Medical experts in Japan are warning of a surge in "hidden heatstroke" as extreme temperatures trigger a rise in misdiagnosed cases.

This trend is critical because early symptoms are frequently mistaken for general summer fatigue, which can delay life-saving treatment during periods of extreme heat.

On Sunday, temperatures across Japan reached dangerous levels, with more than 300 locations recording temperatures above 30 degrees Celsius [4]. Three locations nationwide exceeded 40 degrees Celsius [3], including Tanabe City in Wakayama Prefecture, which recorded a high of 40.6 degrees Celsius [2]. This marked the first time in five days that temperatures surpassed the 40-degree threshold [1].

Dr. Hiromichi Ito said that the similarity between heatstroke and summer fatigue makes early detection difficult. He said that while general fatigue is common, specific red flags indicate a more serious condition. "If there is a fever and muscle pain, I want people to suspect heatstroke," Ito said [2].

In one case, an 82-year-old patient presented symptoms that were not immediately obvious as heatstroke. Ito said the patient felt a stinging or cooling sensation in the throat, along with a heavy head and dizziness. He said that the patient did not have a cough [1].

These extreme conditions have been observed across the country, particularly in the Kanto and Kyushu regions. High-density events, such as the Nagaoka Fireworks Festival in Nagaoka City, have also seen increased vigilance regarding heat-related illnesses [1].

Doctors said that the inability of the body to regulate temperature during such heatwaves leads to these hidden cases. Because patients may not feel the classic symptoms of a heat stroke immediately, the presence of muscle aches and fever serves as a vital warning sign for those who might otherwise ignore their condition.

“"If there is a fever and muscle pain, I want people to suspect heatstroke,"”

The rise of 'hidden heatstroke' suggests that traditional public health warnings may be insufficient for aging populations or those in extreme urban heat. By highlighting non-traditional symptoms like muscle pain and fever, health officials are attempting to shift the public's diagnostic framework to prevent fatalities during an increasingly volatile summer season.