Every four or five months or so I get blurry peripheral vision.  The
periphery of my vision becomes brighter and reading can become a
little bit harder.  It lasts about 30 minutes than passes.

I have no other symptoms, except perhaps a slight headache, I say
perhaps because I notice a headache this time, but do not recall it
happening before.

Is this a form of cateracts, the fact it passes makes me think not?

Anybody any idea if it could be serious?  It causes me no pain and
vision is only slightly impaired.

Thanks.