丹溪曰。内经谓诸痿起于肺热。又谓治痿独取阳明。盖肺金体燥而主气。畏火者也。脾土性
湿而主四肢。畏木者也。若嗜欲无节。则水失所养。火寡于畏而侮所胜。肺得火邪而热矣。
肺受热则金失所养。木寡于畏而侮所胜脾得木邪而伤矣。肺热则不能管摄一身脾伤则四肢不
能为用。而诸痿作矣。泻南方则肺金清而东方不实何脾伤之有补北方则心火降而西方不虚。
何肺热之有。故阳明实则宗筋润能束骨而利机关矣。
天产属阳。浓味发热。凡病痿者。必淡泊食味。至于食少肌瘦。或泄泻者。虽有内热必
以芳香甘温先复胃气为主则饮食进而痿弱自健。若拘于寒凉。谷气益衰。四末益枯矣。
湿热痿者雨湿浸淫。邪气蒸脾。流于四肢。自觉足胫逆气上腾。或四肢酸软肿痛。或足指麻
木顽痒。小便赤涩脉来沉濡而数此皆湿热在下之故。所谓湿热不攘大筋 短。小筋弛长 短
为拘。弛长为痿也。宜升阳燥湿禁用填补之剂。
湿痰痿者肥盛之人湿痰内停。客于经脉。使腰膝麻痹。四肢痿弱。脉来沉滑。此膏粱酒湿之故。宜燥脾行痰。
气虚痿者。因饥饿劳倦。百骸溪谷皆失所养。故宗筋弛纵骨节空虚。凡人病后手足痿弱者。
皆属气虚所谓脾既病不能为胃行其津液四肢不得禀水谷气而不用也宜补中益气。治痿独取阳明此为气虚者立法也。
阴虚痿者酒色过度。下焦肝肾之火燔灼筋骨。自觉两足极热上冲腿膝酸弱痿软。行步艰
难脉来涩弱。或左脉虽大。按之无力宜补养肝肾。
血虚痿者。凡产后失血后。面色痿黄。手足无力。不能行动者也。宜滋养营血往往用养
血药而痿如故者。脾虚不能生血也。
血瘀痿者。四肢痛而不能运动。致脉涩而芤者宜养血行瘀。
食积痿者。饮食太过。升降失常。脾气不得运于四肢手足软弱或腹膨胀痛或恶心嗳气。宜消导食积。然后补脾。
痢后脚软胫疼。或膝肿者。此下多亡阴所致宜补脾兼升。举之剂间有痢后兜涩太早积瘀
不清。下注隧道而成痿者。此又当行气逐瘀。与前症迥异矣。
朱丹溪说：《内经》指出各种痿证都起因于肺热，又说治疗痿证应专门取法阳明经。这是因为肺属金，体性干燥而主司气机，畏惧火邪；脾属土，性质湿润而主司四肢，畏惧木邪。如果嗜欲没有节制，就会使肾水失去滋养，导致火邪因缺乏制约而反侮其所胜之脏（即火克金），肺受到火邪侵袭而产生热象。肺受热后，金失去滋养，导致木邪因缺乏制约而反侮其所胜之脏（即木克土），脾受到木邪侵袭而受伤。肺热则不能统摄全身，脾伤则四肢无法正常使用，于是各种痿证就产生了。泻南方心火，则肺金得以清肃，东方肝木就不会过实，脾又怎么会受伤呢？补北方肾水，则心火下降，西方肺金就不会虚弱，肺又怎么会有热呢？所以，当阳明经气血充实时，宗筋得到滋润，就能约束骨骼并利于关节活动了。
天然生长的食物多属阳，味道浓厚易生热。凡是患有痿证的人，饮食必须清淡。至于那些进食少、肌肉消瘦，或者伴有腹泻的人，即使体内有热，也必须以芳香甘温的药物先恢复胃气为主，待饮食增进后，痿弱自然强健。如果拘泥于使用寒凉药物，会使谷气更加衰弱，四肢末端更加枯槁。
湿热型痿证，是由于雨湿浸淫，邪气蒸腾损伤脾胃，流注于四肢。患者自觉足胫部有逆气上冲，或四肢酸软肿痛，或脚趾麻木顽痒，小便赤涩，脉象沉濡而数。这都是湿热在下焦的缘故。正如所说：“湿热不除，大筋收缩变短，小筋松弛变长。”大筋短缩会导致拘挛，小筋松弛会导致痿废。适宜用升阳燥湿的方法，禁用填补滋腻的药剂。
湿痰型痿证，多见于形体肥胖之人，湿痰内停，客居于经脉，导致腰膝麻痹，四肢痿弱，脉象沉滑。这是由膏粱厚味和酒湿引起的。适宜用燥脾化痰的方法。
气虚型痿证，是因为饥饿劳倦，导致全身筋骨关节都失去滋养，因此宗筋弛缓纵弛，骨节空虚。凡是病后出现手足痿弱者，都属于气虚。这就是所说的“脾既已病，不能为胃行其津液，四肢得不到水谷精气的供养而无法使用”。适宜用补中益气汤治疗。“治疗痿证独取阳明”，这是针对气虚者确立的治疗法则。
阴虚型痿证，是因为饮酒色欲过度，下焦肝肾之火燔灼筋骨。患者自觉两脚极热，向上冲击腿部，膝盖酸弱痿软，行走艰难，脉象涩弱，或者左脉虽然大，但按之无力。适宜补养肝肾。
血虚型痿证，多见于产后或失血之后，面色萎黄，手足无力，不能行动。适宜滋养营血。往往有人用了养血药而痿证依旧，这是因为脾虚不能生血所致。
血瘀型痿证，表现为四肢疼痛而不能运动，导致脉象涩而芤。适宜养血行瘀。
食积型痿证，是因为饮食过量，升降功能失常，脾气不能运化至四肢，导致手足软弱，或者腹部膨胀疼痛，或者恶心嗳气。适宜消导食积，然后补脾。
痢疾之后出现脚软、小腿疼痛，或者膝盖肿胀的，这是由于下利过多导致亡阴所致，适宜补脾兼用升举之剂。间或有痢疾后过早使用收敛涩肠药，导致积滞瘀血不清，下注经络而成痿证的，这又应当行气逐瘀，与前症截然不同。
Zhu Danxi stated: The *Neijing* (Classic of Internal Medicine) asserts that all types of Wei syndrome (flaccidity syndrome) originate from Lung heat, and further states that in treating Wei syndrome, one should exclusively focus on the Yangming meridians. This is because the Lung belongs to Metal, has a dry nature, governs Qi, and fears Fire; the Spleen belongs to Earth, has a damp nature, governs the four limbs, and fears Wood. If desires are unrestrained, Kidney Water loses its nourishment, causing Fire to lack restraint and overact upon its controlled element (Metal), leading to Heat in the Lung due to Fire pathogen invasion. When the Lung is heated, Metal loses nourishment, causing Wood to lack restraint and overact upon its controlled element (Earth), injuring the Spleen due to Wood pathogen invasion. Lung Heat prevents the regulation of the whole body, and Spleen injury renders the four limbs unusable, thus giving rise to various Wei syndromes. By draining the South (Heart/Fire), Lung Metal becomes clear, and East Liver/Wood does not become excessive; how could the Spleen be injured? By tonifying the North (Kidney/Water), Heart Fire descends, and West Lung/Metal does not become deficient; how could Lung Heat exist? Therefore, when the Yangming channels are full, the Zongjin (major tendons) are moistened, enabling them to bind the bones and facilitate joint movement.
Naturally produced foods tend to be Yang in nature, and rich flavors generate Heat. For those suffering from Wei syndrome, diet must be bland. In cases where food intake is low, muscles are emaciated, or diarrhea occurs, even if there is internal Heat, priority must be given to restoring Stomach Qi using aromatic and sweet-warm herbs. Once appetite improves, the flaccidity will naturally strengthen. If one rigidly adheres to cold-cooling remedies, Grain Qi will further decline, and the extremities will become increasingly withered.
In Damp-Heat Wei syndrome, rain-dampness invades, pathogens steam and damage the Spleen, flowing into the four limbs. Patients feel counter-flowing Qi rising from the calves, or experience soreness, softness, swelling, and pain in the four limbs, or numbness and stubborn itching in the toes. Urine is red and scanty, and the pulse is deep, soggy, and rapid. These are all caused by Damp-Heat in the lower jiao. As it is said: "If Damp-Heat is not dispelled, the large sinews shorten, and the small sinews elongate." Shortening of large sinews leads to contracture, while elongation of small sinews leads to flaccidity. The appropriate treatment is to raise Yang and dry Dampness; filling and cloying agents should be strictly avoided.
In Damp-Phlegm Wei syndrome, this condition is common in obese individuals where Damp-Phlegm stagnates internally, lodging in the meridians, causing numbness in the waist and knees, and flaccidity in the four limbs. The pulse is deep and slippery. This results from rich food and alcohol-dampness. The appropriate treatment is to dry the Spleen and resolve Phlegm.
In Qi-deficiency Wei syndrome, it arises from hunger and fatigue, causing the entire body's sinews and joints to lose nourishment. Thus, the Zongjin becomes lax, and bone joints feel empty. All cases of post-illness flaccidity in hands and feet belong to Qi deficiency. This refers to the concept that "when the Spleen is diseased, it cannot transport fluids for the Stomach, and the four limbs do not receive water and grain Qi, rendering them useless." The appropriate treatment is Bu Zhong Yi Qi Tang (Tonify the Middle and Augment the Qi Decoction). "Treating Wei syndrome by exclusively focusing on Yangming" establishes the principle for Qi-deficiency cases.
In Yin-deficiency Wei syndrome, it results from excessive drinking and sexual indulgence, causing Fire in the Lower Jiao (Liver and Kidneys) to scorch the sinews and bones. Patients feel extreme heat in both feet rushing upward to the legs, with soreness, weakness, and flaccidity in the knees, making walking difficult. The pulse is choppy and weak, or the left pulse may appear large but feels powerless upon pressure. The appropriate treatment is to nourish the Liver and Kidneys.
In Blood-deficiency Wei syndrome, it commonly occurs after childbirth or blood loss, presenting with sallow complexion, weakness in hands and feet, and inability to move. The appropriate treatment is to nourish Ying-Blood. Often, Wei syndrome persists despite the use of Blood-nourishing herbs because the Spleen is too deficient to generate Blood.
In Blood-stasis Wei syndrome, symptoms include pain in the four limbs preventing movement, resulting in a choppy and hollow pulse. The appropriate treatment is to nourish Blood and move stasis.
In Food-stagnation Wei syndrome, it is caused by excessive eating, disrupting ascending and descending functions, so Spleen Qi cannot transport nutrients to the four limbs. Symptoms include weakness in hands and feet, abdominal distension and pain, nausea, and belching. The appropriate treatment is to guide out food stagnation, followed by tonifying the Spleen.
After dysentery, if there is leg weakness, calf pain, or knee swelling, it is caused by excessive downward purging leading to Yin exhaustion. The appropriate treatment is to tonify the Spleen combined with lifting agents. Occasionally, if astringents were used too early during dysentery, causing retained accumulation and stasis to flow down into the collaterals and form Wei syndrome, the treatment should involve moving Qi and expelling stasis, which is completely different from the previous conditions.