Normal intraocular pressure should be within the range of 11~21mmHg. However, due to the different degrees of tolerance of each optic nerve on the intraocular pressure, some people’s intraocular pressure is higher than the normal value, but it do not damage optic nerve and visual field, this is called ocular hypertension, but rather glaucoma. Some people may have glaucomatous, the optic nerve and visual field are damage, but their intraocular pressure is in the normal range, which called normal tension glaucoma. That is to say high intraocular pressure is not necessarily glaucoma, people who have glaucoma can also have a normal intraocular pressure.