Sub-threshold laser refers to the application of sub-visible burns during retinal laser therapy, thereby producing an effect only at the cellular level without causing any visible clinical burns. Subthreshold laser application requires scanning pattern laser where the duration of a burn can be reduced to 10 milliseconds.
Conventionally, photocoagulation, [duration (pulse) of the laser, typically 100 milliseconds and above] is done to produce a visible white burns in the retina generally classified as light, moderate or intense. Doctors generally aim for moderate intensity while lasering retinal periphery as in pan-retinal photocoagulation (PRP) or light burns while lasering the sensitive macular area. ( macular grid/ focal laser).
These fresh laser burns undergo scarring and gradually enlarge in size over a period of years. This can sometimes cause late visual loss (visual field damage, decreased contrast sensitivity etc.). Hence the conventional laser is best avoided while lasering the sensitive macular area.
All the complications of the conventional laser are offset by using the subthreshold laser protocol and laser system . Since they do not produce any subclinical burns these can be repeated whereas conventional laser produced scarring cannot be repeated.
In this era of intravitreal injections, there are still indications for focal laser in the macula area especially in diabetic macular edema as a combination therapy or in cases of clinically significant macular edema and cases of central serous retinopathy. Sub-threshold laser treatment protocol is ideal for these conditions. In this treatment, there are no visible lesions since the goal is to cause the laser effect at the cellular level without causing visible lesions in the sensitive foveal or macular area.
Ideal cases for subthreshold/ subvisible lasers
- Pathology close to the fovea
- Acute CSCR
- Chronic CSCR
- Microaneurysms surrounding FAZ
- The combination with anti-VEGF for DME
- Diabetic macular edema non-responsive to anti-VEGF
- Small vein occlusions/ branch retinal vein occlusions
Advantages of subthreshold /subvisible laser include
- Lack of scarring
- Lack of scotomas
- Ability to treat foveal areas
- Preservation of colour and contrast sensitivity
- Permits high-density treatment
Figure 1. Clockwise from top left: color fundus photograph, red free photograph, early phase and late phase of fluorescein angiography of a case of diabetic macular edema with centre-involving (fovea) macular edema
Figure 2. Two months after the patient underwent subthreshold focal laser with yellow laser shows resolution of macular edema and no clinically visible laser burns. Clockwise from top left: color fundus photograph, red free photograph, and fundus autofluoresence image two months after subthreshold/ subvisible focal laser.