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Syndrome differentiation according to the six meridians

When a TCM physician consults patients, the first step is to differentiate exogenous disease (illnesses originating form outside of the body) from endogenous disease s (illnesses originating form inside the body); because, they have different principles of diagnosis. There are two common methods for identifying exogenous diseases. These are "syndrome differentiation according to the six meridians" and "syndrome differentiation according to defense(wei), vital energy(qi), nutrient(ying) & blood(xue)" discussed in the next section.

Zhang Zhongjing (150~219AD)
The six meridian method was developed early in the Eastern Han Dynasty by Zhang Zhongjing (150-219AD) in the book of Shanghan Zabzabinglun (Discourse on Fevers and Miscellaneous Illnesses), which is a monograph about many types of external infectious diseases. Later this book became an important guide for identifying this kind of diseases. The method investigates the pathological changes over the course of an exogenous disease, especially for cold induced diseases referred to as "shanghan".

Based on the distribution of the three yang meridians (Tai Yang, Yang Ming, Shao Yang) and the three yin meridians (Tai Yin, Shao Yin, Jue Yin), symptoms and signs of exogenous diseases are classified into six stages that correspond to one of these particular meridians. The three yang meridians are located on the exterior parts of the body and as a disease or evil progresses the disease's symptoms will progress along the more interior yin meridians. Therefore, a TCM practitioner is able to identify what stage of a disease a patient has by the signs and symptoms displayed upon one of these six meridians. The more interior the disease has progressed the more severe the illness and damage to the organs.

In cold-induced exogenous diseases (shanghan), the external cold and wind evils enter the body through the skin and muscular striae. They pass along the meridians and eventually attack the organs. There are particular pathways of transmission for the evils that invade the body. They usually pass from the exterior parts of the body to the interior and in this process gradually deplete the body of its qi (vital energy).

In exogenous disease, the usual progression of disease along the meridians is:

Tai yang (small intestine & bladder meridians)
Yang Ming(stomach & large intestine meridians)
Shao Yang (gall bladder & triple burners meridians)
Tai Yin (lung & spleen meridians)
Shao Yin (heart & kidney meridians)
Jue Yin (liver & pericardium Meridians)

Tai yang disease
refers to conditions caused by exogenous evils invading the body surface; which is also known as exterior syndrome.

Yang Ming disease refers to conditions where exogenous wind and cold evils have converted into heat evils and transmit directly into the interior, attacking the Yang Ming meridians, which run through the stomach and intestines. This may also be caused by delayed treatment leading to body fluid consumption. Yang Ming diseases exhibit the greatest conflict between evils and the body's disease preventing forces.

Shao Yang disease refers to conditions due to unrelieved exterior syndromes that have transmitted into the interior. The evils turn toward the interior but have not completely reached the interior. The pathological changes are neither on the exterior nor in the interior but stay in between. Areas like the gall bladder and stomach are affected. They are known as half exterior-half interior syndromes.

Tai Yin disease refers to a cold-deficient(xu) type condition resulting from a deficiency of spleen qi and retention of cold-dampness in the interior. It is mostly due to cold evils attacking the middle burner in a constitutionally weak patient, or from delayed treatment of the three yang meridians diseases.

Shao Yin disease refers to pathological changes in the heart and kidneys, which are extremely deficient conditions and characterized by systemic weakness.

Jue Yin disease indicates the terminal stage of the six-meridian diseases, in which the body's disease preventing forces are exhausted. There is a derangement in the yin yang balance. This condition is complex with syndromes of extreme heat or extreme cold, or syndromes of alternating cold and heat.

Syndromes of the six-meridians
Diseased meridian Presentations
Tai Yang Wind invasion Fever, aversion to wind, headache and spontaneous sweating. On examination, the pulse is floating and slow
Cold invasion Fever, aversion to cold, painful and stiff neck, body aching, unable to sweat and asthma. On examination, the pulse is floating and tense.
Yang Ming Meridian type High fever, profuse sweating, extreme thirst, flushed face and chest fullness. On examination, a dry, yellow tongue coating and a floating and forceful pulse are present.
Fu-organ type Feverishness of body, which is more pronounced in the afternoon, sweating, constipation, abdominal distention and pain that worsens when pressed, restlessness and delirium. On examination, a dry yellow tongue coating or yellow coating with thorns on the tongue and a deep and forceful pulse is present.
Shao Yang Bitter taste in mouth, dry throat, blurred version, alternating chills and fever, fullness in chest and under ribs, poor appetite, chest fullness and nausea. On examination, a white tongue coating and taut pulse is present.
Tai Yin Abdominal distension with a preference for warmth and feels better when pressed, vomiting, anorexia and diarrhea. On examination, a pale tongue with a white coating and a slow and weak pulse is present.
Shao Yin Cold type Aversion to cold, spiritual fatigue, cold limbs, diarrhea, stool containing undigested food, nausea, thirst, preference for hot drinks and profuse and clear urine. On examination there is a pale tongue with a white coating and a deep and weak pulse.
Heat type Irritability, insomnia, dry mouth and throat and yellow and scanty urine. On examination there is a red tongue tip or a deep red tongue with little coating and a deep, thready and rapid pulse.
Jue Yin Extreme cold type Cold limbs, low body temperature and aversion to cold. On examination, there is a pale tongue and an extremely thready, feeble pulse that can hardly be felt.
Extreme heat type Cold limbs, irritability, feverish sensation, thirst and dark yellow urine. On examination, there is a yellowish tongue coating and rolling pulse.

According to the eight principles theory, diseases in the yang meridians belong to the exterior and are more often seen as heat and excess(shi) syndromes. Diseases in the yin meridians belong to the interior and are more often seen cold and deficiency(xu) syndromes. When there are changes in any of these six meridians, symptoms associated with the malfunctioning of the connected organs or other interrelated meridians will appear. The six-meridian method is regarded as an incomplete method for identifying all organs or meridians diseases; because, this method mainly focuses on cold-induced diseases (shanghan). Although syndromes of the six meridians differ, they are interrelated due to a constant changing process.