Is betahistine a vestibulosuppressant and how to treat and diagnose presbyastasis?

Betahistine is not a vestibulosuppressant - it does not inhibit the activity of the labyrinth, but improves its blood supply. In high doses, it can be used in the therapy of presbyasia.

Presbyastasis can be considered as a cause of dizziness only after other sources of the observed symptoms have been ruled out. The assessment is done by a neurologist.
According to scientific sources:

"A age-specific cause of vertigo headache is presbyasia, also referred to as multisensory vertigo. It is thought that it may account for about 18% of cases of vertigo in older people. The cause of vertigo in presbyasia depends on the presence of involutional or degenerative changes associated with the aging process vision, vestibular system and sensation proprioceptive. Eye diseases cause significant deterioration of vision in the elderly, and consequently difficulties in judging distances and impaired visuospatial orientation. Age-related changes in the peripheral and central parts of the vestibular system lead to impaired reflexes vestibulo-ophthalmo, responsible for the maintenance of the fixation of the gaze with head movements, as well as abnormal saccades and abduction movements, making it difficult to fixate the gaze on moving objects. Muscle diseases, polyneuropathy or degenerative changes in the joints result in impaired sense of position. As a consequence of the above changes, balance and gait disorders occur, which increases the risk of falls. Presbyastasis can be considered as a cause of dizziness only after other sources of the observed symptoms have been ruled out.

It may be worth consulting an ophthalmologist at the same time.