that are being stimulated in an aberrant way due to some extraneous force or problem to give you such a sensation which you know cannot be true since your other arm feels normal temperature and sensation.

A physician, or better yet, neurologist would be the very best specialist you could seek out in order to perform an examination of your neck, shoulder, and arm to see if they can determine what might be causing these strange sensations. Also, depending how the numbness is traveling (to the outside, to the inside, over the top of the shoulder, or through the axilla) it is possible to know exactly which nerves are likely involved so that those nerves could be targeted by testing methods to try and solve the puzzle of cause.

Tests that you should speak to your doctor about would be the following:

1. Serum levels of Vit. B12, Vit. D, and folate
2. Serum levels of thyroid hormones (TSH, FT4)
3. Serum Hemoglobin A1C, ESR, CRP

If you suffer from any neck pain at all that is new or even long standing there could be a compression going on causing these symptoms therefore, a plain film or CT of the cervical spine could lead to the discovery of a pinched nerve or nerve root.

An electrical study known as an EMG/NCV test can be performed by a neurologist who specializes in this type of test which can be very precise at localizing the problem though the cause may not be as easy to detect by this method.

I hope this addresses your concerns and that you'll keep me in mind for future questions regarding these or other neurological/medical issues. If so, I'd appreciate your rating this interaction with a HIGH STAR SCORE and look forward to a few words of feedback. In addition, I'd appreciate your CLOSING THIS QUERY if you're satisfied with my response.

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