Thank you for your query.
1. The most common causes for this would include:
a. Muscle spasms pressinfg on nerve
roots or causing nerve entrapment
b. Cervical spondylosis
c. Raised intra-cranial tension.
2. Get your Cervical Spine
X-rays done. Get a Neurologist's opinion to identify the exact dermatome (area). This will help identify the nerve supply involved.
3. Anti-histamines will help reduce the sneezing
while the investigations are going on.
4. Does this also happen while coughing?
8. Avoid coughing hard, sneeze with the mouth open and use a thinner pillow.
I hope that I have answered your queries. If you have any further questions, I will be available to answer them.