Hisao's condition became evident when he suffers a heart attack at the beginning of the story when meeting Iwanako. The doctors expressed their amazement that it didn't show itself until he was a teenager. He was diagnosed with it shortly thereafter. He struggled to live with the symptoms for a while after before eventually overcoming and learning to live with them.
Arrhythmia is often detected at an early age, but in Hisao's case, it did not become evident until he suffered a heart attack in his teen years. During moments in the story he would often have heart palpitations, a common symptom that can be dangerous. The heart may beat too quickly, too slowly or erratically.