Shortly after the introduction of the ophthalmoscope, which enabled ophthalmologists to observe the characteristic glaucomatous excavation of the disc (initially thought to be an elevation), von Graefe1 realized that cupping could occur without a digitally measured pressure elevation. It is alleged that von Graefe was persuaded by Donders' view that all glaucoma was due to elevated intraocular pressure, but he continued to believe that some optic nerve heads must be more susceptible to elevated intraocular pressure than others. The debate as to whether there are two diseases separable by a pressure level or whether it is all one and the same disease continues to this day. I support the concept that the intraocular pressure does not separate two diseases but that there is probably more than one pathophysiologic mechanism producing damage.
See also p 1145.
Glaucoma with normal pressure (the pressure is practically never "low" but is "normal" or "high
Drance SM. Low-Tension GlaucomaEnigma and Opportunity. Arch Ophthalmol. 1985;103(8):1131-1133. doi:10.1001/archopht.1985.01050080043016