In a small percentage of cases, the chemical changes are caused by a hereditary enzyme defect, trauma to the eye., diabetes, or use of certain drugs, such as the steroid prednisone.

• 其中在很少一部分病例，化学成份的变化的原因则是遗传性酶缺陷，或眼睛外伤，或糖尿病，或是使用了某种药物，如类固醇的强尼松（一种肾上腺皮质激素亦称波尼松）。