Comprehensive Notes on Photobiomodulation, Glare, and Light Impacts in Optometry

Photobiomodulation (PBM) and LLLT

• Definition: Non-invasive Low-level laser therapy (LLLT) using low-intensity LED visible and IR light delivered continuously or in pulses.

• Mechanism: Photons are absorbed by mitochondrial structures, specifically cytochrome c oxidase, to stimulate RPE cells and reduce oxidative stress.

• Clinical Application: Used for tissue regeneration and reducing inflammation. In optometry, it targets dry AMD progression (typically $9$ sessions over $3$ weeks, $3$ cycles per year).

• Evidence: Research is emerging/inconsistent.

  • LIGHTSITE I: $46$ eyes; showed improvement of $4$ letters in best corrected VA (BCVA) at months $1$ and $7$. Note: Conflict of interest from LumiThera.

  • LIGHTSITE II: $29$ eyes; used $590\,nm$, $660\,nm$, and $850\,nm$. Showed GA lesion growth in both PBM and Sham groups.

  • LIGHTSITE III: $148$ eyes; letter gain at $13$ months, but no difference between groups in drusen volume or contrast sensitivity (CS).

Glare: Discomfort vs. Disability

• Discomfort Glare: High-luminance light causing annoyance or pain without obscuring vision. Components include Glare potential (measurable), Glare sensation (retinal response), and Glare perception (subjective brain interpretation).

• Disability Glare: Physical reduction in visual performance due to light scattering in the eye media. Reduces contrast sensitivity and affects the retinal image.

• Clinical Relevance: Conditions like cataracts (increased scatter), AMD, and uveitis increase glare sensitivity.

• Practical Recommendations:

  • Use wrap-around frames and wide-peak caps.

  • Optical settings: $2700K-3000K$ warm light temperatures to reduce glare.

  • Use polarized sunglasses to remove veiling glare.

Clinical Testing and Tint Effectiveness

• Tints: Yellow and orange tints reduce haze and enhance contrast in low light/fog but can reduce color perception if too dark.

• Oculus Mesotest II: Measures low-light vision and glare sensitivity using Landolt rings; used for driver's license exams and occupational requirements.

• C-Quant: Quantifies light scattering in the eye (expressed as log) to evaluate forward straylight for clinical decisions like cataract surgery.

Light and the Visual System

• Adaptation Glare: The retina adjusts sensitivity via RPE regeneration when moving between photopic vision ($555\,nm$ peak) and scotopic vision.

• Photostress Testing: Measures macula recovery time after bright light exposure.

  • Healthy: $15-30\,seconds$.

  • Macular Disease: Delayed recovery up to $5\,minutes$.

• Visual Considerations: Intensity, contrast, glare control, and uniform direction of light are essential to prevent eye strain.

Optimal Lighting Standards and Selection

• Standards by Task:

  • General areas: $100-300\,lux$.

  • Office tasks: $300-500\,lux$.

  • Precision (e.g., surgery): $750-1500\,lux$.

• CRI (Color Rendering Index): Measures accuracy against natural daylight ($100$ scale).

• Performance: High illuminance speeds up processing for difficult tasks but does not significantly change raw performance levels.

Effects of Aging on Vision

• Optical Changes: Thicker/yellowed lenses (acting as blue light filters), reduced pupil size, and lower photoreceptor volume.

• Visual Decline: Contrast sensitivity and glare tolerance decline much faster than standard visual acuity.

• Lens Fluorescence: The lens converts UV to visible light; this increases with age and diabetes, contributing to light scatter.

Fluctuating Light and Visual Stress

• Flicker: Rapid intensity changes from fluorescent sources or stroboscopic effects. Humans are most sensitive to spatial frequencies of $2-6\,cycles$ per degree.

• Clinical Sensitivity: Migraine and epilepsy patients have higher intolerance to flickering light.

• Meares-Irlen Syndrome: Brain struggles to process high-contrast patterns, causing shimmering or bending text; managed with specialized rose-tinted lenses (Comfort $41$).

Circadian Rhythms and Biological Regulation

• Mechanism: Regulated by the Suprachiasmatic Nucleus (SCN) via ipRGCs (intrinsically photosensitive Retinal Ganglion Cells) containing melanopsin.

• Physiological Cycle:

  • $6\,AM$: Peak cortisol; rising HR/BP.

  • Night: Melatonin peaks; body temperature reaches minimum in early morning.

• Dysfunction: Disruptions like jet lag or shift work lead to sleep issues and Seasonal Affective Disorder (SAD).

• High Melanopsin Displays: Research (Allen et al. 2018) indicates these reduce melatonin secretion and sleepiness compared to low-melanopsin displays.

Myopia and the Impact of Outdoor Light

• Singapore Statistics: $15\%$ prevalence in $4$-year-olds, rising to $70\%$ after education. Severe myopia (> -6.00) affects $10\%$ of adults.

• Outdoor Light Study (Read et al. 2018):

  • Singapore children: $61 \pm 40\,min/day$ of exposure (> 1000\,lux).

  • Australian children: $105 \pm 42\,min/day$ of exposure.

• Conclusion: Light exposure, rather than near work duration, is the primary differentiator in myopia rates between these groups.