We carried out a retrospective review of records from ROP clinic over a period of one year (October 2015 to September 2016) at AIIMS, New Delhi, India. ROP was classified according to International Classification of Retinopathy of Prematurity (ICROP) 2005 [4]. Gestational history, previous treatment history, stage and zone of ROP, and management details (follow-up or laser photo-coagulation or vitrectomy) were noted. Retinal falciform fold and disc macular drag were classified as ‘sequalae’. Final outcome was categorized into: no ROP, regressed and attached, regressed but dragged, ROP sequalae or Stage 5 disease.

Out of 410 infants with ROP registered during the study period, 16 (3.9%) had asymmetric disease (Web Table I). Mean (SD) gestational age (GA) was 29.6 (2.3) weeks and mean (SD) birthweight was 1269.3 (426.7) grams. Mean (SD) post-conceptional age (PCA) at presentation was 44.6 (10.9) weeks. Five infants were previously treated either with laser photocoagulation (n=3) or intravitreal Bevacizumab (n=1) or both (n=1). Ten infants had a similar zone but different stage of disease. Management differed between fellow eyes in ten babies (four were previously treated). Final outcome differed in between the fellow eyes in eleven babies.

One-fifth of the threshold ROP patients in Cryotherapy for Retinopathy of Prematurity (CRYO-ROP) study had asymmetric threshold disease at presentation [1]. Fielder, et al. [2] reported asymmetric ROP in 25.4% of patients with a difference of at least one stage according to ICROP (1984). Quinn, et al. [3] in a retrospective review of CRYO-ROP cohort, reported that acute-phase ROP had asymmetry ranging from 5.8% to 25% between 32 to 38 weeks PCA. Varied ROP course has been previously reported in Indian setting between twins but not among fellow eyes [5].

The probable reasons for low incidence noted in our study could be either a difference in classification systems used and/or regional geographic differences. CRYO-ROP study had classified based on threshold disease, where variation in number of clock hours or quadrant involvement in the same zone was taken as asymmetry while these will be considered similar as per ICROP 2005.

Local ocular factors might play a role in difference in disease activity between fellow eyes [2]. Possible mechanisms include regional variations of retinal neuro-vascular development and retinal light dose [6].

High concordance between fellow eyes in ROP often helps us prognosticate and plan treatment accordingly. However, ROP might present with asymmetric disease, and so it is essential to examine both eyes thoroughly and treat accordingly at each visit.

Contributors: DK, PC: conception/design of work; acquisition, analysis and interpretation of data; and drafting the manuscript. RT: interpretation of data for the work, and revising the manuscript critically for important intellectual content. All authors approved the final version and agree to be accountable for all aspects of work.

Funding: None; Competing interest: None stated.

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2. Fielder AR, Shaw DE, Robinson J, Ng YK. Natural history of retinopathy of prematurity: A prospective study. Eye Lond Engl. 1992;6:233-42.

3. Quinn GE, Dobson V, Biglan A, Evans J, Plotsky D, Hardy RJ. Correlation of retinopathy of prematurity in fellow eyes in the cryotherapy for Retinopathy of Prematurity Study. Arch Ophthalmol. 1995;113:469-73.

4. International Committee for the Classification of Retinopathy of Prematurity. The International Classification of Retinopathy of Prematurity revisited. Arch Ophthalmol. 2005;123:991-9.

5. Azad R, Chandra P, Patwardhan SD, Gupta A. Profile of asymmetrical retinopathy of prematurity in twins. Indian J Ophthalmol. 2010;58:209-11.