Red-free visualization enhances ease of laser therapy for retinopathy of prematurity

Dear Editor,

Laser therapy of the avascular retina using a laser indirect ophthalmoscope (LIO) delivery system is the standard treatment for retinopathy of prematurity (ROP). However, it is technically difficult, has a steep learning curve, and may take an unduly long time, especially in a moving infant under sedation and in cases of aggressive ROP (AROP) or persistent avascular retina (PAR), where there is an absence of clear demarcation between vascular and avascular retina. The LIO systems commonly used are solid-state, 532-nm, green lasers, with a broad-spectrum white light source for illumination. We observed that switching to the red-free filter in the LIO headset, in conjunction with LIO laser safety filters, offers unique visualization benefits during ROP laser therapy.

The red-free filter present in the LIO headsets allows a narrow spectrum of transmission between 540 and 570 nm, which belongs to the intermediate light spectrum. This spectrum is less scattered, penetrates deeper than the shorter wavelengths, is reflected by all the retinal layers, and is absorbed by melanin.[1] These properties increase the transparency of ocular media and retina against the uniform background reflectance from RPE and facilitate viewing of retinal layers and their pathology.[1] When used in ROP cases, this spectrum is more readily absorbed by the melanin, and due to variable transmission through the thinner retinal layers in the avascular retina compared to the more reflective vascular retina, it gives a contrasting appearance. This eases the differentiation between vascular and avascular retina for the ophthalmologist. It also helps to prevent inadvertent encroachment of the laser therapy into the macular area by new trainees, as the macula appears darker than the surrounding retina in this spectrum due to increased visibility of granular pigmented epithelium.[1]

In addition, hemoglobin in retinal blood vessels has a high absorption of this spectrum, which, combined with increased transparency of superficial layers in this spectrum, provides good negative contrast and effectively highlights the retinal vessels.[1] This helps demarcate the vascular retina from the avascular retina, identify terminal bulbs, vascular loops in AROP, flat neovascularization, and PAR [Fig. 1]. Moreover, the reduced scatter from the cornea and vitreous optimizes retinal visualization and enhances infant comfort.[2] However, we did not observe significant visualization advantage in cases with corneal edema, mild cataract, or dispersed vitreous hemorrhage. The visibility benefit also varies significantly depending on the surgeon’s laser safety filters in different equipment, as they exhibit different transmittance rates across various light spectra, depending on their optical density and visual light transmittance. We use the Omega 500 binocular LIO (Heine Optotechnik GmbH and Co.KG, Gilching, Germany) attached to GYC500 green laser console (Nidek Co. Ltd., Aichi, Japan).

Figure 1.

(a) Optos wide-field imaging of AROP captured in flying baby position. (b) Representative red-free (green channel) mode showing enhanced visualization of retina vessels, avascular loops (black star), and peripheral vascular–avascular junction (white star), which will enhance further when viewed through LIO headset. AROP = aggressive retinopathy of prematurity, LIO = laser indirect ophthalmoscope

Red-free examination has proven to be useful in localizing retinal nerve fiber layer damage in glaucoma, vascular changes in diabetic retinopathy, vitreoretinal interface abnormalities, and level of pigmented lesions.[3] Red-free light was also reported to be useful in differentiating retinal vessels from choroidal vessels in a case of AROP in oculocutaneous albinism during laser treatment.[4] Patients examined with red-free light have reported lesser pain and light sensitivity, which increased their compliance.[2] This is particularly beneficial for ROP laser treatment in preterm babies, which requires prolonged light exposure and extensive peripheral retinal treatment, and can reduce procedure time.

We believe that monochromatic red-free visualization offers superior contrast and better patient compliance compared to broad-spectrum light during laser treatment of ROP; its use can not only enhance efficiency, but also ease the learning curve. We currently find this technique very useful in day-to-day ROP laser therapy at our center and would recommend it, especially for new trainees.

Conflicts of interest:

There are no conflicts of interest.

Funding Statement

Nil.