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Homeostasis of the Eye

The human body is immensely complex. Sympathetic systems, parasympathetic systems, hormones, and homeostasis. Everything in the body happens for a reason– releasing chemicals, breaking down wastes– and then comes back to its regular form. In this homeostatic form, all is presumably well and life progresses as usual.

But what does that mean for the eyes and vision? In a perfect world, we would all have 20/20 vision but in reality, many of us do not. Emmetropia, or the condition of not having a refractive error, should be the homeostatic state for humans, or one may think. Nearsightedness and farsightedness, unfortunately, is not something that the negative feedback system can correct in those with myopia or hyperopia. Instead, the set-point system in those with refractive errors is our physiologic refractive error; someone can accommodate to slightly improve their vision but their biological set point is their myopia or hyperopia (Wallman & Winawer, 2004). Struggling to see better through accommodation can eventually cause strain and headaches, these symptoms resolve once back in the homeostatic set-point.

Whereas the near- and farsightedness is thought to be caused by how the eye grows within the orbit and forms in early life, the set-point for the extraocular muscles is thought to be more genetic (Wallman & Winawer, 2004; Altick et al, 2012). Normal ocular alignment is the homeostatic state for most people but those with congenital strabismus are thought to have a genetic cause that causes their exo- and esotropia which the body recognizes as the set-point (Altick et al, 2012). Prisms can correct muscle misalignment when glasses are being worn but the eye(s) with the misalignment will revert back inward or outward through the negative feedback system ultimately. While a misalignment caused by trauma or nerve palsy is not genetic, the body adopts this as being the new set-point and will regain homeostasis again when the is not correctly aligned.

When all is well in the body, we are expected to thrive, but there are some cases where homeostasis is not exactly where it should be. For those with refractive errors, corrective lenses and refractive surgery can help get vision to 20/20. Strabismus surgery can repair the extraocular muscle misalignments in most cases, overruling the negative feedback system and creating a new normal. Through man-made change, the set-point can be altered to achieve homeostasis the way it should be.

Wallman, J. & Winawer, J. (2004, August 19). Homeostasis of eye growth and the question of myopia. Neuron. 43(4): 447-468. https://doi.org/10.1016/j.neuron.2004.08.008

Altick, A., Feng, C., Johnson, L., Schlauch, K., & von Bartheld, C. (2012, August). Differences in gene expression between strabismic and normal human extraocular muscles. Invest. Ophthalmol. Vis. Sci. 53(9):5168-5177. doi: https://doi.org/10.1167/iovs.12-9785.

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