What Causes Left Posterior Neck Numbness?
Most probably it is due to cervical spondylitis.
Detailed Answer:
HI
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After going through your query I concluded that probably you are having cervical spondylitis with compression symptoms of nerve roots as it is common in your age. It can be confirmed by cervical spine x ray. Compression can be seen by MRI of cervical spine.
Its treatment is neck exercises, Diacerine (you might need a prescription for this), glucosamine and Mecobalamine. Analgesic such as ibuprofen is useful for pain relief. It is advised to use a thin pillow or no pillow during sleep.
You should exclude other important causes of numbness such as vitamin B12 deficiency by doing vitamin B12 level test and Diabetes by blood sugar level test. You can discuss with your Doctor about it.
Hope your query get answered. If you have any clarification or additional questions then don't hesitate to write to me. I will be happy to help you.
Wishing you a good health .
Take care.