Definition of intraocular pressure: IOP is the pressure of the contents of the eye acting on the inner wall of the eye. Normal IOP: 1. From a clinical perspective, the definition of normal IOP should be the range of IOP that does not cause optic nerve damage. The mean value of IOP in normal people is 16 mmHg (1 mmHg = 0.133 kPa), with a standard deviation of 3 mmHg. From a statistical concept, normal IOP is defined as 10-21 mmHg, but in practice, IOP in normal people is not normally distributed. Therefore, IOP >21mmHg cannot be mechanically considered as a pathological value. 2. Normal IOP is not only reflected in the absolute value of IOP, but also in the symmetry of both eyes and the relative stability of diurnal pressure. In normal people, the difference in IOP between eyes should not be >5mmHg in general, and the fluctuation range of IOP in 24 hours should not be >8mmHg. High IOP and normal IOP glaucoma: High IOP is not always glaucoma, and normal IOP cannot exclude glaucoma. 1. Hypertensive IOP: Some patients have an IOP that exceeds the upper limit of statistical normality but do not develop optic nerve or visual field damage on long-term follow-up. 2. Normal IOP glaucoma: Some patients have typical glaucomatous optic nerve atrophy and visual field defects even though their IOP is in the normal range. In addition, there are also some patients with optic nerve atrophy and visual field defects that still progress after IOP is controlled, suggesting that other factors besides IOP are involved in the development of glaucoma. Race, age, myopia, and family history, as well as any condition that can cause inadequate blood supply to the optic nerve, such as cardiovascular disease, diabetes mellitus, and abnormal blood rheology, may also be risk factors for glaucoma. The pathological process of both experimental glaucoma and secondary glaucoma confirms that elevated intraocular pressure is an important factor in causing optic nerve and visual field damage. The higher the IOP and the longer the duration of high IOP, the greater the risk of causing optic nerve damage. Moreover, for the treatment of glaucoma, IOP is still the most definite and controllable risk factor; at the same time, the fact that the development of optic nerve damage is slowed down in most glaucoma patients after IOP control also proves the danger of high IOP. Therefore, it is important to recognize normal IOP and pathological IOP for both diagnosis and treatment of glaucoma.