Purpose: To investigate the effects of subliminal micropulse yellow laser application on central macular thickness and best corrected visual acuity in cystoid macular edema secondary to retinitis pigmentosa patients.

Retinitis pigmentosa (RP) is a progressive photoreceptor and retinal pigment epithelial (RPE) degeneration that begins as a night vision loss, resulting in narrowing of the visual field and legal blindness. RP is a heterogeneous genetic disorder, affecting 1/3000- 8000 people worldwide. RP is the result of mutation in one of more than 260 genes. These genes are responsible for the synthesis of peptides involved in the visual cycle. These genes are also responsible for the synthesis of growth factors responsible for the conversion of glucose to adenosine triphosphate (ATP) or responsible for the removal of metabolic wastes.

The incidence of cystoid macular edema (CME) in RP has been reported as between 10% - 50%. There are several hypotheses about the pathogenesis of cystoid macular edema in RP. The first hypothesis is explained by Müller cell hypertrophy and its paracrine effects. Mutations in the retinal pigment epithelium disrupt the synthesis of some growth factors. Stress caused by apoptosis in rod cells in the periphery leads to ectopic synaptogenesis of Müller cells in the central. Müller cells undergo copensatory hypertrophy and synthesize excessive growth factors. This paracrine effect provides protection of central vision. Edema in a certain level is considered to be protective to photoreceptors and should not be treated. But if edema is excessive and prolonged leads to a break in synaptic connections in the neural retina and an increase in neurodegeneration. CME also deteriorates central visual quality in patients with impaired peripheral vision. Treatment should be considered only if the edema is excessive and disrupts central vision. According to our clinical experience, when central macular thickness exceeds 500 microns, central visual quality of the patient decreases and requires treatment.

Other pathophysiological causes of CME in RP are explained by low grade inflammation and retinal autoantibodies. In some genetic mutation types of RP, such as the X-linked RPGR gene mutation; vitritis, lipofuscin deposits and inflammation are predominant. Ciliopathy leads to inflammation and CME, which increases photoreceptor loss rate. Immediate treatment of inflammation-induced edema can slow disease progression. Inflammatory edema appears as cystoid macular edema, whereas compensatory edema due to Müller cell hypertrophy is seen as separeted intraretinal cysts. Inflammatory edema responds well to carbonic anhydrase inhibitors, while compensatory edema does not.

The results of the treatment of CME in RP are controversial, as the compensatory or inflammatory distinction is not made clearly. Treatments such as grid laser photocoagulation, oral acetazolamide, topical carbonic anhydrase inhibitors, intravitreal therapy with corticosteroids or anti-vascular endothelial growth factor agents and pars plana vitrectomy may be effective in some cases with CME secondary to RP. Most of these treatments have either insufficient response or excessive side effects.

To our knowledge, so far, we have not found a scientific publication about the use of micropulse yellow laser for treatment cystoid macular edema secondary to retinitis pigmentosa.

Subliminal micropulse laser (SL-MPL) is a method developed to reduce the laser-induced thermal damage caused by conventional laser therapy for treatment some macular diseases. In the micropulse mode, laser is applied in short pulses, thereby reducing the thermal energy generated in the target area. The coagulation scars do not form with SL-MPL treatment. Sublethally injured RPE cells induce an up- and down regulation of various growth factors (GFs) [pigment epithelium-derived factor (PEDF), vascular endothelial growth factor (VEGF) inhibitors, VEGF inducers, permeability factors, etc.] which restores the pathologic imbalance.

The aim of this study was to investigate the effect of yellow (577 nm) SL-MPL therapy on central macular thickness (CMT) and on best corrected visual acuity (BCVA) in patients with cystoid macular edema secondary to retinitis pigmentosa.

Inclusion Criteria:

• Diagnosis of any phenotypic variation of RP
• Presence of separeted intraretinal cysts
• Central macular thickness ≥ 500 µm
• Unresponsive to systemic or topical carbonic anhydrase inhibitors
• No interventional treatment for macular edema in the last 3 months

Exclusion Criteria:

• Patients with signs of inflammation, such as cells in the vitreous, intraretinal white dots, septa-free edema
• Patients responding to systemic or topical carbonic anhydrase inhibitors
• Any interventional treatment has been applied for macular edema in the last 3 months
• The presence of other causes that may lead to CME such as epiretinal membrane, vitreous traction, diabetes or uveitis.