Though there are only three PubMed-listed EY studies in glaucoma, there is growing interest in relaxation techniques to lower IOP [51]. Following our earlier demonstration that vessel diameters are reduced in POAG patients which compromises OBF, our study is the first controlled trial showing that EY is safe and effective to lower IOP, normalize VD in microvessels, and improve VF function. Though a 3% diameter recovery seems small, according to Poiseuille’s law, vessel diameter has a 4th-power influence on blood flow. Therefore, an EY-induced 2.5 MU increase in recovered vessels translates to a 12.6% OBF restoration [(100% + 3%)4–100% = 12.6%], i.e., an increase reaching almost the 15% that POAG patient lost compared to healthy controls [38]. We hypothesize that this normalization of vasoconstriction has a (secondary) down-stream effect of reactivating hypo-metabolic (“silent”) neurons and provides the biological mechanism of vision restoration.

Interestingly, even squeezing the eyelids for a few minutes is enough to increase IOP [52], while ocular massage lowered mean IOP immediately by − 6.2 ± 1.9 mmHg, but this effect persisted only 5 min after the ocular massage with a mean reduction of − 3.8 ± 2.0 mmHg [53]. In contrast to the brief changes in IOP associated with eyelid squeezing or ocular-facial massage, our eye yoga involved slow, deliberate eye movements, ocular-facial massage, and deep breathing meditation to promote relaxation and reducing stress on a daily basis over the course of 1 month. We hypothesize that slow and deliberate eye movement contracting and stretch oculomotor and ciliary muscles reduces muscle tension (like body postures of yoga asanas). This then also relaxes retinal vessels and increases outflow of aqueous humor, thus lowering the IOP. Meditation, on the other hand, is a more general method to lower mental stress which activates the parasympathetic nervous system which alone can already lower IOP in glaucoma patients [45].

From a basic science perspective, our results confirm the hypothesis that stress should be considered a key cause, not only an effect, of glaucoma and that relaxation is a complementary technique to improve vision. This causality is justified because the independent variable (treatment using relaxation) altered the dependent variables IOP, vessel diameter, and VF function. Therefore, the widespread opinion that stress is a “non-issue” in glaucoma care needs revision for two reasons: first, glaucoma patients with VD tend to have personality dispositions of low-stress resilience, being worrisome, perfectionistic, ambitious, compulsive, and/or neglecting their own needs [54,55,56]. This, in turn, creates sudden or long-term mental stress triggering a psychosomatic response [57]. Second, the diagnosis of progressing vision loss triggers a pessimistic life perspective because the continuous anxiety and fear of going blind possibly accelerate progression (“dark thoughts—cloudy vision”), a downward spiraling (vicious) cycle [58, 59].

That stress negatively impacts all bodily organs is the basis of psychosomatic medicine, and, as we confirm, the eyes are no exception: first, stress increases stress hormones such as glucocorticoids [60], pro-inflammatory cytokines [61], and endothelin-1 [30, 62,63,64], and it reduced nitric oxide (NO) [65, 66], a vasodilatory neuropeptide lining rich in peri-vascular neurons and in the endothelium. Second, stress triggers sympathetic activation, increasing muscle tone (tension/constriction) not only of the perivascular muscles but possibly also of the oculomotor muscles. We submit that chronic vasoconstriction in POAG therefore continuously deprives retinal and brain neurons of oxygen and nutrients, lowering neuronal energy states and reducing visual processing [38, 58].

The most effective, long-known remedy for stress is relaxation. It is not only used for thousands of years in traditional medicine, but many relaxation programs are standard care in psychosomatic medicine, psychotherapy, psychiatry, and neuro-rehabilitation. But so far they are not used in standard care of low vision rehabilitation. Our study results confirm the benefits of relaxation techniques (here: eye exercises and meditation), and it provides the biological explanation why vision can be improved or restored: it improves vascular regulation and blood flow. This is in line with others showing that yogic ocular exercises lower IOP in the short- [47] and long-term [46] as does breathing (mindfulness) meditation which reduces stress- and inflammation biomarkers (cortisol, IL6, TNF-α, and ROS), it elevates β-endorphins, BDNF, and TAC and even alters gene expression [44, 45].

Based on our observations, we propose the following two-pronged theory of glaucoma which we term the “eye ball retraction theory”: first, increased (sympathetic) muscle tension shortens oculomotor muscles, pulling the eyes slightly backward against the eye socket of the skull which, in turn, increases IOP and constricts OBF in the optic nerve head (paling) and choroid, especially when intracranial and optic nerve subarachnoid space pressure is low [67, 68]. Second, stress may increase retinal perivascular smooth muscle tension, adding to the eye pressure-induced vasoconstriction.

This “eyeball-retraction-theory” might explain why relaxation by EY is a remedy for glaucoma: stretching the oculomotor muscles during eye movement exercises lowers their tension which has the advantage of lifting the pressure against the choroid, possibly reducing IOP [44, 45], normalizing the eye globe’s shape, and/or enhancing aqueous outflow. Relaxation of the perivascular smooth muscles on the other hand increases vessel diameter, normalizing OBF. This leads us to the proposition that the mechanism of vision restoration after eye yoga is primarily due to vascular recovery. If true, neuronal recovery (neuroplasticity) would only be an effect secondary to improved blood flow.

Though we recognize that the lumen regulation of venous vessels is different from that of arteries [69], the dilation of venous vessels observed in our study can be explained by eye yoga relaxation which may have reduced the perivascular smooth muscle tension. Venous dilation, unlike arterial regulation, is more passive but can still be influenced by changes in local blood flow and pressure dynamics [30]. Thus, relaxation-induced venous dilation could be a contributing factor to the overall improvements in ocular blood flow and visual function, though further studies are needed to clarify this issue.

Overall, although further study is needed to confirm the proposed mechanism and long-term effect of EY, our study offers potential avenues for personalized POAG therapy within the framework of 3PM: in contrast to an IOP-centric focus to explain glaucoma, we agree with the proposal of Josef Flammer [27, 29] that VD or vascular dysfunction is a key mechanism of open angle glaucoma which, according to our own suggestion, is compatible with the view that mental stress may be the underlying cause. We propose that in the future, glaucoma treatment should be tailored to the patients’ individual characteristics (blood flow, mental stress, whole body status, lifestyle, environmental factors, etc.) rather than focusing on the one and only risk factor, IOP. It would open the door for a more personalized, holistic approach to preventing progression of vision loss and optimization of restoration and recovery in low vision rehabilitation.

Though our study sample is small, yet significant benefits of EY were observed which argues for a strong effect. Furthermore, some of our “active control” patients showed some vision recovery as well, possibly due to either normal variation, placebo effects, or the (non-specific) benefit of daily “time-out” relaxation. Whatever the mechanism, relaxation by EY is “complementary” (not an alternative) to standard IOP management. Future studies could test if EY can increase efficacy of drug therapies and/or improve a more optimistic mental state of mind.

Stress-related VD is a risk factor not sufficiently considered in clinical practice. The discovery of vascular recovery in this study once again reaffirms the role of the blood flow theory in the prevention of glaucoma. Future studies should therefore also consider the use of EY for preventing the occurrence and progression of glaucoma.

In this study, we validated that EY, a psychosomatic therapy, can effectively reduce IOP, increase blood flow, and improve vision. Because of our results, we propose that EY is an effective complementary therapy for glaucoma. It offers the advantages of being harmless, non-invasive, side-effect free, and low-cost, enriching our means of treating glaucoma and contributing to personalized treatment of glaucoma.

In addition to these considerations, blood flow factors are one of the mechanisms underlying glaucoma. As we demonstrated, it is the improvement of stress-related vascular dysfunction which may be a key factor contributing to VF recovery in glaucoma. Future research should develop and establish models to predict the onset, prognosis, and possible recovery of glaucoma based on patients personalized assessment of blood flow function. In this manner, our findings may contribute to a paradigm shift from medicine which is just reactive to increase IOP or VF dysfunction in glaucoma by considering a more personalized (3PM) approach which goes beyond the traditional state of the art.