Why should valium not be used if i have narrow angle glaucoma?

Valium can be used: I am not aware of any connection between glaucoma and the use of valium.

Pressure can rise: Valium is in a class of drugs that the fda has listed as a contraindication because of a potential further reduction in the angle of the eye responsible for passage of the eye fluid. If that angle is already narrow, any further narrowing may result in blockage of fluid flow causing the eye pressure to build up and cause redness, pain, and glaucoma damage to the eye nerve, which can be permanent.

It depends: Valium belongs in a class of medications that, in theory, could precipitate an angle closure attack (as a weak anticholinergic). In practice, this is not an absolute contraindication. What is more important is to ensure that your narrow angles are managed by your ophthalmologist (either with laser+/-medication).