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Heatstroke is essentially a heat disease of summer. However, there are cases where illness arises from directly contracting the pathogenic factor of Summerheat (Shu), and cases where illness arises in the summer environment due to cold contraction caused by seeking coolness. Although the etiologies differ, both originate from the summer season. Therefore, Summerheat diseases are divided into two syndromes: Yin Summerheat and Yang Summerheat. Their treatments are as opposite as ice and charcoal, and they must be distinguished carefully.

Yin Summerheat refers to contracting cold pathogens during the summer. People, fearing the heat and seeking coolness, fail to avoid cold. They may stay in deep halls or large buildings, sit in windy places or under tree shade, or dress inadequately during times of fluctuating temperatures. This allows cold pathogens to invade the muscle and surface, resulting in symptoms such as fever, headache, absence of sweating, aversion to cold, bodily stiffness, and limb soreness. Because this occurs in the summer month due to cold exposure, it is named Yin Summerheat; it is essentially Cold Damage (Shanghan). Treatment should primarily focus on warming and dispersing, following the methods for treating Cold Damage. Additionally, some individuals who are careless with their diet consume excessive raw and cold foods, causing cold damage to the zang-organs, leading to symptoms like vomiting, dysentery, diarrhea, and abdominal pain. This is also due to cold contraction in summer, but since the cold pathogen is internal, treatment should prioritize warming the middle jiao. This also belongs to the category of Yin Summerheat.

Yang Summerheat refers to contracting heat pathogens due to summer conditions. In Zhang Zhongjing's works, this is termed "Zhong Ye" (Heatstroke). When working hard in the scorching sun or fields without regard for fatigue, toxic heat damages yin fluids, resulting in symptoms such as headache, irritability, high fever, extreme thirst, profuse sweating, floating pulse, panting, or lack of energy to move. Because this occurs in the summer month due to heat exposure, it is named Yang Summerheat. Treatment should assess the deficiency or excess of qi and the severity of fire, employing either tonification or clearing methods to consolidate qi. This differs significantly from the treatment of Yin Summerheat. If one treats based solely on the name "Summerheat" without distinguishing exterior/interior or yin/yang, it will cause severe harm to patients.

First Point: Yin Summerheat syndromes may manifest in the exterior or the interior. They are common among wealthy and sedentary people, caused by indulging in comfort and failing to guard against wind-cold. Yang Summerheat syndromes are common among laborers, caused by unavoidable exposure to summer heat. When summer heat presses upon people, they fear it and can avoid it, so few contract it. However, when yin-cold invades, it feels pleasant and goes unnoticed, so many contract it. Thus, among those suffering from Summerheat diseases, Yang Summerheat is rare, while Yin Summerheat accounts for eight or nine out of ten. Nowadays, practitioners see symptoms like fever and headache and immediately diagnose it as Summerheat, using only cooling herbs. This demonstrates a profound lack of understanding.

Second Point: The disease named Cold Damage (Shanghan) involves cold pathogens, yet its names vary. The condition named Injury by Summerheat (Shangshu) involves Summerheat pathogens, yet its manifestations vary. Regarding the varying names of Cold Damage: winter cold is called Proper Cold Damage; spring warmth is called Warm Disease; summer heat is called Heat Disease. Thus, Warm Disease and Heat Disease are also aliases of Cold Damage. The Classics state: "If one is injured by cold in winter, one will suffer from Warm Disease in spring." It also states: "For those whose Cold Damage transforms into Warmth, if it occurs before the Summer Solstice, it is Warm Disease; if after the Summer Solstice, it is Heat Disease." This explains the variation. Regarding the varying nature of Injury by Summerheat: contracting cold due to summer leads to cold injuring the form, which is Cold Damage; contracting heat due to summer leads to heat injuring qi, which is Injury by Summerheat. This distinguishes the different types of Summerheat diseases involving internal injury and external contraction. The Classics state: "Excess qi with cold body results from Cold Damage; deficient qi with hot body results from Injury by Summerheat." Excess qi with cold body means the body has contracted cold while qi remains intact, hence Cold Damage. Deficient qi with hot body means the body has exposed itself to summer heat, and heat has damaged qi, hence Injury by Summerheat. Many do not understand this principle, mistakenly believing that Cold Damage always presents with a cold body, which is logically inconsistent and contradicts the Classics.

Third Point: During the peak of summer heat, the body should have slight perspiration; this is the way to preserve health and aligns with the season. If one insists on having no sweat at all, it implies an excess of yin over yang, often leading to disease. The Classics state: "Summerheat should be expelled along with sweat; do not stop it." This indicates that summer sweat should not be suppressed. It also states: "If one does not sweat during summer heat, wind-malaria will develop in autumn." This indicates the pathology of not sweating during summer. Thus, the propriety of sweating in summer is clear.

Fourth Point: The continuation of the discussion on latent yin in summer is found in Volume II, "Chuan Zhong Lu".

Fifth Point: There are eight signs of Summerheat: deficient pulse, spontaneous sweating, body heat, coldness of the back, greasy face, irritability and thirst, slightly cold hands and feet, and heavy body. Treatment should primarily regulate primordial qi, with clearing and draining as secondary measures.

Sixth Point: For those who have died from heatstroke, one must not expose them to cold; exposure to cold will cause death. One should only use warm objects to protect the umbilicus and treat the patient gradually.