Retinal Physiopathology

There are 10 layers in the retina ^{2, 14, 15}, listing from the light path as follows:

internal limiting membrane, nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer (INL), outer plexiform layer, outer nuclear layer (ONL), external limiting membrane, photoreceptor (rod and cone) layer, retina pigment epithelium (RPE). “Photoreceptor cells are differentiated postmitotic retinal neurons uniquely adapted for the efficient capture of photons and for the initiation of visual transduction” ¹⁶ (see Figure 13). In order to perform these functions, photoreceptors are being maintained in an oxygen rich environment and interact with high levels of incident light. The rod outer segment (ROS) membranes also contain high levels of polyunsaturated fatty acids, which stand a high potential for cell injury. “Yet, normally, most visual cells survive into the seventh decade and beyond” ¹⁷. The constant renewal of ROS, disk displacement and phagocytosis activities in the RPE layer may help ROS membrane turnover every nine to ten days ¹⁵ . “Photoreceptors also have the ability to adapt electrophysiologically to a wide range of incident light and metabolically to long-term environmental light conditions” ¹⁵. Additionally, repair mechanisms may also greatly prevent cell death. The delicate balance between the light interaction and overload-self-protecting mechanism determines the photoreceptor’s survival or irreversible damage.

Figure 13 Light path within the retina layers ¹⁵

Moreover, specific wavelength has its different light absorption and transmittance rates for each tissue. Demonstrating in Figure 14 ⁴, While the retinal transmission extends from 400 to 1200 nm, the retinal absorption peaks between 400 and 600 nm, which is the PC-LED white light dominated region.

Although the outer ocular tissues (such as cornea and lens) absorb majority of the toxic UV lights, it is another research subject and therefore is excluded from this report.

Figure 14 Light absorption and transmittance in the eye ^{4, 18, 19} 2.2.2 Retina Light Injury

The retinal light injury induced by environmental light exposure was first described in 1966 ²⁰. Many associated investigations have been added to researcher’s understanding of its pathological process since. The initial intention of this type of study was to determine and define the injury, and it has expanded to more recent mechanism oriented pathological analyses. The light interacts with visual system through different mechanisms. Some of the ocular tissues or pigments are designed to absorb photons to reduce retinal exposure as a self-defense function. Moreover, some other ocular structures can increase oxidative stress and lead to retinal injuries through photochemical and photodynamic effects. Three mechanisms have been categorized: photothermal-,

radiant energy, a photon, from light to the retinal tissue” ². When the intense light is converted into heat, the pigmented tissue (mostly melanin in the RPE and choroid) raises its temperature and causes photocoagulation.

(2). Photomechanical injury: the tissue damage is caused by mechanical compressive or tensile forces that generated by rapid introduction of energy into RPE melanosomes. Is occurred when extremely high retinal irradiances (typically laser beam) cause tissue heating and expansion that triggers instant retina alteration and bleeding.

(3). Photochemical injury: the tissue damage is caused by free radicals that generated from light exposure. This type of injury associates with both long-duration and short-wavelength light exposure ^{22, 23}. When the retinal intrinsic protective mechanism is overdoing by defending the light insult, the retinal injury may occur.

Noell et al. suggested this hypothesis in 1966, after learning that the albino rat retinas were irreversibly injured by continuous exposure to ambient light.

This finding motivated extensive studies, further elucidating this mechanism different from mechanical and thermal retinal injury. This also is the most common type of retinal light injury with two classes ^{16, 24}. According to Kremers and his colleagues’ research in 1988, these two classes of retinal injury have been shown in both rodent and primate models ²⁴.

(a) Class I injury is characterized as exposure to white light (irradiance below 1 mW/cm²) for hours to weeks. Despite there are some debates on the initial site of injury from low-level light exposure, it is generally believed the initial injury site

(b) Class II injury is characterized as exposure to relatively high irradiance white light (above 10 mW/cm²). The initial injury site at the RPE with action spectrum peaking at shorter wavelengths.

2.2.3 Action Spectrum of Retinal Light Injury

It is well recognized that retinal light injury is determined by the exposure duration and light intensity reaches the retina (retinal irradiance). The wavelength dependency is also convinced with action spectra in its pathological process. As shown in Figure 15, the action spectrum of retinal light injury ranges from 400 nm to 580 nm and peaked around 480 nm to 500 nm. This action spectrum closely overlapping the PC white light LED emitting range and signifying a warning for ONL injury.

As mentioned previously, LED is becoming the primary light sources for many lighting applications. White LEDs make retina exposure to violet, indigo and blue light at much higher levels than in conventional light sources. This is the first time in history that human will be exposed to such substantial bright light regardless day or night. Moreover, photochemical injuries usually progressively induce photoreceptor loss long after the initial exposures.

Therefore, when evaluating all the knowledges and risks on blue-light hazard, its cumulative effect should also be carefully considered for this unavoidable chronic exposure.

Figure 15 Action spectrum of retinal light damage ¹⁷

2.2.4 Progress of Photoreceptor Light Induced Injury

As demonstrating on Figure 16, a typical development of retinal injury by light exposure is shown schematically ²⁵. As first explained by Kuwabara and Gornss in a 1968 publication, only the tip of photoreceptor shows vacuoles at stage 1. At stage 2, outer segment is tortuous and swollen. Myelin membranes are separated from each other and form vesicular and tubular structures. The pathological changes can be found at the synaptic end of photoreceptic cells.

Pigment epithelium also shows clear increase in myeloid bodies. At stage 3, damaged outer segment is isolated from inner segment and becomes large round or pear-shaped body filled with tubular material, followed by cellular

Müllei’s cell at stage 5.

Figure 16 Scheme of the photoreceptor light induced injury progress

Diagram concept modified from (Kuwabara and Gornss) ²⁵

2.2.5 Animal Model for Retinal Light Injury

Animal models is frequently used for light-induced retinal degeneration and mechanisms analysis. Rodent retina is a good alternative, if primate is not available for light induced retina degeneration study. To test the light induced retina injury in rodent species became the most convenient model due to its accessibility, capacity to induce photoreceptor-specific cell death, oxidative stress recognition, blood-retina barrier breakdown, and other inflammation marker identification ²⁶.

Cell bodyPhotoreceptor sensory cilium Nucleus Inner segment Outer segment RPE GCL

INL

ONL

RPE CH

Normal 1 2 3 4 5

Injury Stage

exposure associated (acute); the second type of the injury is induced by less-bright-cyclic light exposure (chronic). Despite both types can lead to serious retinal degeneration, their injury mechanisms and phenotypes can be very different. Giving the credit to professor Mandal ²⁷, Figure 17A shows the typical retinal injury by acute light exposure in a Sprague Dawley (SD) rat. The photoreceptor cell death can easily be observed (white arrows) juxtaposing by those normal-appearing cells (yellow arrow). On the other hand, the chronic light injured retina appears to be “normal” morphologically as shown in Figure 17B. Its ONL is thinning gradually without regional differences. The gentle ONL thinning process could protect the retina from massive retina destruction ²⁷. This is a strategical self-defense mechanism so called “retina remodeling”, which should be observed closely and interpreted systematically.

Figure 17 Sprague-Dawley (SD) rats acute and chronic retinal light injury ²⁷

(A) (B)

2.3 Potential retinal injury induced by chronic exposure to LED light