LETTERS

Perspective on Visible Light Therapy As Prophylaxis Against DR

In the January/February 2014 issue, Geoffrey B. Arden, PhD, FRCOphth, eloquently summarized the case for the role of retinal hypoxia, and particularly the possible influence of increased oxygen demand dark-adapted rod photoreceptors generate in the pathogenesis of diabetic retinopathy (“Phototherapy for Diabetic Eye Disease,” pages 22-29). Pursuing this theory to its logical conclusion, he suggests diabetics therapeutically avoid dark adaptation to slow and even reverse the progression of DR.

Employing ideal visible light wavelengths to avoid light toxicity and sleep disturbance to minimize the potential for adverse treatment effects, he suggests light therapy as a preventive treatment for DR. This is an interesting theory worthy of further investigation, as we cannot have too many useful options in the treatment of DR.

EARLIER TREATMENT

Dr. Arden recognizes that despite the wide availability of new potent pharmacologic agents to treat DR and continued judicious use of retinal photocoagulation, management of the sight-threatening complications of DR remains unsatisfactory.

Due to the risks and adverse effects of these effective therapies, not to mention expense, they cannot be well used as preventive treatments in asymptomatic diabetics who have not yet developed visual loss or “clinically significant” or “high-risk” DR.

He notes that the difficulties we continue to have, despite modern advances, in improving and preserving normal visual function in DR suggest strongly that our focus should move to earlier, prophylactic treatments, aimed at preventing visual loss. Many others join him in this opinion.^1,2

DEFINING EARLY

But what is “early”? In general, chronic physiologic dysfunction precedes anatomic derangement. Thus, with all due respect to the ETDRS, even “mild” nonproliferative DR actually represents advanced disease with end-organ damage. Thus, the risks of treatment (such as argon laser photocoagulation), as much as the physical findings, defined “early” in the ETDRS as now. ³

It is clear, then, that to intervene prior to the development of clinical DR and visual loss, effective measures must be developed that have, ideally, no treatment risks or adverse effects. Light therapy as proposed by Dr. Arden may be one. What about subthreshold diode micropulse (SDM) laser, which Dr. Arden singled out as suggesting a gap in the hypoxia theory he proposes?

OTHER APPROACHES

Before we get to SDM, we should note other therapeutic approaches based on the theories Dr. Arden described. Short-pulse continuous-wave lasers, such as the PASCAL, aim to reduce retinal oxygen demand by selective destruction of retinal photoreceptors.

As with conventional thermal argon laser photocoagulation, it works.⁴ But at what cost? Even if selective destruction of photoreceptors can improve DR, is it not somewhat oxymoronic that we intentionally sacrifice the vehicles of vision to preserve vision?

The RPE, not mentioned by Dr. Arden, is increasingly appreciated as the most influential part of the retina in DR, mediating the retinal response to diabetes via elaboration of innumerable cytokines, known and unknown, including VEGF and PEDF. ²

HURTING TO HELP

Ultra short-pulse continuous-wave lasers now offer treatment consisting of selective destruction of the RPE while largely sparing photoreceptors.⁵ It works, too. However, this approach must solve the same philosophic problems as selective photoreceptor destruction.

In both, tissue damage occurs, as does at least temporary breakdown of the blood-retinal barrier. Healing is by debridement and filling in from adjacent tissue, not regeneration of new replacement elements. Retinal function is lost, and inflammation, however slight, is incited; the potential for retreatment is limited.

Thus, while even these minimally destructive approaches might be justifiable relative to the risk of visual loss in the untreated diabetic eye or the adverse effects of conventional photocoagulation, neither appears suitable — as inherently damaging to the retina — as an early treatment to prevent DR.

SDM LASER THERAPY

So we can now treat DR with full-thickness chorioretinal ablation (ETDRS photocoagulation), selective photoreceptor destruction, selective RPE destruction, and possibly simply avoidance of dark adaptation. All may conceivably function, at least in part, by reducing retinal oxygen demand, as Dr. Arden described. What about SDM?

As Dr. Arden pointed out, SDM is the notable exception. With SDM the RPE is selectively treated but not damaged in any currently detectable way (“low-intensity” treatment). ⁶

Subthreshold diode micropulse treatment actually improves retinal sensitivity by microperimetry at the locus where the treatment is applied.⁷ The neurosensory retina does not absorb SDM, ideally via an 810-nm laser, so it has no direct effect on photoreceptors.

By exclusion, then, SDM appears to work by sustainably influencing the treated, preserved, and viable RPE cells to normalize cytokine expression. The “high-density” treatment (recruitment) of all of the RPE in areas of retinal pathology to achieve a therapeutic effect maximizes the “low-intensity” treatment effect. ⁶

Subthreshold diode micropulse has been demonstrated as clinically effective in the treatment of DR and other disorders in the absence of any known risk or adverse treatment effect.^6-8 SDM can be safely treat the fovea in eyes with fovea-involving DME and 20/20 vision to eliminate DME and decrease the risk of vision loss. ^9-10

STUDY RESULTS

A randomized clinical trial found SDM superior to ETDRS photocoagulation, with results comparable to anti-VEGF therapy. ¹¹

In a retrospective study, SDM panretinal photocoagulation appeared to arrest the progression of DR without any treatment associated retinal damage or adverse effect ¹² — all of this, without directly affecting retinal oxygen demand.

It has become clear that all therapeutically effective treatments for DR also slow, arrest, or reverse DR severity.^1,2 This should surprise no one. However, only one therapeutically effective treatment — SDM — has been shown to do this in the absence of any retinal damage or any adverse treatment effect or treatment risk. ^6-12

IN CONCLUSION

The future of DR management lies in prevention. Light therapy, as Dr. Arden proposed, may be one such strategy. More will certainly emerge.

However, SDM has been shown in more than 13 years of clinical practice and multiple studies to be effective and clinically harmless (without known risk or adverse treatment effect) in the treatment of DR.

The unique safety profile of SDM makes it not only the obvious first-line treatment for DR, but also the only suitable tool available to begin the transition to preventive treatment of DR. It is available to us now.

Jeffrey K. Luttrull, MD
Ventura, CA

REFERENCES

Sneezing and PVD

I read with interest the article by Peter A. Karth, MD, MBA, and Theodore Leng, MD, MS, in the October 2013 issue (“Antiobiotic Controversies in Vitreoretinal Practice,” pages 22-25). I’m writing to share my thoughts on the conundrum they raised regarding the absence of an effect of recent fluoroquinolone use on rates of retinal detachment.

The obvious flaw in the study they cited by Etminan et al¹ is that the risk-set sampling the authors used to select controls was based on matching age and month and year of cohort entry, with no effort to match for a similar proclivity for Valsalva maneuvers.

Valsalva maneuvers, such as coughing and sneezing, are associated with posterior vitreous detachment, which in turn is associated with retinal RD. Upper respiratory illnesses (URIs) cause coughing and sneezing, and they are also a common reason for patients to be prescribed oral fluoroquinolones.

It could be that the sneezing and coughing URI bring on are the precipitating events for a PVD that leads to RD, and it just so happens that fluoroquinolones get caught up in the statistics because the patients’ primary care doctors prescribe them for URIs.

In other words, the causal link to RD is URIs, and the association of URIs with fluoroquinolones creates a confounding link between fluoroquinolones and RD.

The absence of a causal link seems all the more likely given the observed absence of an association with recent or past fluoroquinolone use. RP

Rodger Bodoia, MD, PhD
Olympia, WA

REFERENCE