Introduction

Primary open angle glaucoma (POAG) is a chronic optic neuropathy affecting approximately 2 % of adults above the age of 70 years [1]. It is one of the leading causes of blindness worldwide and represents a major public health issue.

There is growing awareness that environmental factors, behaviors, and diet are intimately related to patient health and may play a role in the pathogenesis and progression of glaucoma. Evidence from recent years has shown that some behaviors and environmental factors can affect the intraocular pressure (IOP), the primary risk factor for POAG [2–5].

Patients are interested more than ever in conducting a healthy lifestyle and taking an active role in the management of their ailments. Many seek lifestyle, diet, and other advice from their doctor and other sources. Medical information is readily available on the internet with variable quality, and many POAG patients implement it in their daily lives [6]. It is important, therefore, to be aware of the evidence and potential risks involved in various lifestyle and diet recommendations concerning POAG patients.

The amount of medical literature on the subject of modifiable lifestyle interventions (MLS) in the management of POAG is growing steadily with new aspects explored in the last few years. The conclusions are not always clear and sometimes even contradictory. Several reviews have been published on this subject, the latest by Welte et al. in 2016 published in German, which focuses mainly on alternative medicine for POAG [7]. The most recent English review has been published by Bhartiya in 2014, which also focuses on alternative medicine and only to a lesser extent on lifestyle modifications [8]. Other reviews evaluate the effects of MLS on the risk of developing glaucoma in healthy subjects [9, 10].

As of yet, there are no guidelines available about what lifestyle advice should be offered to POAG patients, if any. This article aims to review the literature regarding MLS in the management of POAG patients focusing on the last 3 years.

Materials and methods

Literature searches

A systematic search was conducted using Cochrane Library and MEDLINE, PubMed, ClinicalTrials.gov, metaRegister of Controlled Trials (www.controlled-trials.com), WHO International Clinical Trials Registry Platform (http://www.who.int/ictrp/search/en) and Google Scholar with the following keywords: glaucoma, POAG, diet, supplement, extract, coffee, caffeine, alcohol, exercise, weight-lifting, yoga, valsalva, sleep, position, supine, complementary, CAM, marijuana, smoking, tobacco, and lifestyle. Additional records were identified by hand-searching bibliographies of relevant studies.

Eligibility criteria

The aim of this review was to identify studies which relate to effects of different MLS interventions on patients affected by POAG. We included studies meeting the following criteria: 1. The study population included a POAG patient group. 2. Studies examining any objective outcome measure relating to POAG (e.g., IOP, visual field (VF) changes, pattern electroretinogram amplitude, or foveal sensitivity). 3. Examining any MLS such as diet change, food supplements, sleeping position, caffeine intake, physical activities. 4. Written in English. 5. Full publications (not an abstract or letter to the editor). 6. Published between 2013 and 2016. Our exclusion criteria were 1. Studies focused on incidence or prevalence in healthy populations 2. Studies performed on animals, cadavers, healthy subjects, or those affected by conditions other than POAG. 3. Case reports and non-empirical opinion articles were also excluded.

Screening and synthesis

The review process was conducted under the guidance of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) criteria to support reporting [11]. Two reviewers (IH, AA) independently conducted the search for relevant publications. Selected publications were then approved by a senior investigator (ZB). Individual studies were graded using the Scottish Intercollegiate Guideline Network (SIGN) assessment system for individual studies as implemented for Preferred Practice Patterns by the American Academy of Ophthalmology [12]. Figure 1 shows a flow diagram of the inclusion process.

Fig. 1
figure 1

Flow diagram of inclusion process based on Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [11]

Full size image

Results

Data was extracted from 12 publications that met our criteria. Most were cross-sectional or retrospective in nature, and four were randomized controlled trials. Table 1 shows a summary of the studies. Table 2 presents a comparison of the recent evidence to previous evidence with clinical correlations.

Table 1 Summary of studies
Full size table
Table 2 Comparison of recent evidence to previous evidence
Full size table

Diet

Previous studies have shown that higher intake of fruit, especially fresh oranges and peaches, may be associated with decreased odds of glaucoma in healthy subjects [13]. Vegetables such as fresh carrots, green collards, kale, and iceberg or romaine lettuce may also be associated with a decreased risk of glaucoma in some populations [14, 15]. One study showed that dietary supplementation with oxidants such as iron and calcium may increase the risk of glaucoma [16].

Three randomized controlled studies published in recent years explored different food supplementation options in the management of POAG.

A study focusing on dietary supplementation was recently published by Mutolo et al. [2]. Twenty-two POAG patients were randomized into a control group and a food supplement group which was given two tablets a day containing homotaurine, coleus forskohlii root extract, l-carnosine, folic acid, vitamins B1, B2, B6, and magnesium. The groups were then followed for one year. The treatment group showed a progressive decrease of IOP peaking at about 2 mmHg after 12 months, while the control group showed no significant reductions (P < 0.01). Furthermore, a significant improvement of pattern electroretinogram amplitude (2.9 μV, P < 0.01) and foveal sensitivity (4.8 dB, P < 0.05) was observed in treated patients, but not in controls.

Another study conducted by Garcia-Medina et al. showed less promising results [17]. One hundred and seventeen POAG patients were randomized into a control group and two treatment groups who were given slightly dissimilar tablets once a day, containing a combination of vitamins, lutein, zinc, copper, magnesium selenium, and ω-3 fatty acids. Patients were then examined every 6 months for a period of 2 years. No significant difference was found between the groups in any of the outcome measures which included visual field examinations and optical coherence tomography scans.

Finally, in 2014 Bonyadi et al. published a study focusing on the effect of saffron [18]. Thirty-four POAG patients were randomized into a control group, who were given a placebo, and a treatment group, given oral aqueous saffron extract for 1 month, both groups were then followed for an additional month. The treatment group showed a significant decrease of IOP relative to the control group (saffron: from 12.9 ± 3.7 to 10.6 ± 3.0 mmHg vs. control: from 14.0 ± 2.5 to 13.8 ± 2.2 mmHg; P = 0.001) with IOP values returning to normal once the treatment was discontinued.

Sleeping position

The way glaucoma patients sleep may also influence IOP. It has been demonstrated that IOP is higher at night when patients are asleep and in the supine position [19]. The extent of IOP increase may be mitigated by sleeping with a slight head elevation [20]. In recent years several groups investigated the significance of specific sleeping positions relative to others.

A retrospective study on 692 POAG patients was conducted by Kim et al. in 2014, investigating the relationship between preferred sleeping position and VF loss [21]. In a questionnaire survey approximately a third of patients preferred a lateral sleeping position; 66–72 % of those habitually sleep on the worse eye-dependent side.

Lee et al. demonstrated a significant increase in IOP (1.3 ± 0.99, P < 0.001) of the dependent eye relative to the non-dependent eye of 20 POAG patients after five minutes in a lateral position, regardless of the eye in which the glaucoma was worse or the head elevation [22].

These results seem somewhat at odds with the work of Kaplowitz et al. who examined sleep behavior of POAG patients using sleep laboratory videos, surveys, and home recordings of 261 POAG patients in total [23]. The authors have found that a lateral sleeping position was preferred by most POAG patients and that surprisingly the non-dependent eye was the one correlated with a worse VF loss (P = 0.002). The authors speculate that this may be a result of reduced perfusion in the non-dependent eyes.

Flatau et al. explored the possibility that IOP elevation is influenced by mechanical force created by the pillow on the eye [24]. They examined the eyes of 22 glaucoma patients and 11 controls in the “face down” sleeping position. The IOP of the glaucoma patients was increased by an equivalent of 2.5 ± 1.1 mmHg in the “face down” position (P = 0.03), however, was unchanged in the controls (P = 0.33).

Exploring the effect of head elevation, Lazzaro et al. examined 30 patients in a sleep laboratory, half of which had glaucoma [3]. On two separate nights they slept either lying flat or lying on a wedge pillow at a 20-degree head elevation. They measured IOP before and during the night at 2-h intervals. Patients sleeping with a head elevation demonstrated a lesser elevation of IOP throughout the night (1.51 mmHg) compared with those sleeping supine (supine: 16.02 ± 1.65 vs. head-elevation: 14.5 ± 1.36; P < 0.001). These results are in line with previous studies on the subject.

In 2016 Park et al. investigated whether different methods of head elevation achieve the same outcome [25]. Their results show that a bed head angle elevation of 30 degrees significantly lowered the IOP of POAG patients by an average of 2.0 mmHg (P < 0.001), while using multiple pillows for the same elevation caused no significant change in IOP (P = 0.081).

Caffeine

Caffeine is a popular pharmaceutically active substance found in many beverages, food products, and medications. Most studies indicate that caffeine consumption equivalent to one cup of coffee elevates IOP about 1–2 mmHg for a 2-h period in both healthy patients and those affected by POAG [26–29]. Several studies have demonstrated a relationship between heavy caffeine consumption (≥3 cups a day) and the risk of glaucoma in susceptible individuals [30, 31], yet the implications in POAG patients themselves has hardly been studied.

A recent study by Aziz et al. addresses this question [32]. Ninety-one POAG patients had their coffee consumption habits recorded and analyzed with respect to their disease severity and progression. The results show that despite no significant association between coffee drinking and the disease severity (P = 0.863), frequency of coffee intake was significantly associated with disease progression (P = 0.001). In this small cross-sectional study daily coffee consumption was associated with an eightfold increase in the risk of progression (95 % confidence interval; OR 2.5 to 9.6).

Exercise

In healthy individuals aerobic exercise such as jogging, cycling, and walking reduces IOP, whereas exercises such as heavy-weight lifting may elevate IOP [4, 33, 34]. In POAG patient these responses to exercise seem to be similar or even more pronounced [35]. There is some debate over the benefits of exercise in POAG patients [36], yet few prospective studies exist.

In 2015 Agrawal et al. randomized 90 newly diagnosed POAG patients into three groups, two groups received medical treatment only while the third was also advised to exercise 30 min daily [37]. The exercise group achieved significantly lower IOP values compared with the patients treated identically, but without exercise (9 mmHg and 6 mmHg reductions respectively; P value not reported) and noted better visual quality of life after 30 days.

Yoga

Yoga on the other hand, might be detrimental. The head-stand position performed by yoga practitioners has been shown to elevate IOP approximately twofold during and following the exercise, particularly in glaucoma patients [5, 38]. A recent study by Jasien et al. on POAG patients showed similar IOP increases in yoga positions such as the “downward facing dog” position (17 ± 3.2 mmHg to 28 ± 3.8 mmHg; P < 0.01) and the “forward bend” position (17 ± 3.9 mmHg to 27 ± 3.4 mmHg; P < 0.01), amongst others [39].

Swimming goggles

Swimming goggles may elevate IOP up to 4.5 mmHg and may decrease blood flow to the optic nerve head [40–42]. They have also been reported to extend drainage blebs in patients who have undergone trabeculectomy [43]. However, a study from 2015 by Franchina et al. found no association between frequent swimming goggles use and glaucoma [44]. The long term effect of swimming goggles on POAG patients is yet unresolved.

Additional modifiable factors

Wearing a tight neck-tie may elevate IOP in POAG patients for a short period, yet extended wear has practically no effect [45].

We identified no studies on the effect of alcohol on POAG patients; however, some studies show alcohol consumption does not affect the risk of glaucoma [46].

Similarly, we identified no studies concerning the influence of tobacco smoking on patients affected by POAG. Some studies have suggested smoking might be a risk factor for the development of glaucoma, yet a recent meta-analysis of six observational studies found no causal association [47, 48].

Additionally, no studies regarding the effects of marijuana met our inclusion criteria. Several small studies from the 1980s, however, showed that cannabinoids may reduce IOP transiently [49].

Discussion

There is currently insufficient evidence that changing lifestyle habits has a proven effect on the progression of glaucoma. It is possible that lifestyle plays an important role, yet the lack of robust randomized controlled trials precludes any definite conclusions. Awareness of the possible influences of certain habits, however, should help guide clinical advice on an individual patient basis.

Moderate aerobic exercise and a high fiber diet containing fruits and vegetables may favorably influence glaucomatous disease progression and is also consistent with a healthy lifestyle. Moderating caffeine intake and changing sleeping positions may also have a positive impact. Regarding weight-lifting and yoga exercises, patients should be informed of the potential negative effects these activities might have.

Each intervention discussed in this review may provide a modest reduction in IOP or a reduction for a modest period of time. Exercise and diet might reduce IOP merely 2–3 mmHg each, caffeine restrictions might reduce IOP only 1–2 mmHg for a short period, and sleeping with a head elevation may achieve just 1–2 mmHg IOP reductions during sleep. The cumulative effect, however, might be significant.

The risk of progression in glaucoma is dependent on IOP. In the Early Manifest Glaucoma Trial the magnitude of this dependence was estimated at 10 % decrease in risk of progression with each IOP reduction of 1 mmHg [50]. Using simple and inexpensive methods to achieve even a modest reduction in IOP may be a worthwhile cause and have a dramatic impact on POAG patients.

Most measurements of IOP are performed in an office setting during the patient’s visit. They provide only a brief glimpse of the actual average pressure the patient is exposed to. Over a 24-h period IOP fluctuates markedly and may vary up to 10 mmHg [51, 52]. Many behaviors and environmental factors may cause this fluctuation. Understanding these factors and advising patients regarding their potential effects might influence average IOP and disease progression even with seemingly no change in IOP measured by conventional means.

Certain MLS have been shown to have a large impact on other eye conditions. In 2001 the Age-Related Eye Disease Study Research group showed that supplementation with their formula led to a significant decrease in progression of age-related macular degeneration [53]. Some of the components of their formulae (vitamins C, E, and zinc) are discussed in this review in the context of glaucoma. The fact that well known simple dietary compounds can produce such outcomes in other conditions provides ample justification for investigating their effects in respect to glaucoma.

Given the retrospective nature of the evidence and the limited amount of randomized controlled trials on this subject, drawing clinical correlations is difficult. This work may provide the basis for future studies to test the clinical correlations gathered in this review and to initiate randomized controlled trials examining the effects of MLS on patients affected by POAG.