at the earliest time possible, well before the median, but still within the known range. This tends to occur within very immature retinas in very tiny premature infants. It also tends to occur in a zone I location. And the progression is very fast, hence AP-ROP. However, even this plays out within the known window and follows a linear pattern. Zone I disease is recognized to be harder to clinically delineate. The early stages can be overlooked, neovascularization can be primarily intraretinal rather than obviously extraretinal and it can progress from stage 1 to stage 3 very fast (Fig. 4.4). So AP-ROP does exist, it is fast and it is dangerous, but doesn’t present as a normal retina ­followed by threshold ROP in 1 day. It progresses rapidly through the difficult to recognize stages and perhaps most importantly, develops plus disease quickly and early. So we now know that ROP follows a linear progression in a tight time frame that must consider the range (standard deviations) as well as medians and be mindful of how the individual patient fits into this known range. We will revisit timing issues in the screening session.

Prognostic factors within the retina were detailed in two major CRYO-ROP publications [72, 73]. Major prognostic indicators were the presence of plus disease, the location by zone in which ROP develops, and ROP status. Minor prognostic indicators were the circumferential extent of stage 3 disease, and the more difficult to assess rate of progression. ET-ROP confirmed the major importance of plus disease and zone. They did not confirm the independent parameter of extent of stage 3 by clock hours or involvement [35]. Hence the more posterior the disease, the ­presence of plus disease, and the presence of stage 3 disease all increase the risk of progression to an unfavorable outcome. We can think of ROP as having natural breaks of escalating risk of unfavorable outcomes as follows:

Disease with Little or No Risk

•Immature vascularization, zone II or III

•Stage 1, zone II or III, no plus

•Stage 2, zone II or III, no plus

Disease with Moderate Risk

•Immature vascularization zone I

•Stage 3, zone II or III, no plus

•Stage 1 or 2, zone I, no plus

Disease with High Risk

•Stage 3, zone II, plus

•Stage 3, zone I, no plus

•Stage 3, zone I, plus

This is another way of saying plus disease and zone I disease are the critical retinal risk factors with stage 3 playing an important role. This was well documented by CRYO-ROP and confirmed and refined by ET-ROP.

Special attention should be given to zone III. Zone III is defined by full nasal vascularization. As we stated in the classification section, the temporal retina is irrelevant to differentiating between zone II and zone III. This has produced misunderstanding in the natural history of zone III events and unfortunately clinical misinterpretations. Again as previously stated, retinal vascularization follows a narrow time frame. CRYOROP and LIGHT-ROP provide information on normal vascularization patterns. Figure 4.32 illustrates the timing of onset of zone III vascularization in no ROP patients [74] (Fig. 4.32). The two studies produced identical curves. Nasal vascularization occurs at a median of about 35 weeks PMA. The range of onset is from 31 weeks to just over 40 weeks [74]. Zone III vascularization should rarely be present the first time a screening exam is performed. The achievement of zone III vascularization in a normally maturing retina is indeed a very positive retinal prognostic indicator. However, the development of ROP prior to this negates this positive prognosis. Full nasal vascularization can develop in the presence of preexisting temporal zone II ROP. In that event the prognostic factors apply to zone II ROP not zone III ROP [36]. This is how the misconception surrounding zone III arises. Zone III normal vascularization or zone III ROP without preexisting zone II ROP are both very favorable prognostic signs. But zone III ROP following zone II ROP must continue to be followed as zone II ROP.

Regression of ROP is also part of the natural history of the disease. However, since effective treatment exists, the natural regression patterns of serious ROP now are no longer observable. Obviously most low risk and much moderate risk ROP disease regresses without incident and without an unfavorable outcome. Yet it is important to point out that risk is a relative term and that low risk does not equate to zero risk. As a corollary, all favorable outcomes are not necessarily normal outcomes. We will discuss this more in the treatment section.

Fig. 4.32  Cumulative onset of zone III vascularization for no ROP patients by postmenstrual age and chronologic age for CRYO-ROP and LIGHT-ROP. (Reprinted from [74] copyright, American Medical Association, 2002. All rights reserved)

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4.6  Pathogenesis

ROP is a disease of VLBW infants. Birth weight and GA are the overwhelming risk factors for this disease [44]. Pathophysiologic theories must account for this. There are many other observed risk factors in the literature, most from small sample size reports. The larger or wellcontrolled the trial, the fewer risk factors rise to statistical significance [75–78]. Several risk factors have been studied extensively and discarded as blind alleys. Two of those are ambient light [32, 34, 79–85] and vitamin E [86–90] Oxygen remains a frustratingly difficult risk factor to decipher [15, 18, 22–28, 91, 92]. A facetious report even speculated that ROP would correlate with the weight

of the infants’ charts [93]. This latter contains within it the realization that ROP is indeed associated with the sickest children when birth weight remains constant. These sick children have multiorgan, multisystem compromise and the external support is the most aggressive. It is very difficult to isolate the retina from these other organ failures.

We have discussed the normal embryology of the retina within the natural history section presented earlier. The pathogenesis of ROP therefore begins with premature birth and is superimposed on retinal embryologic development. It is this superimposition on retinal embryology and the consequent tight correlation with GA rather than chronologic age that makes ROP

a combined disease of extrauterine environmental factors and inherent retinal and perhaps genetic factors.

We know the retinal embryology, so what happens to an infant after premature birth? At birth, fetal circulation is transformed by the switch from placental oxygenation to lung oxygenation. Oxygen saturation rises from mixed venous levels to arterial levels. However, these fetal lungs are immature and are not capable of fully mature oxygen transfer. Medical intervention provides inhaled supplemental oxygen, enhancing the oxygen transfer. These fetal lungs can sometimes respond to this more efficiently in the initial neonatal period than later due to the superimposition of chronic lung disease. So several factors lead to a potential initially hyperoxic state: mixed venous oxygenation to arterial oxygenation; supplemental inspired oxygen; immature but as yet undamaged lungs; and a low retinal metabolic rate of oxygen consumption.

At some point after birth this relative hyperoxia begins to change. The lungs become damaged, alveolar – blood oxygen exchange is compromised, and retinal metabolic demand for oxygen rises precipitously according to rigidly timed embryologic events. This gives rise to relative hypoxia. And this transition is not a smooth, linear one. There are undoubtedly dramatic swings during the gradual change over from hyperoxia to hypoxia. So again ROP pathogenesis must account for retinal developmental imperatives and these observed physiologic changes.

Retinal vascularization is modulated by VEGF, which is constructed by highly regulated VEGF MRNA [62–65]. This process is acutely sensitive to relative states of hyperoxia and hypoxia. Hypoxia up regulates VEGF and hyperoxia down regulates its production. VEGF levels are therefore not only going to be influenced by extrauterine environmental factors, but also the intrinsic metabolic demands of a developing retina. The onset of any subclinical injury in a premature retina is not known, but the onset of the clinical disease correlates with this embryology. Perhaps there is an environmentally determined injury, perhaps at the cellular level. But the manifestation of ROP is dependent on the development status of the retina, specifically metabolic activity.

VEGF is not the only vasoactive molecule within the retina. There are insulin like growth factor, IGF-1, basic fibroblast growth factor, and transforming growth factor, plus any isoforms [60, 66–69]. These cytokines create a complex milieu in which relative oxygen concentrations drive vascularization. And this vessel

formation and tissue invasion create modifiable changes in the extracellular matrix [94–97]. Astrocyte development and migration parallel this vascularization and may be the source of VEGF [98, 99].

Flynn and Chan-Ling have offered a unique take on this process [70]. They maintain that retinal vascularization is indeed sequential. Early, initial, vascularization occurs from 14 to 21 weeks GA, arises exclusively from vascular precursors, and is limited to zone I retina. Further vascularization transitions to the budding process and forms the vascular bed in zones II and III. More importantly, the first phase is independent of VEGF while the second phase is VEGF determined. They present solid evidence for this and propose it as a physiologic explanation for the different pathophysiologic manifestations and treatment responses of zone I versus zone II ROP.

This is a very attractive theory. It provides an explanation for the very different behavior of AP-ROP in true zone I versus transitional ROP versus zone II ROP. It fits with much of the basic science data, and it is not inconsistent with the VEGF/oxygenation theory of ROP but adds to it. However, if zone I AP-ROP is VEGF independent, then it would seem to be independent of the relative retinal hypoxia/metabolic demand relationship that seem integral in explaining the uniformity of time of onset. On the other hand, AP-ROP in zone I do tend to occur earlier in PMA, especially true zone I AP-ROP, i.e., 360° of zone I and not just one clock hour of zone I. But what does drive the insult of this form of ROP? This remains to be seen.

Zone I uniqueness notwithstanding, typical ROP appears to occur in two phases, a vasocessation phase and a vasoproliferative stage. Phase I, the vasocessation phase, begins with the initial hyperoxia that occurs in the immediate neonatal period. Arterial oxygen, increased FIO2 (fraction of inspired oxygen) and low retinal oxygen utilization secondary to a low metabolic rate all combine to produce this relative hyperoxia. The relative nature of the hyperoxia is important. The tissue saturation may not be higher than the normoxic levels of a mature retina, but retinal vascularization should be normally occurring under the in utero hypoxic conditions characteristic of mixed venous blood supply. So, the tissue oxygen tension is high, VEGF production is diminished, and normal vascularization ceases. Ashton and co-workers first hypothesizedtheconnectionamongvascularization,oxygenation, and retinal metabolic demand [14]. But he thought of it as a vaso-obliteration phase. There is no evidence for mature

endothelial cell breakdown [100]. Therefore vasocessation is a more accurate term.

Phase II is the vasoproliferative stage. As one would expect, this phase is driven by hypoxia and increasing VEGF levels. The relative hypoxia is fueled by decreasing alveolar oxygen exchange and increasing retinal oxygen utilization. It is important to recognize that these two phases do not transition abruptly. It is undoubtedly a phased transition albeit within a probably tight time frame. During this possibly short transition period, the retina probably undergoes frequent and potentially wide swings of hyperoxia/hypoxia. This could be even more meaningful for ROP development [101, 102].

This two phase view of ROP development is completely compatible with the observed homogeneity of onset of ROP, the VEGF model of ROP, and at least in theory the correlation between severity of ROP and overall “sickness” of the infant. The sickest infants undoubtedly have the most volatile transition phase of retinal oxygenation with medical intervention chasing the tissue oxygenation needs. It suggests that the ultimate cause of ROP is related to the mismatch of tissue oxygen need and tissue oxygen supply. This need not be an identical oxygenation level ideally suited for other organ systems. And certainly it is not the same or nearly as simple as the measured peripheral oxygen saturation. When one appreciates the need for microenvironmental sampling and assessment, one understands why medical oxygen saturation measurements are far too gross to be so far meaningful.

There is much to learn of ROP pathophysiology. Is it all a VEGF/other cytokines/variable oxygenation process superimposed on rigidly timed natural embryologic retinal development? Are there cellular damage components that are so far anatomically not identifiable? Why does the vasoproliferative phase result in abnormal neovascularization with devastating cicatricial consequences? Does true zone I physiology (360° of zone I) really mandate a different mechanism of disease? The answers to these and more questions will undoubtedly be determined in the future.

4.7  Screening

Screening for acute ROP is a critical and absolutely necessary clinical activity. Its primary goal is to detect all treatment level disease at a stage appropriate for

intervention. This means that a level of retinal disease must be detected at a point where treatment is indicated but not beyond where treatment would be effective. This concept introduces a time factor and is therefore commonly referred to as a treatment window of opportunity. Disease must be detected during the scientifically determined intervention point. Detecting disease earlier has no inherent advantage, but detecting disease that has progressed beyond the optimal intervention point has grave implications. To paraphrase, ROP is a progressive disease and in early, not serious disease, treatment is not indicated, but in late, advanced acute/cicatricial disease treatment response is not optimal. So each retinal disease state has a narrow window along the progression trajectory and has a critical timing component to it.

Screening for acute ROP is unlike most other disease screening tools or protocols. Most screening is conducted by nonphysicians e.g., vision screening, and a false negative rate is not only inherent, but calculably necessary. ROP screening makes no allowance for false negatives. Screening blood tests or other laboratory screens are also typically imperfect. ROP screening makes no allowance for imperfection. So screening is really a misnomer. ROP screening is really a series of complete, thorough, professional eye examinations and the practitioner is held to a very high standard. Any system devised to screen infants utilizing nonophthalmologists or remote ophthalmologists should be highly cognizant of this fact.

On the other hand, while being held to standards well above those expected in typical screening situations, more mundane issues such as cost, efficiency, and exam complications are bundled into the process. So many outside experts treat ROP screening the way one would preschool vision screening or a blood test, analyzing cost efficient methodology. But these analyses do not have the same applicability when this is recognized for what it is, i.e., definitely not simple disease screening. We will cover this in some detail in the medico-legal section.

Medical evaluation for ROP involves defining an at risk population, determining a time to initiate examination, conducting ongoing examinations while maximizing disease recognition and minimizing complications, assessing an appropriate conclusion point for the evaluations, and having a treatment plan available. The problem in doing this has been that most guidelines were developed by consensus or utilization of empirical data.

All the organizational official guideline publications have traditionally been consensus documents. Previous examples of this include the British, American, and Canadian documents [103–105]. This is no longer the case. The CRYO-ROP and LIGHT-ROP investigators teamed up to produce the first evidence based screening criteria utilizing large multicenter databases [74]. Unfortunately, even these trials can have data holes and therefore cannot answer every medical evaluation question rigorously. A glaring omission involves data on infants over 1,250 g birth weights. These trials did not collect data on larger infants and so empirical data is still necessary to complete a screening guideline. But the

above paper provided a robust platform upon which criteria could be based.

The recently published 2006 U.S. screening guidelines is based primarily on the findings of the 2002 ­evidence based CRYO-ROP and LIGHT-ROP work [41, 106]. It has already been discussed that ROP progression is predictable. Large sample sizes allow us to use this predictability with confidence. Figures 4.33 and 4.34 illustrate this. Figure 4.33 shows the timing of onset for various serious ROP states in CRYO-ROP patients. Figure 4.34 takes one of those disease states, i.e., prethreshold, and graphs its onset for both CRYOROP and LIGHT-ROP. The steep curve demonstrates

Fig. 4.33  Timing of onset of serious ROP by postmenstrual age and chronologic age.

(Reprinted from [74] copyright, American Medical Association, 2002. All rights reserved)

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Fig. 4.34  Timing of onset of prethreshold ROP by postmenstrual age and chronologic age for CRYO-ROP and LIGHTROP. (Reprinted from [74] copyright, American Medical Association, 2002. All rights reserved)

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the tight time frame but also the entire range of onset. The near superimposition of the LIGHT-ROP data confirms the original CRYO-ROP data validity over time. The authors used this data to predict when to initiate screening exams and when to safely conclude them. The following recommendations are from the 2006 consensus paper but were based directly on the 2002 paper as well as incorporating the new ET-ROP data.

Table 4.6 presents the suggested timing for the initiation of acute ROP evaluation examinations based on GA at birth and PMA and CA of the first exam. It arises directly from an analysis of Fig. 4.6 data. Follow-up examination recommendations were based on ET-ROP

findings and are presented in Table 4.7. Conclusion of acute screening recommendations was based on the 2002 paper as well as CRYO-ROP regression data [36, 74] (Table 4.8 provides this).

The above guidelines apply to infants with birth weights of 1,250 g or less and only to the U.S. Likely the data could be extrapolated to other high wealth countries. But what of bigger birth weight babies or international concerns? There is little large scale data on babies over 1,250 g. Several recent publications have tackled screening inclusion criteria [40, 107– 114]. These studies do not use large rigorous multicenter databases although may use pooled data. These studies definitely do not agree on inclusion criteria.

Table 4.6  Timing of initiation of acute ROP screening

Table 4.7  Recommended follow-up exam intervals

One week or less

 Stage 1 or 2 ROP: zone I

 Stage 3 ROP: zone II

One to two week

 Immature vascularization: zone I – no ROP

 Stage 2 ROP: zone II

 Regressing ROP: zone I

Two week

 Stage 1 ROP: zone II

 Regressing ROP: zone II

Two to three week

 Immature vascularization: zone II – no ROP

 Stage 1 or 2 ROP: zone III

 Regressing ROP: zone III

Table 4.8  Conclusion of acute ROP evaluation based on age and retinal findings

Zone III retinal vascularization attained without previous zone I or II ROP (if there is examiner doubt about the zone or if the PMA is less than 35 weeks, confirmatory examinations may be warranted)

Full retinal vascularization

PMA of 45 weeks and no prethreshold disease (defined as stage 3 ROP in zone II, any ROP in zone I) or worse ROP is present

Regression of ROP (care must be taken to be sure that there is no abnormal vascular tissue present that is capable of reactivation and progression)

The 2006 U.S. guidelines paper suggested including all infants with a birth weight of 1,500 g or less OR GAs of 30 weeks or less. The 2001 guidelines paper from the same sponsoring organizations recommended 1,500 g and/or 28 weeks . The other eight above have varied recommendations. Since the CRYO-ROP/ LIGHT-ROP evidence based data only applied to 1,250 g or less, which protocol is correct for bigger infants? There is no good answer to that question. Reynolds conducted a rough retrofitted analysis of these various guidelines. He analyzed fourteen patients that had severe ROP with relatively unfavorable outcomes derived from medical malpractice cases [40]. The various screening guidelines above would not have uniformly defined each of the 14 infants as eligible for screening inclusion. If a protocol would have failed to include all 14 infants, its validity is open to interpretation. At the very least it would be shown to be flawed in its ability to include all proven at risk patients. All 14 obviously at risk patients would have been screened according to the inclusion criteria of the 2006 guidelines paper and the 2002 CRYO-ROP/LIGHT-ROP paper [41, 75, 106]. Three of the above referenced papers would also have included all [107–111]. If the primary goal of screening is to detect potentially blinding ROP, the 2002 evidenced-based guidelines and the 2006 consensus guidelines do just that. International concerns were discussed in the incidence section. Suffice to say here that U.S. standards do not apply to middle income countries. Larger, older babies can develop serious disease requiring treatment.

So we have defined our at risk population. We have determined when to initiate exams, how frequently to conduct exams, and when to conclude them. What about an examiner’s ability to detect disease, potential complications, and treatment plan availability. It is essential to recognize that examiner error occurs and the implications of that [39, 75]. The 2006 US guidelines incorporated both the recognized error potential and necessary protective measures into its recommendations. It goes without saying that examiners should be well versed in ROP natural history and the exam techniques necessary e.g., scleral depression. But even experienced, highly trained, protocol adherent examiners can make errors. Guidelines need to address the ramifications of this. And treatment support must be present, if not on site then within reach of transport capabilities. Accurate screening must be tied to treatment availability.