This is an important diagnostic theory for differentiation syndromes, mainly used for the exogenous syndromes in TCM. It was originally recorded in Treatise on Cold Diseases and Miscellaneous Disorders 
, written by Zhang Zhongjing (150–219 A.D.), a distinguished physician of the Eastern Han dynasty. Based on the knowledge of Basic Questions of the Inner Canon of the Yellow Emperor, Zhang studied the transmitting characteristics of febrile diseases caused by cold and created the clinical methodology system pattern of TCM. The exogenous pathogenic factors are classified into three-Yang (Taiyang, Yangming, Shaoyang) and three-Yin (Taiyin, Shaoyin, Jueyin) syndromes according to the different stages of development for diagnosis as well as for determining the treatment.
This theory is universally applicable to diseases caused by exogenous pathogenic factors. Actually, the principle and methodology of Yin and Yang can be utilized on every kind of patient and situation. It has been one of the typical and standard theories with the methodology system for differentiation syndrome practice for almost 2000 years. The standard package of prescriptions for Chinese herb formulation created by Zhang has been well used all over the world as a treasure of Chinese culture .
In accordance with the holistic principle of TCM and the close relationship between meridians and viscera, some doctors further developed Zhang Zhongjing’s theory of Six Meridians as one of the TCM Zang Xiang 
model called the “Six-Meridian Zang Xiang System Theory.” It added brand new meanings to “Differentiation of Syndromes with Six Meridians,” which is useful for complicated medical situations.
Introduction
Six-meridian syndrome differentiation classifies the stages of exogenous febrile diseases into six types according to the Yin–Yang principle: the Taiyang, Yangming, and Shaoyang syndromes: and the Taiyin, Shaoyin, and Jueyin Syndromes. It reflects the pathological development of the meridians and viscera. From the viewpoint of the location of a disease, Taiyang is compared to the exterior, Yangming to the interior, and Shaoyang to the half interior and exterior, while the three Yin meridians are all compared to the interior. There is a regular and orderly path of syndrome development from the Taiyang Syndrome to the Jueyin Syndrome, or a possible path of development. There are meridian syndromes and Zang–Fu organ syndromes.
Three-Yang meridian disorders are based on pathological changes of six Fu organs, and are manifested as conditions of sthenia and heat. Three Yin meridian disorders are based on pathological changes of the five Zang organs and are manifested as conditions of asthenia and cold or by symptoms of insufficient anti-pathogenic Qi. The six-meridian differentiation includes pathological changes of 12 meridians and viscera. The principle of treatment for disorders of the three Yang meridians may focus on elimination of pathogenic factors, whereas treatment for disorders of the three Yin meridians may strengthen anti-pathogenic Qi and restore the normal function of the body.
In the regular pattern of transmission of febrile diseases among the six meridians, the three Yang meridians are affected first, followed by the three Yin meridians. However, the pathogenic cold may directly attack the three Yin meridians. The above conditions are closely related to the nature of a disease, the strength of pathogenic factors and body resistance, and what treatment the patient has received before.
Syndromes According to the Six-Meridians Theory
Taiyang syndrome
Taiyang Meridians occupy the biggest area of the human body, on the back part exterior of the whole body, governing both nutrient Qi and defensive Qi, and controlling the opening and closing of more than half of the pores. When exogenous pathogenic factors attack the body first via the superficial portion of the body, the Taiyang meridian serves as a screen for the six meridians. When the exogenous pathologic factors invade the body, the Taiyang meridian is the first to be affected. The defensive Qi of the body rises against the pathogenic factors. Then the syndrome of Taiyang disease appears. If the pathogenic factors are not relieved, it penetrates into the Fu syndrome. It can be divided into the Blood Retention and Water Retention.
Taiyang meridian syndrome
This disease caused by invasion of pathogenic factors, may be divided into Taiyang wind and Taiyang cold syndrome.
Taiyang wind syndrome
The Symptoms: Fever, aversion to wind, stiffness and pain in the head and neck, spontaneous or no sweating, floating slow pulse or harsh breathing and retching, and whitish thin tongue fur.
Treatment: Dispel pathogenic wind from the superficial muscles and regulate the function of the Ying and Wei systems.
Taiyang cold syndrome
This disease is caused by invasion of cold, and obstruction of defensive and nutrient Qi.
Symptoms: Aversion to cold, fever, no sweating or dyspnea, stiffness and pain in the neck, head and joints, being asthmatic, floating tight pulse, and whitish thin tongue.
Treatment: Relieve the exterior syndrome and facilitate the lung Qi circulation to relieve the asthma.
Taiyang Fu syndrome
This refers to the syndrome of failure to relieve Taiyang meridian syndrome and transmission of this syndrome into the bladder, where it may divide into two syndromes: Taiyang water accumulation and blood accumulation.
Taiyang water retention case
This is caused by dysfunction of the bladder in transforming Qi and retention of water.
Symptoms: Fever, aversion to cold, dysuria, distension and fullness in the lower abdomen, vomiting, thirst, whitish watery tongue, and floating pulse.
Taiyang blood retention case
This is caused by transmission of pathogenic factors in the Lower Jiao and mixing with blood.
Symptoms: Lower abdominal spasm, fullness, even mania, regular urination, deep and rough or knotted pulse, and reddish tongue with sticky fur.
Yangming syndrome
This represents both strong anti-pathogenic Qi and pathogenic factors, which invade the Yangming meridian, and show excessive interior Yang heat in the stomach and intestines. The heat invades inside and transforms into Yangming, and leads to consumption by body fluids and dryness. The syndrome may be divided into Yangming Meridian Syndrome and Yangming Fu Syndrome.
Yangming meridian syndrome
This appears as a syndrome without retention of feces in the intestines.
Symptoms: High fever, profuse sweating, aversion to heat, extreme thirst, preference for cold drinks, flushed face, dysphonia, reddish tongue with dry yellow fur, and bounding pulse.
Yangming Fu syndrome
This syndrome appears as retention of dry feces in the intestines.
Symptoms: Reddish tongue with dry yellow fur, rapid forceful pulse, afternoon tidal fever, polyhidrosis on the hands and feet, abdominal pain and distension, constipation, irritability, even delirium, polydipsia, and so on.
Shaoyang syndromes
Shaoyang is a half-exterior and half-interior syndrome, due to pathogenic factors attacking the gall bladder when Taiyang syndrome is not relieved yet. The struggle between pathogenic factors and anti-pathogenic Qi may appear between the exterior and the interior with heat and excess, leading to stagnation of Qi. It may transmit from Taiyang or Yangming, or by direct invasion of pathogenic factors.
Symptoms: Alternating chills and fever, bitter taste in the mouth and no appetite, dry throat, vertigo, fullness and discomfort in the costal and hypochondriac regions, restlessness, vomiting, blurring vision, dizziness, and taut pulse.
The treatment should be focused on relieving symptoms from Shaoyang meridians.
Taiyin syndromes
The Taiyin controls dampness and is the protective line of the three Yin meridians. Due to the decline of spleen Yang and the abundance of cold dampness, which develop either from the disease of three Yang meridians by improper treatment or from a direct attack by cold. A febrile disease of exogenous pathogenic factors involving Taiyin Meridians appears as a radical change. Taiyin disease is an interior, asthenia, and cold condition.
A pale tongue with white fur and a weak pulse, fullness of the abdomen, vomiting, poor appetite, diarrhea, frequent abdominal pain, and preference for warmth and pressure may be caused by declined and stagnated spleen Yang or retention of cold and dampness; absence of thirst, a pale tongue with a smooth white coating and a slow weak pulse indicate the deficiency condition or asthenia cold syndrome.
Treatment should be focused on warming the middle Jiao to dispel cold, reinforce the spleen Yang, and release dampness.
Shaoyin syndromes
Shaoyin syndromes are connected with the heart and kidney, which maintain the dynamic balance of Yin and Yang of the body. Shaoyin may be caused by improper treatment of other syndromes and damage to kidney Yang, or direct attack by pathogenic factors. Both of them show the asthenia condition. According to the situation of Yin and Yang, Shaoyin syndromes can be divided into the Cold Syndrome and the Heat Syndrome.
The cold syndrome of Shaoyin
This is caused by the asthenia of heart and kidney Yang and attack by pathogenic cold on Shaoyin. A pale tongue with white fur and a deep feeble pulse, aversion to cold, curling up in sleep, lassitude, pallor, cold limbs, dispiritedness, diarrhea with undigested food, profuse urine, nausea, and no thirst or preference for hot drinks may show excess Yin cold.
Treatment should be focused on restoring Yang from collapse.
The heat syndrome of Shaoyin
This is caused by the asthenia of heart and kidney Yin, appearing as restlessness, a deep-red tongue with scanty fur and a reddish tip and a thready rapid pulse, vexation and insomnia and dry mouth and throat.
Disharmony between Water and Fire indicates Yin asthenia and Yang hyperactivity in the Shaoyin meridians.
Treatment should be focused on nourishing Yin to reduce heat.
Jueyin syndromes
Jueyin syndrome may appear in the last stage of the six-meridian disorder by cold attack, which is considered a result of improper treatment of Yang meridians or Shaoyin syndrome; or direct attack on the liver by pathogenic factors. There is a constant struggle between pathogenic factors and healthy Qi in the Liver Meridian and Pericardium Meridian.
There is a mixture of heat and cold, persistent thirst, Qi going upward to attack the heart, chest pain with a sensation of heat, hunger but no appetite, cold limbs, diarrhea, and possibly vomiting of Ascaris. No typical pulse condition is seen, because it is a complicated syndrome.
Treatment should be identified by the complicated situation, and both heat and cold are treated simultaneously.
Summary
The theory of six-meridian syndrome differentiation is an important component of TCM. It uses the three Yin and three Yang meridians as guidelines for detecting the conditions of Yin and Yang, exterior and interior, cold and heat, and asthenia and sthenia. Thus, it is not only suitable for diseases due to exogenous pathogenic factors but also for any internal disease. The change in strength of the pathogenic factors and healthy Qi, viscera and meridians, blood and body fluid are characterized by the transmission, combination, complication, and direct attack with changes of various syndromes. In clinical practice, one should always consider the relationship between the six meridians and the eight principal syndromes with the change of Zang–Fu functions.
In CWM’s view, the viscera are individual organs, but in TCM every viscus is inseparable from the system, which includes several Yin or Yang meridians and viscera to form as a union. One of the TCM Zang Xiang 
models, called the six-meridian Zang Xiang System Theory, is classified mainly by meridian. It adds a brand new idea to Differentiation of Syndromes with Six Meridians, which is very useful for the complicated medical situation as follows:
| (1) | Taiyang meridian – Zang Xiang system: including hand Taiyang, foot Taiyang with hand Shaoyin Meridians and the connected Heart, Brain, Small Intestine, Bladder, blood vessel, and tongue as a gather-up system. |
| (2) | Yangming meridian – Zang Xiang system: including hand Yangming, foot Yangming with hand Taiyin Meridians and the connected Lung, Large Intestine, Stomach, skin and hair, and nose as a gather-up system. |
| (3) | Shaoyang meridian – Zang Xiang system: including hand Shaoyang, foot Shaoyang with hand Jueyin Meridians and the connected Gall Bladder, San Jiao, Pericardium, Subcutaneous tissue, and larynx as a gather-up system. |
| (4) | Taiyin meridian – Zang Xiang system: including hand Taiyin, foot Taiyin with foot Yangming Meridians and the connected Spleen, Stomach, Lung, four limbs, muscle, and lips as a gather-up system. |
| (5) | Shaoyin Meridian – Zang Xiang system: including hand Shaoyin, foot Shaoyin with foot Taiyang Meridians and the connected Kidney, Heart, Bladder, skeleton, and ears as a gather-up system. |
| (6) | Jueyin meridian – Zang Xiang system: including hand Jueyin, foot Jueyin with foot Shaoyang Meridians and the connected Liver, Gall Bladder, Pericardium, tendons, and so on as a gather-up system. |
TCM uses comprehensive diagnostic methods and is capable of viewing a disease or syndrome on several levels. The emphasis of the internal harmony and integrity in the human body, is one of its key guiding principles.