Long-term effects of lens removal as the vitreous ages?

Question

In a case of pediatric cataract surgery where the lens was not replaced, there is no lens to serve as a barrier between the aqueous and the vitreous. I once asked an ophthalmologist what keeps them from mixing together in that case -- are they of different viscosities, like oil and water, for instance? She said it's something like that, but that with aging the vitreous gradually thins (causing floaters, among things) and they end up blending together. She said this isn't really a problem in practice, as each humour's main function is to deliver nutrients to the right parts of the eye and they can still do that.

I understand that the aqueous drains through a duct, so that part isn't a closed system. But the vitreous usually is a closed system, as I understand it, which is why floaters don't go away. A remedy for floaters that is sometimes brought up is vitrectomy, but that carries risk of retinal detachment (according to Wikipedia, and I think I remember hearing that from an ophthalmologist too).

Putting all of these together, it seems possible that with aging, as the vitreous becomes less distinct from the aqueous, some of the vitreous might end up draining through the same duct that the aqueous drains through, in an eye with no lens to serve as a barrier. Is that the case? And if it is, does it carry with it a higher risk of the things that can go wrong with vitrectomy (especially retinal detachment)? Or does the risk that comes with that procedure not arise with a slow, gradual aging process?

Update: I've seen a reference to the hyaloid membrane, which, in addition to the lens, sits between the vitreous and aqueous. I don't know if this membrane is also removed when a lens is removed or if it remains.

Answer 1

...it seems possible that with aging, as the vitreous becomes less distinct from the aqueous, some of the vitreous might end up draining through the same duct that the aqueous drains through, in an eye with no lens to serve as a barrier. Is that the case?

In a normal (i.e. lens intact) eye, that would, I think, be a problem.

The aqueous humor is a self-replenishing system, whereas the vitreous humor is made and completed before birth. There is no replenishment of it, although, because it is near a capillaries bed, there is diffusion of small molecules like glucose back and forth that takes place between the blood and the vitreous humor.

You're correct that the vitreous humor (initially a very well organized gel-like substance) liquifies as aging occurs, but the predominant effect of this is floaters (vitreous detachments*), retinal pulls, tears, hemorrhages and detachments.

Fluid (aqueous humor) is produced by cells in the ciliary body (which is part of the ciliary muscle, with ligamentous connections to the edge of the lens) behind the iris. This fluid-filled space is called the posterior chamber. The fluid then passes from behind the iris into the anterior chamber, between the iris and the cornea, and finally drains through a permeable network (the trabecular meshwork) near the base of your the anterior part of the iris. So, there is constant production and drainage of aqueous humor, keeping the fluid level balanced.

When too much fluid is made, or not reabsorbed, the anterior chamber bulges, deforming the shape of the vitreous humor and placing pressure on the optic nerve. There is no fluid exchange between the two unless there is disease of the posterior chamber.

When the vitreous humor liquifies, the volume stays the same. If it could drain out of the trabecular meshwork, I would imagine there would be a much greater risk of more serious vitreous detachments and subsequent retinal detachments.

In the case of a lens removed in childhood, the vitreous humor, being a highly organized gel, is probably little affected by lens removal. With age, because liquifaction happens from the middle of the vitreous; the effect of loss of the lens is still probably small, but I imagine - I'm not an opthalmologist - it might be different for someone who has had a lifetime to accommodate.

(I might have to read and edit this answer again.)

*_{Not the sole cause of floaters
cross-section image from Patient.co.uk: Acute Angle-closure Glaucoma
https://www.dartmouth.edu/~humananatomy/part_8/chapter_46.html
J Clin Pathol. 2003 Sep; 56(9): 720: Concurrent vitreous disease may produce abnormal vitreous humour biochemistry and toxicology}