Since the lamina cribrosa of the optic nerve head forms the border between the intraocular space with a higher pressure and the retrobulbar space with a lower pressure, a pressure gradient exists across the lamina cribrosa which can be expressed as the difference between intraocular pressure and the pressure in the retrobulbar cerebrospinal fluid and optic nerve tissue space.1,2 This trans-lamina cribrosa pressure gradient is important for ocular diseases in which the pressure on one or on both sides of the lamina cribrosa is either abnormally high and/or abnormally low.3,4 An abnormal pressure gradient influences the physiology and pathophysiology of the optic nerve head,5–9 including the orthograde and retrograde axoplasmic flow.10,11 In that context, it should be kept in mind that the term ‘intraocular pressure (or IOP)’ is a misnomer. What we call ‘IOP’ is just the transcorneal pressure difference.2 The true pressure in an eye with an IOP of 20 mm Hg is 780 mm Hg (760 mm Hg (atmospheric pressure) plus 20 mm Hg). For the optic nerve, however, it is not the transcorneal pressure difference which counts, but the trans-lamina cribrosa pressure difference and the trans-lamina cribrosa pressure gradient.
The trans-lamina cribrosa pressure gradient depends on the pressure difference and the distance between the intraocular compartment and the retrobulbar fluid filled compartment. The distance between both compartments markedly depends on the thickness of the lamina cribrosa.12,13 Consequently, thinning of the lamina in highly myopic eyes may be a reason for their susceptibility to glaucoma.13,14 In addition, histomorphometric studies have shown that in non-highly myopic eyes, the lamina cribrosa gets thinner in advanced stages of the disease.13 This glaucoma-related thinning of the lamina cribrosa may be one of the reasons for the increased risk of progression in eyes with advanced glaucoma.15,16
The trans-lamina cribrosa pressure difference depends on the IOP and the retrobulbar cerebrospinalfluid pressure (CSFP). It is elevated if either the IOP is elevated and/or if the CSFP is reduced. Over 30 years ago, Volkov pointed out that a low CSFP could pathogenetically be associated with glaucomatous optic neuropathy.17 The same idea had earlier been expressed by Szymansky and Wladyczko.18 Yablonski, Ritch and Pokorny postulated that an abnormally low CSFP around the optic nerve may be the reason for a barotraumatically induced optic nerve damage in normal-tension glaucoma.19 In an experimental study, they decreased the intracranial pressure to 5 cm H2O below the atmospheric pressure by cannulation of the cisterna magna. The IOP of one eye was reduced to slightly above atmospheric pressure by cannulation of the anterior chamber. After 3 weeks, the optic nerve heads of the eyes in which the IOP was unaltered showed typical features of glaucomatous optic neuropathy. In contrast, in the eyes in which the IOP was also lowered, no changes occurred. The authors hypothesized that reducing intracranial pressure would have the same effect as increasing the IOP for the development of glaucoma.
Berdahl et al retrospectively reviewed medical records of over 50,000 patients who had undergone lumbar puncture for primarily non-ophthalmological reasons.20,21 The CSFP was significantly (P<0.0001) lower in the subjects with normal-tension glaucoma (8.7±1.16 mm Hg) and in the primary open-angle glaucoma group (9.1±0.77 mm Hg) than in the control group (11.8±0.71 mm Hg). Additionally, the CSFP was higher in the ocular hypertension group than in age-matched controls (12.6±0.85 mm Hg vs. 10.6±0.81 mm Hg; P<0.05). These results were confirmed by a prospective study by Ren et al.22–24 In their study, the lumbar CSFP was significantly (P<0.001) lower in the normal-tension glaucoma group (9.5±2.2 mm Hg) than in the high-IOP glaucoma group (11.7±2.7 mm Hg) or the control group (12.9±1.9 mm Hg). The trans-lamina cribrosa pressure difference was significantly (P<0.001) higher in the normal-IOP glaucoma group (6.6±3.6 mm Hg) and the high-IOP glaucoma group (12.5±4.1 mm Hg) than in the control group (1.4±1.7 mm Hg). In a multivariate analysis, the amount of glaucomatous visual field loss was mainly associated with the trans-lamina cribrosa pressure difference (P=0.005) while IOP and CSFP as single parameters were no longer significantly (P>0.50) associated with the perimetric loss. In a parallel study, the CSFP was significantly (P<0.001) higher in an ocular hypertensive group of 17 patients (16.0±2.5 mm Hg) than in the control group (12.9±1.9 mm Hg).24 In the control group, CSFP was significantly correlated with both systolic blood pressure (P=0.04) and IOP (P<0.001). Since the IOP also showed a tendency toward an association with blood pressure (P=0.09), as was also shown in a population-based study,25 the trans-lamina cribrosa pressure difference was not significantly (P=0.97) related with the blood pressure. In the control group, the pressure in all three fluid compartments was thus correlated with each other, with the systemic blood pressure being the highest, followed by the IOP and finally by the CSFP. In the control group of the study by Ren et al, the CSFP pressure (P=0.01; correlation coefficient r=−0.26), IOP (P=0.001; r=−0.34) and trans-lamina cribrosa pressure difference (P=0.004; r=−0.31) decreased significantly with older age. The CSFP was additionally associated with higher body mass index (P<0.001).26 These findings confirmed the association between an abnormally high body mass index in patients with idiopathic intracranial hypertension.27
In conclusion, taking into account (1) that it is the trans-lamina cribrosa pressure difference (and not the transcorneal pressure difference, i.e. the so-called ‘IOP’) which is most important for the physiology and pathophysiology of the optic nerve; (2) that studies have suggested physiologic associations between the pressure in all three fluid filled compartments, i.e. the systemic arterial blood pressure, the CSFP and the IOP; (3) that an experimental investigation suggested that a low CSFP may play a role in the pathogenesis of normal-tension glaucoma, and (4) that clinical studies have reported that patients with normal-tension glaucoma had significantly lower CSFP and a higher trans-lamina cribrosa pressure difference when compared to normal subjects, one may infer that a low CSFP may be associated with normal-tension glaucoma in some patients. A low systemic blood pressure, particularly at night, could physiologically be associated with a low CSFP, which leads to an abnormally high trans-lamina cribrosa pressure difference (with barotraumatically induced optic nerve damage) which is similar to a situation where the CSFP is normal and the IOP is elevated. This model could explain why patients with normal-tension glaucoma tend to have a low systemic blood pressure, and why eyes with normal- and high-pressure glaucoma, in contrast to eyes with a direct vascular optic neuropathy, can share profound similarities in their optic nerve head appearance.
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