Sir,
We read with interest the correspondence on ‘Quicker painless diabetic laser,’ whereby a pulse duration of 20 ms with corresponding higher power in argon laser panretinal photocoagulation resulted in less painful treatment sessions.1 The reduced pain during treatment is thought to be due to lower heat conduction to the choroid and sclera.
Early studies of the effect of pulse duration and laser wavelength showed a narrower safety margin with argon laser between retinal burn and retinal haemorrhage for short pulse durations (<50 ms).2, 3, 4, 5 More recently, a semiautomated argon laser delivery system has been developed and tested on rabbits. Using a pulse durations of 20 ms, the threshold for a visible burn was 110–120 mW while that for retinal haemorrhage was 600 mW; suggesting an adequate safety margin.6 Also, light retinal burns produced using pulse durations of 10 and 100 ms had similar histological appearances at 1 week.6 However, whether the histological changes in the long-term are similar for both pulse durations is not known. It is also not known if shorter pulse duration burns have the same therapeutic effect in controlling proliferative diabetic retinopathy as longer pulse durations burns.
Prior to promoting a shorter pulse duration for panretinal photocoagulation on anecdote alone, sufficient evidence should be gathered to show there is a significant reduction in pain during treatment, that treatment is equally effective at controlling proliferative disease and that the shorter pulse duration treatments have an acceptable side effect profile.
References
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Obana A, Lorenz B, Gässler A, Birngruber R . The therapeutic range of chorioretinal photocoagulation with diode and argon lasers: an experimental comparison. Lasers Light Ophthalmol 1992; 4: 147–156.
Birngruber R, Gabel V-P, Hillenkamp F . Fundus reflextometry: a step toward optimization of the retina photocoagulation. Mod Probl Ophthalmol 1997; 18: 383–390.
Mainster MA, Sliney DH, Belcher III CD, Buzney SM . Laser photodisruptors: damage mechanisms, instrument design and safety. Ophthalmology 1983; 90: 973–991.
Blumenkranz MS, Yellachich D, Andersen DE, Wiltberger MW, Mordaunt D, Marcellino GR et al. Semiautomated patterned scanning laser for retinal photocoagulation. Retina 2006; 26: 370–376.
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Day, A., Davies, N. Reply to T Rimmer and W Wyke. Eye 21, 140–141 (2007). https://doi.org/10.1038/sj.eye.6702569
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DOI: https://doi.org/10.1038/sj.eye.6702569