Allometry and Scaling of the Intraocular Pressure and Aqueous Humour Flow Rate in Vertebrate Eyes

In vertebrates, intraocular pressure (IOP) is required to maintain the eye into a shape allowing it to function as an optical instrument. It is sustained by the balance between the production of aqueous humour by the ciliary body and the resistance to its outflow from the eye. Dysregulation of the IOP is often pathological to vision. High IOP may lead to glaucoma, which is in man the second most prevalent cause of blindness. Here, we examine the importance of the IOP and rate of formation of aqueous humour in the development of vertebrate eyes by performing allometric and scaling analyses of the forces acting on the eye during head movement and the energy demands of the cornea, and testing the predictions of the models against a list of measurements in vertebrates collated through a systematic review. We show that the IOP has a weak dependence on body mass, and that in order to maintain the focal length of the eye, it needs to be an order of magnitude greater than the pressure drop across the eye resulting from gravity or head movement. This constitutes an evolutionary constraint that is common to all vertebrates. In animals with cornea-based optics, this constraint also represents a condition to maintain visual acuity. Estimated IOPs were found to increase with the evolution of terrestrial animals. The rate of formation of aqueous humour was found to be adjusted to the metabolic requirements of the cornea, scaling as , where Vac is the volume of the anterior chamber. The present work highlights an interdependence between IOP and aqueous flow rate crucial to ocular function that must be considered to understand the evolution of the dioptric apparatus. It should also be taken into consideration in the prevention and treatment of glaucoma.

在脊椎动物中，眼内压（intraocular pressure, IOP）是维持眼球形态、使其可作为光学器官行使功能的必要条件。其稳态由睫状体产生的房水与房水从眼球流出时所受阻力之间的动态平衡所维系。眼内压调节失常通常会对视力造成病理性损害，高眼压可引发青光眼——青光眼是人类第二大常见致盲病因。本研究通过对头部运动过程中作用于眼球的作用力及角膜的能量代谢需求开展异速生长与缩放分析，并将模型预测结果与通过系统综述整理得到的脊椎动物实测数据集进行比对，探讨了眼内压与房水生成速率在脊椎动物眼球发育中的重要性。研究结果表明，眼内压与体重仅存在较弱的相关性；而为维持眼球焦距，眼内压需较重力或头部运动引发的眼球内外压差高出一个数量级。这一约束是所有脊椎动物共有的进化限制。对于以角膜作为光学成像主体的动物而言，该约束同时也是维持视敏度的必要条件。研究发现，估算得到的眼内压随陆生动物的演化而升高。房水生成速率需适配角膜的代谢需求，其缩放关系为，其中Vac为前房体积。本研究揭示了眼内压与房水流量之间的相互依存关系，这一关系对眼球功能至关重要，若要理解屈光装置的演化过程，必须将其纳入考量范畴。该结论在青光眼的预防与治疗中也具有参考价值。