Retinopathy of Prematurity- Mechanism and Treatment

• It is a multifactorial vasoproliferative retinal disorder. 65% of infants with birthweight less than 1250 grams and 80% below the weight of 1 kg will develop some degree of ROP. After developing the sclera and choroid, the retinal elements migrate from the optical disc to the periphery. The retinal vessels start to migrate at 16 weeks; this process is complete by 40 weeks.

Possible Mechanisms Of Injury

• In the first stage, hyperoxia, hypoxia, and hypotension at a critical point of vascularisation of the retina lead to vasoconstriction and subsequent arrest in retinal vascular development. In the second stage, i.e., neovascularization. Hypoxic avascular retina leads to excess VEGF, hemorrhage, edema, and extraretinal fibrovascular proliferation. Extensive fibrovascular proliferation leads to retinal detachment and abnormal retinal function. Risk factors of ROP include low birth weight, prematurity, prolonged oxygen exposure, sepsis, mechanical ventilation, blood transfusion, and poor postnatal weight gain.

Zone

• The eye is divided into three zones. Zone 1 is an imaginary circle with the optic nerve at the center, and the radius will be twice the distance from the optic nerve to the macula. Zone 2 is the edge of zone 1 to the ora serrata on the nasal side and approximately half the distance to the ora strata on the temporal side. Zone 3 is the crescent-shaped area extending from zone 2 to the ora serrata on the temporal side.

Stages

• Stage 1 is the demarcation line and marks the beginning of the process.
• Stage 2, there is fibrovascular tissue proliferation with height and width.
• Stage 3, there is extraretinal fibrovascular proliferation.
• Stage 4 is partial retinal detachment. Stage 4A involves the macula, and stage 4B does not involve the macula.
• Stage 5 is complete retinal detachment.

Extent

• Circumferential location in the clock hours.

Plus Disease

• There is vascular dilatation and tortuosity in at least 2 quadrants. In pre-plus disease, there is milder dilation and tortuosity.

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Types Of Rop

• Threshold ROP- The ROP needs treatment, and the risk of blindness without treatment is 50%. It is reduced to 25% with treatment. It is seen in zone 1 or 2 and should be stage 3 with plus disease and 5 or more contiguous or 8 cumulative clock hours.
• Prethreshold ROP- Any ROP in zone 1 below threshold ROP or zone 2 with age 2 plus disease or zone 3 with plus disease and a few clock hours. Type 1 is zone 1 of any ROP with plus disease or stage 3 with or without plus disease. It can also be zone 2, stage 2, or 3 ROP with plus disease. There will be a 32% of regression. Type 2 is zone 1 stage 1 or 2 without any plus disease, or it can be zone 2 stage 3 without any plus disease. There will be a 77% chance of regression. Prethreshold ROP type 1 should be treated, and type 2 is milder, so only close observation is required.
• Prognosis- Any patient with stage 1 or stage 2 ROP will undergo spontaneous regression, and zone 3 will have a good prognosis. Significant ROP will increase the risk of myopia, anisometropia, astigmatism, strabismus, amblyopia, late retinal detachment, and glaucoma. A cicatricial disease with residual scarring may be associated with retinal detachment years later. The prognosis for stage 4 ROP depends upon macular involvement. If the macula is not involved, then the outcome is good. Once detachment of the retina has occurred, the outcome is overall poor. The target SpO2 levels are 91-95%. If lower SpO2, there will be less ROP and more mortality.

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Treatment

• The treatments include LASER, cryo, antiVEGF, and retinal reattachment.
• LASER- It is delivered through an indirect ophthalmoscope and applied to the avascular retina. An average of 1000 spots are placed in each eye. The numbers may range from a few hundred to 2000. Both argon and diode photo laser coagulation is used. The development of cataracts, glaucoma, or anterior segment ischemia following laser or cryotherapy has been reported.
• Cryotherapy- It is applied to the external surface of the sclera, and areas peripheral to the ridge of ROP are frozen till the entire avascular retina has been treated. 35-75 applications are done in each eye and must be done under GA. It causes more inflammation and requires more analgesia. It is used in special cases like poor pupillary dilatation or vitreous hemorrhage.
• Anti-VEGF therapy- It is done by giving an intravitreal injection, particularly for zone 1 or aggressive posterior ROP cases after laser and with vitreoretinal surgery. It is a short duration with topical anesthesia. It has less destruction of the retina and lower long-term myopic rates.
• Retinal reattachment- Once the macula detaches in stage 4B or 5 ROP, surgery is the option. Vitrectomy with or without lensectomy is the treatment of choice. Scleral buckling is done for peripheral detachment. Poor visual outcome and will fall under the range of legal blindness.

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