Whenever i have muscle spasm in neck i develop headache and blurred vision. why?

Cervicogenic HA: Is a very common headache secondary to neck involvement. Cervicogenic headache is a unilateral pain provoked by neck movement or by pressure on the neck. These headache patients usually had a reduced cervical range of motion in association with the headache, dizziness, blurred vision, etc. Rec.: see orofacial pain or headache specialist for evaluation and treatment.