Simultaneous measurements of IOP and ICP in supine, sitting, and HDT have shown that the ICP is more sensitive to body posture than IOP, leading to a posture-dependent TLCPD.
7,34 These studies assumed a fully communicating ONSAS and did not take a potential ONSAS collapse into account. In the current study, the collapsible ONSAS disrupted the link between ICP and ONSAS pressure, preventing the ONSAS pressure from dropping to the same level as the ICP in the upright posture (
Fig. 6), suggesting that the postural change in the TLCPD is not as large as previous studies suggested. The results support the collapse as a possible safety mechanic against large TLCPDs in the upright posture,
5 with the effectiveness of the safety mechanism being highly sensitive to the ONS distensibility (
Fig. 7). This could have direct bearing on various TLCPD-related diseases as differences in the distensibility between individuals or groups of individuals could lead to substantially different TLCPDs. One example is glaucoma, specifically normal tension glaucoma, in which a high TLCPD is believed to be an underlying cause.
1–4 A decreased distensibility would result in a lower ICP and an increased TLCPD in the upright posture and could thus contribute to the development of the disease. Such a decrease in distensibility could occur naturally with aging, as the dura mater stiffens as we grow older,
35 eventually compromising the postural ONSAS collapse and increase the risk of developing glaucoma. Another ocular disease related to the TLCPD is the spaceflight-associated neuro-ocular syndrome that develops in astronauts on extended spaceflights, in which the clinical findings include globe flattening, choroidal folds, and papilledema, not unlike findings in patients with increased ICP.
8 To explain these clinical findings it has been suggested that microgravity eliminates the ICP reduction provided by the upright posture on earth, leading to a chronic increase in ICP in microgravity
7,9 and thus a decrease in the TLCPD and an increased force posterior to the eye. If there is an ONSAS collapse in the upright posture, the effect of this chronic increase in ICP on the TLCPD will be less and less with increasing ONS distensibility.