But anxiety is a symptom of sporadic fatal insomnia that i know?

Anxiety: Isolated symptom is not sufficient to diagnose a disease. As common as anxiety is, sfi is as rare.

Health Anxiety: Health anxiety can make people fear having serious physical illnesses when these are not actually present. Such people worry a lot about physical sensations, and misinterpret them to mean scary things. They persist in asking about particular rare diagnoses despite repeated reassurance of not having them. Family physicians and psychiatrists can work together well to help such people find relief.