4.1. Structure–function relationships
As detailed in Sections 2 and 3, a number of neurovascular structure–function relationships have already been demonstrated in human ROP and rat models of ROP. Further delineation of these relationships is needed to advance our understanding of the disease process and its consequences. Improved management of ROP can be built on this foundation.
To date, studies of structure and function have provided evidence that the rods are injured in ROP. The proximal cause of the injury remains unknown. Both hyperoxia and hypoxia, which are injurious to the immature photoreceptors (Fulton et al., 1999b
; Wellard et al., 2005
), are encountered during the course of clinical management of the preterm infant or in the creation of rat models of ROP (Cunningham et al., 1995
; Penn et al., 1995
; Liu et al., 2006a
; Akula et al., 2007a
). The retinal vasculature has been studied extensively in ROP, but less is known about the role of the choroid. The choroid supplies oxygen to the photoreceptors. The swiftly flowing choroid, with oxygen diffusing freely through its fenestrated vessels, may allow local oxygen to reach levels that damage the immature rods. If this is the case, the molecular crosstalk between choroid and pigment epithelium would be altered in the ROP eye (Gogat et al., 2004
; Saint-Geniez et al., 2006
). Molecular hypotheses based on this idea are testable in animal models of ROP.
In human subjects, additional quantitative information about the microscopic structure of the macula is urgently needed in three specific areas: 1) the array of the cone photoreceptors in the macula, 2) the relationship between these cones and the pigment epithelium, and 3) the neural laminae in the central retina. Currently, it is not clear if altered cone and cone bipolar distributions are the basis for the altered mfERG topography, altered visual acuity, and color vision deficits in ROP. This can be elucidated using a combination of adaptive optics ultra-high resolution imaging, multifocal electroretinography, and visual psychophysics in children.
Understanding of the ROP disease process and its lasting consequences, both in children and in rat models of ROP, also calls for further investigation of the cellular and molecular basis for dysfunction in the post-receptor ROP retina. It is the superficial retinal vasculature on which the criteria for clinical diagnosis of ROP at preterm ages is based. More complete information about the deep retinal vasculature is needed to understand the neurovascular interplay in ROP. As detailed in Section 2.4, in older subjects, the locus and characteristics of the deep capillaries are abnormal years after acute ROP has resolved (Hammer et al., 2008
). And in rat models of ROP (Section 3.2.1), an intimate association of the vascular and neural characteristics have been demonstrated through analysis of post-receptor retinal activity and anatomic and molecular biological data, not only after the peak of the acute ROP rat disease but also during its evolution (Akula et al., 2008b
; Favazza et al., 2009
In rat models, the proteins (growth factors) that share in the regulation of development of the post-receptor neurons and the retinal vasculature, both superficial and deep, will be studied in animals with well characterized retinal and vascular function and morphology. This will advance our understanding of the intra-laminar reorganization of the post-receptor retina (Jones et al., 2003
; Xu and Tian, 2004
) during the evolution and resolution of ROP. In human ROP subjects, the structure and function of post-receptor central retina will be studied using adaptive optics imaging (Hammer et al., 2008
) and by analyzing higher order signals in the mfERG (Hood, 2000
; Bearse et al., 2006
). Rod-driven post-receptor function will be further studied using psychophysical assessments of adaptation and spatial processing. In some retinal regions, oximetry (Pournaras et al., 2008
) is expected to yield further insights into the neurovascular function of the ROP retina and its relation to vision.
4.3. Therapeutic targets
Even in our current state of knowledge, it is clear that both photoreceptor and post-receptor retina are involved in the ROP disease process. Both are potential targets for intervention.
Persistent rod dysfunction is documented in human ROP subjects (Section 2). Structure and function of ROP rat rods are altered acutely and chronically (Section 3). ROP has its onset at the ages during which the developing rod outer segments elongate with consequent escalation of energy demands. Early ROP rat rod dysfunction predicts the severity of the retinal vascular outcome. Thus, interventions that target the rods are strongly motivated. Theoretically, interventions that support either the metabolic and nutritional needs of the immature rods or protect those rods from injurious exposures, or both, would reduce the neurovascular abnormalities of ROP. The photoreceptor's turnover of outer segment material, maintenance of the circulating current, and operation of the visual cycle demand a level of energy use that exceeds that of any other cell in the body (Steinberg, 1987
). This extreme metabolic demand makes the photoreceptor vulnerable to injury. Interventions that could be safely administered very early in the clinical course of ROP, such as at the first detected sign of neurovascular disease, should be the most beneficial.
Light exposure modulates the turnover of outer segment material and suppresses the rods' circulating current and, accordingly, decreases the photoreceptor's energy requirement. Arden et al. (2005)
have proposed light as a safe, effective treatment for ROP. Light level of the habitat in which infant rats are reared affects long term the ERG photoresponse parameters in both normal control and ROP rats (Fulton et al., 1998a
). Thus, a pre-clinical test of the Arden hypothesis is feasible. Following a longitudinal design, with outcome measures being ERG receptor and post-receptor components and integrated curvature of the retinal arterioles, it can be determined if moderate, non-damaging light levels, rather than very dim light, improve the neurovascular outcome in rat models of ROP. For success, such a strategy must consider rod photoreceptor development (), which was not necessarily the case in a clinical study of effects of light on ROP (Reynolds et al., 1998
Control of ambient oxygen may be hypothesized to damp hyperoxia-hypoxia swings that damage the rods and to supply, via the choroid, optimal oxygen levels to cope with the rods' energy demands. Tight regulation of supplemental oxygen has reduced the frequency and severity of ROP in some studies. (Askie et al., 2003
; Chow et al., 2003
; Saugstad, 2006
; Vanderveen et al., 2006b
). Interestingly, Sears and colleagues reported lower oxygen saturation targets at less than 34 weeks (85% to 92%) and higher targets at greater than 34 weeks (92% to 97%) decrease the severity and incidence of ROP (Sears et al., 2009
). Although the age for shift in oxygen management was not based on the rhodopsin growth curve, it appears well chosen ().
Based on this evidence, one can argue that extensions of current practice to evidence-based standardization of light and oxygen interventions, plus vigorous screening programs, are the components of a practicable, global program that will relegate ROP to the history books. However, to date, adjustments in light and oxygen and faithful adherence to screening and treatment guidelines do not prevent clinical ROP or residual visual deficits that occur even when ROP has been mild. Thus, consideration of additional approaches is warranted. We theorize that improved management, or better yet, prevention of ROP, may be achieved through timely pharmaceutical interventions.
Goals of pharmaceutical treatment of ROP rods include suppression of turnover of outer segment material and support of the circulating current. By following this line of reasoning, many possible pharmaceutical agents may hold realistic promise for treatments of ROP. A few are mentioned below.
Members of the family of carbonic anhydrase inhibitors (CAI) suppress conversion of carbon dioxide to bicarbonate and are commonly used to treat glaucoma by decreasing production of aqueous humor. CAIs, such as acetazolamide that is administered systemically, also alter circulating current of the rods (Donner et al., 1990
; Findl et al., 1995
). In a pilot study on mature rats, a topically administered CAI, dorzolamide (Trusopt, Merck), doubled the saturated amplitude of the rod photoresponse, RROD
, derived from the ERG a-wave (Chang et al., 1997
). Thus, dorzolamide is forecast to relieve the immature photoreceptor from one of its most energy demanding tasks (generation of the circulating current) and to reduce injury to the photoreceptor and hypoxia in the post-receptor ROP retina. We also note in our recent ROP rat experiments (Section 3), a visual cycle modulator that at early ages increased the saturated amplitude of the rod photoresponse (RROD
) improved both rod sensitivity (SROD
) and integrated curvature (ICA
) of the arterioles at older ages (Akula et al., 2008a
Another likely mechanism of injury to the rod includes generation of reactive oxygen species, or free radicals. Oxidative stress increases the production of reactive oxygen species that can damage intracellular lipids, proteins, and DNA, including mtDNA (Corral-Debrinski et al., 1991
; Cadenas and Davies, 2000
). In the retina, 90% of the mitochondria are in the photoreceptor inner segments. Damage to mitochondria reduces their ability to produce cellular energy, which exacerbates oxidative stress and eventually activates the apoptotic cascade. Activation of the apoptotic cascade leads to death of photoreceptors in retinal degenerative disorders (Delyfer et al., 2004
). Because antioxidants scavenge free radicals, shrewdly compounded antioxidants administered in a timely fashion to at-risk infants may protect the mitochondria and the rod itself from oxidative damage (Penn et al., 1997
; Liu and Ames, 2005
; Dorfman et al., 2006
; Ates et al., 2009
) and thus prevent initiation of the apoptotic pathway and attendant cell damage. Although this intervention targets the photoreceptor with abundant mitochondria, it is also anticipated to reduce expression of VEGF and have beneficial effects on post-receptor neural retina and the retinal vasculature.
In addition to the secondary benefits that may arise from treatments targeting the photoreceptors, interventions that target primarily the retinal vasculature and post-receptor neurons, which are intimately associated, both in normal development and in ROP, also warrant consideration for treatment of ROP (Akula et al., 2008b
). VEGF is critical to angiogenesis and neurogenesis (Provis et al., 1997
; Sandercoe et al., 2003
; Gariano et al., 2006
). Thus, lowering VEGF is touted as a promising treatment for neovascular diseases of the retina. It is tempting to inhibit the actions of VEGF as early as possible, thereby preventing the occurrence of ROP altogether (Gariano et al., 2006
). However, timing such treatment of the developing retina poses a number of concerns because neural dysfunction (receptor and post-receptor) antedates the vascular abnormalities and survives their resolution. Thus, it seems unlikely that anti-VEGF treatment holds much hope of restoring good vision to patients with milder forms of ROP (Fulton et al., 2009
). In the mature retina, VEGF is a survival factor for many retinal cells including the photoreceptors (Saint-Geniez et al., 2008
). Considerable concern about treatment of ROP with anti-VEGF pharmaceuticals has already been expressed due to the potential for adverse effects on developing neurons and possible adverse system effects in the growing child (Aiello, 1997
; Yourey et al., 2000
; Hashimoto et al., 2006