Latest Developments in Normal-Pressure Glaucoma: Diagnosis, Epidemiology, Genetics, Etiology, Causes and Mechanisms to Management

Normal-pressure glaucoma (NPG) is part of the spectrum of the open-angle glaucomas and morphologically characterized, as any glaucoma, by a loss of neuroretinal rim parallel to an enlargement and deepening of the optic cup, and development or enlargement of parapapillary beta zone. These morphological characteristics, in addition to the therapeutic benefit of lowering the intraocular pressure (IOP), make NPG differ from vascular-induced optic neuropathy. Based on the anatomy of the optic nerve as a cerebral fascicle, the physiological counter-pressure against the IOP is the orbital cerebrospinal fluid pressure (CSFP), with both pressures forming the trans-lamina cribrosa pressure difference (TLCPD). In contrast to the IOP, the TLCPD is the true pressure exerting force on the optic nerve fibers when passing through the lamina cribrosa. As a theoretical notion, an abnormally high TLCPD due to a low CSFP, in association with a low arterial blood pressure, could therefore be involved in the pathogenesis of NPG. It fits with the finding that the reduction of the IOP (and thus indirectly of the TLCPD) is (the only proven) procedure for NPG therapy. This review additionally highlights the genetic background, diagnostic methods, and therapeutic modalities of NPG.

Noninvasive evaluation of cerebrospinal fluid pressure in ocular hypertension: a preliminary study

PURPOSE

To compare the orbital cerebral spinal fluid pressure (CSFP) and trans-lamina cribrosa pressure difference (TLCPD) determined noninvasively in ocular hypertensive (OH) subjects and controls.

METHODS

Cross-sectional observational study. Magnetic resonance imaging was used to measure orbital subarachnoid space width (OSASW). The CSFP (mm Hg) was estimated from a published formula as 17.54 × MRI derived OSASW at 15 mm behind the globe + 0.47 × body mass index + 0.13 × mean arterial blood pressure -21.52. Estimated TLCPD was calculated as IOP- CSFP.

RESULTS

The orbital subarachnoid space width was significantly wider (p = 0.01) in the OH group than in the control group at all three measurement locations. The MRI derived CSFP value in OH (14.9 ± 2.9 mm Hg) was significantly higher than in the normal group (12.0 ± 2.8 mm Hg; p < 0.01). The estimated TLCPD value in OH (9.0 ± 4.2 mm Hg) was significantly higher than in controls (3.6 ± 3.0 mm Hg; p < 0.01).

CONCLUSION

The wider OSASW and higher estimated CSFP in OH subjects suggest a higher orbital CSFP. Despite a higher orbital CSFP that could be protective, the higher TLCPD in OH may play a significant role in the risk of developing glaucoma.

The effect of increased intra-abdominal pressure on orbital subarachnoid space width and intraocular pressure

In accordance with the trans-lamina cribrosa pressure difference theory, decreasing the trans-lamina cribrosa pressure difference can relieve glaucomatous optic neuropathy. Increased intracranial pressure can also reduce optic nerve damage in glaucoma patients, and a safe, effective and noninvasive way to achieve this is by increasing the intra-abdominal pressure. The purpose of this study was to observe the changes in orbital subarachnoid space width and intraocular pressure at elevated intra-abdominal pressure. An inflatable abdominal belt was tied to each of 15 healthy volunteers, aged 22-30 years (12 females and 3 males), at the navel level, without applying pressure to the abdomen, before they laid in the magnetic resonance imaging machine. The baseline orbital subarachnoid space width around the optic nerve was measured by magnetic resonance imaging at 1, 3, 9, and 15 mm behind the globe. The abdominal belt was inflated to increase the pressure to 40 mmHg (1 mmHg = 0.133 kPa), then the orbital subarachnoid space width was measured every 10 minutes for 2 hours. After removal of the pressure, the measurement was repeated 10 and 20 minutes later. In a separate trial, the intraocular pressure was measured for all the subjects at the same time points, before, during and after elevated intra-abdominal pressure. Results showed that the baseline mean orbital subarachnoid space width was 0.88 ± 0.1 mm (range: 0.77-1.05 mm), 0.77 ± 0.11 mm (range: 0.60-0.94 mm), 0.70 ± 0.08 mm (range: 0.62-0.80 mm), and 0.68 ± 0.08 mm (range: 0.57-0.77 mm) at 1, 3, 9, and 15 mm behind the globe, respectively. During the elevated intra-abdominal pressure, the orbital subarachnoid space width increased from the baseline and dilation of the optic nerve sheath was significant at 1, 3 and 9 mm behind the globe. After decompression of the abdominal pressure, the orbital subarachnoid space width normalized and returned to the baseline value. There was no significant difference in the intraocular pressure before, during and after the intra-abdominal pressure elevation. These results verified that the increased intra-abdominal pressure widens the orbital subarachnoid space in this acute trial, but does not alter the intraocular pressure, indicating that intraocular pressure is not affected by rapid increased intra-abdominal pressure. This study was registered in the Chinese Clinical Trial Registry (registration number: ChiCTR-ONRC-14004947).

Glymphatic stasis at the site of the lamina cribrosa as a potential mechanism underlying open-angle glaucoma

The underlying pathophysiology of primary open-angle glaucoma remains unclear, but the lamina cribrosa seems to be the primary site of injury, and raised intraocular pressure is a major risk factor. In recent years, a decreased intracranial pressure, leading to an abnormally high trans-lamina cribrosa pressure difference, has gained interest as a new risk factor for glaucoma. New research now lends support to the hypothesis that a paravascular transport system is present in the eye analogous to the recently discovered 'glymphatic system' in the brain, which is a functional waste clearance pathway that promotes elimination of interstitial solutes, including β-amyloid, from the brain along paravascular channels. Given that β-amyloid has been reported to increase by chronic elevation of intraocular pressure in glaucomatous animal models and to cause retinal ganglion cell death, the discovery of a paravascular clearance system in the eye may provide powerful new insights into the pathophysiology of primary open-angle glaucoma. In this review, we provide a new conceptual framework for understanding the pathogenesis of primary open-angle glaucoma, present supporting preliminary data from our own post-mortem study and hypothesize that the disease may result from restriction of normal glymphatic flow at the level of the lamina cribrosa owing to a low intracranial pressure and/or a high trans-lamina cribrosa pressure gradient. If confirmed, this viewpoint could offer new perspectives for the development of novel diagnostic and therapeutic strategies for this devastating disorder.

Incident retinal vein occlusions and estimated cerebrospinal fluid pressure. The Beijing Eye Study

PURPOSE

To examine whether the incidence of retinal vein occlusions (RVOs) is associated with estimated cerebrospinal fluid pressure (CSFP).

METHODS

The population-based Beijing Eye Study, which included 4439 subjects (age: 40 + years) in 2001, was repeated in 2011 with 2695 subjects participating (66.4% of the survivors). Fundus photographs were examined for the new development of RVOs, differentiated into central RVOs (CRVOs) and branch RVOs (BRVOs). CSFP was calculated as CSFP [mmHg] = 0.44 × Body Mass Index [kg/m(2) ] + 0.16 × Diastolic Blood Pressure [mmHg]-0.18 × Age[Years].

RESULTS

Incident BRVOs were detected in 50 eyes and incident CRVOs in 8 eyes. BRVOs were located at arterio-venous crossings in 39 eyes. In multivariate analysis, a higher estimated CSFP was associated with a higher incidence of CRVOs (p = 0.004; standardized coefficient beta: 0.06; regression coefficient B: 5.35; 95% confidence interval (CI):1.73, 8.96) after adjusting for urban region (p < 0.001; beta: -0.52; B: -3.93; 95% CI: -4.29, -3.57), higher educational level (p = 0.001; beta: 0.13; B: 0.44; 95% CI: 0.28, 0.60), higher blood concentrations of triglycerides (p < 0.001; beta: 0.08; B: 0.11; 95% CI: 0.05, 0.16) and higher intraocular pressure (p < 0.001; beta: 0.16; B:0.21; 95% CI: 0.16, 0.27). As a corollary, a higher incidence of RVOs as a whole, as well as a higher incidence of CRVOs combined with a higher incidence of BRVOs originating at the optic nerve head, both were significantly associated with higher estimated CSFP (p = 0.002; odds ratio (OR): 1.15; 95% CI: 1.05, 1.25; and p = 0.037; OR: 1.17; 95% CI: 1.01, 1.35, respectively) after adjusting for older age.

CONCLUSIONS

A higher estimated CSFP was associated with a higher incidence of RVOs originating at the optic nerve head (i.e. CRVOs, hemi-central RVOs and BRVOs originating at the optic nerve head), and vice versa, a higher incidence of RVOs was associated with a higher estimated CSFP. It suggested an influence of higher estimated CSFP on higher central retinal vein pressure.

The Beijing Eye Study 2011

PURPOSE

To examine whether an estimated trans-lamina cribrosa pressure difference (TLCPD) better than intraocular pressure (IOP) correlated with markers for glaucoma.

METHODS

The population-based Beijing Eye Study 2011 included 3468 individuals. Cerebrospinal fluid pressure (CSFP) was calculated as CSFP [mmHg] = 0.44 × Body Mass Index [kg/m(2) ] + 0.16 × Diastolic Blood Pressure [mmHg] - 0.18 × Age [Years] - 1.91. TLCPD was IOP-CSFP.

RESULTS

In the non-glaucomatous population, mean TLCPD was 5.8 ± 4.1 mmHg and mean estimated CSFP was 8.9 ± 3.7 mmHg. IOP was higher (p = 0.008), CSFP was lower (p < 0.001), and TLCPD was (p < 0.001) higher in the glaucoma group than in the non-glaucomatous group. The intergroup difference was highest for TLCPD (2.1 mmHg) followed by CSFP (1.7 mmHg) and IOP (0.4 mmHg). Open-angle glaucoma (OAG) was associated with higher TLCPD [p < 0.001; odds ratio (OR): 1.14; 95% confidence intervals (CI): 1.08, 1.19] but not with IOP (p = 0.22; OR: 0.96; 95% CI: 0.89, 1.03). In contrast, angle-closure glaucoma (ACG) was associated with higher IOP (p = 0.03; B: 0.14; OR: 1.15; 95% CI: 1.01, 1.30) but not with TLCPD (p = 0.98), after adjustment for age and anterior chamber depth. Retinal nerve fibre layer thickness was associated with lower TLCPD (p = 0.036) but not with IOP (p = 0.96), after adjusting for gender, age, region of habitation, optic disc area and refractive error. Neuroretinal area and volume were associated with smaller TLCPD (p = 0.002, and p < 0.001, respectively), after adjusting for gender, optic disc area and refractive error, but not with IOP (p = 0.43 and p = 0.25, resp.).

CONCLUSIONS

In OAG, but not in ACG, calculated TLCPD versus IOP showed a better association with glaucoma presence and amount of glaucomatous optic neuropathy. It supports the notion of a potential role of low CSFP in the pathogenesis of OAG.