Sir,

Siaudvytyte et al1 present a literature review and meta-analysis regarding translaminar pressure difference (TLPD) in open-angle glaucoma. Five studies that demonstrated a higher TLPD in open-angle glaucoma have been analyzed in detail.

TLP defined as intraocular pressure (IOP) minus cerebrospinal fluid pressure (IOP−CSF-p)2 is an interesting mechanical concept. However, there are some critical considerations concerning the interpretation and implementation of TLP we would like to mention.

First, as discussed by Siaudvytyte et al1 both IOP and CSF-p used in the equation for TLPD are dynamic and fluctuate independently over time with numerous variables affecting both intracranial pressure and intraocular pressure measurements. Therefore, the estimation of TLPD would be best done by a simultaneous measurement of both IOP and CSF-p. None of the presented studies did it and thus all TLPD studies are dealing with two non-dependent and ever-changing variables to a definite time leaving us without a correlation that would fulfill the requirements of the simple physical equation of pressure, which is force over area at a definite time.

Second, in four out of the five analyzed studies, CSF-p has been measured by lumbar puncture and the lumbar CSF-p was extrapolated to the retrolaminar CSF-p. The assumption the lumbar CSF-p might equal the retrolaminar CSF-p only holds if the CSF-p is homogenous distributed in all CSF spaces, inclusively the subarachnoid space of the optic nerve. However, at least in NTG3 CSF does not communicate freely between the intracranial subarachnoid space and that of the optic nerve. The optic canal is extremely narrow and due to the mechanosensitivity of meningothelial cells4 that line the canal, the anatomy of the canal can change the anatomical pathway for CSF.

Third, as pressure is defined as force over area, the area involved in TLP needs to be known. The area in question is a complex and irregular arrangement of ovaloid circles that are arranged within an annulus5 and is not known in any of the patients presented. Thus, the forces in the equation for TLPD cannot be calculated.

Given these missing information and uncertainties, the concept of TLP seems still premature.