Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Please check out our Photocoagulation Lasers section for more information about the products mentioned in this article.
The treatment of diabetic retinopathy has evolved dramatically over the past century. Before the advent of photocoagulation, surgeons had limited options available to treat retinal complications from diabetes. One approach that surgeons used on patients with proliferative retinopathy was to ablate their pituitary gland. This surgery was developed after Houssay et al. noted in 1930 that hypophysectomy reduced the severity of diabetes in pancreatectomized dogs. In the 1950s, pituitary ablation was attempted on humans using x-ray irradiation, surgical hypophysectomy, and yttrium-90 implantation. Patients treated in this manner demonstrated improvement in their diabetic retinopathy, but severe complications such as postural hypotension, sterility and death prevented its widespread use.
When Beetham and colleagues noted that patients with some retinal conditions showing chorioretinal scarring –- such as retinitis pigmentosa and high myopia –- were relatively protected from developing severe proliferative diabetic retinopathy, they postulated that chorioretinal scarring may prevent neovascular retinopathy in patients with diabetes. Indeed, these investigators noted that in the minority of proliferative diabetic retinopathy patients that had spontaneous resolution of their retinopathy, the fundus showed reticulated tissue with attenuation or obliteration of vessels.
At first, xenon arc photocoagulation was used to create chorioretinal scars in patients with severe diabetic retinopathy. Xenon arc, which produces white light emitting multiple wavelengths, created widespread full-thickness retinal burns on the retina, leading to extensive retinal destruction. Treated retinas showed extensive scarring and fewer exudates, less retinal edema and reduced dilation of the retinal veins. Indeed, patients with non-proliferative diabetic retinopathy who were treated with xenon arc tended to not develop proliferative diabetic retinopathy. Unfortunately, diffuse treatment of the retina with xenon arc photocoagulation also led to many complications, including vitreous hemorrhaging, significant visual-field defects, and fibrous traction.
The advent of the ruby laser allowed, for the first time, the creation of smaller, well-focused chorioretinal scars on the retina. Aiello and colleagues then used the ruby laser to treat individuals with diabetic retinopathy, thereby revolutionizing the use of lasers in Ophthalmology. The ruby laser emits a short (less than 1 millisecond) pulse of monochromatic energy at approximately 694 nm; this wavelength transmits well through ocular media as well as blood, and is absorbed efficiently by the retinal pigment epithelium and choroid. The chorioretinal scars tend to be smaller and more confined than with xenon arc.
When patients were treated with laser photocoagulation, the diabetic retinopathy was noted to be much better controlled. In fact, neovascular nets disappeared, angiopathy improved, retinal hemorrhages decreased, and retinal veins appeared less congested and less leaky by fluorescein angiograms. In time, the Diabetic Retinopathy Study (DRS) would show that the rate of severe vision loss (visual acuity of less than 5 / 200) would be reduced by nearly 60% in eyes treated with photocoagulation compared to untreated eyes.
Today, laser photocoagulation has replaced xenon arc photocoagulation, and a spectrum of wavelengths is available to treat the retina. Argon blue-green laser, which emits at 488 nm and was used for the DRS, has been largely replaced by the argon green laser. Argon green, which emits at 514 nm, has become the most popular wavelength; its absorbance by vessels and retinal pigment epithelium is excellent. Other wavelengths are the dye yellow (emits at 577 nm), which is very well absorbed by blood and may be used to directly treat new vessels, and krypton red (emits at 647 nm), which offers excellent penetration through nuclear sclerotic cataracts. Solid state diode lasers, which emit between 780 and 850 nm, also have longer wavelengths that have excellent penetration. The diode lasers have low power requirements and are often more portable and smaller than other types of lasers.
Today’s laser photocoagulation systems offer a diverse, flexible platform to treat diabetic retinopathy and other retinal disorders. Several photocoagulation systems incorporate multiple lasers at multiple wavelengths, allowing the Ophthalmologist to easily switch back and forth to tailor treatment for each patient. Delivery systems are equally diverse, and many systems have adapters for the slit lamp and indirect ophthalmoscope, as well as endoprobes for surgical endophotocoagulation during vitrectomy. The Ophthalmologist also may consider portability, power requirements, digital displays, voice output, type of foot pedal, and other accessories for customizing their own laser photocoagulation system.