What is the aim of this review?
The aim of this Cochrane Review was to find out if NAC eye drops can prevent or reverse the progression of cataracts (cloudy lens in the eye).
Key messages
It is uncertain whether NAC eye drops prevent, or reverse, the progression of cataracts.
What was studied in the review?
The eye has a clear lens that focuses the light on the back of the eye. As people get older this lens can become cloudy, leading to vision problems. A cloudy lens is known as a cataract. Doctors can remove the cataract and replace it with an artificial lens. This is usually a very successful operation. But any operation has risks and can be an unpleasant experience. Cataracts are common in older populations and cataract surgery is expensive for health care systems. This is why there is interest in preventing, or treating cataract, so that surgery can be avoided.
As part of normal metabolism, our bodies produce chemicals that contain oxygen and are reactive ("reactive oxygen species"). One theory of ageing is that these chemicals may be harmful and might lead to age-related changes in our body, such as cataract. This is known as oxidative stress. N-acetylcarnosine (NAC) is thought to be able to combat some of the effects of oxidative stress as it has anti-oxidant properties. If NAC can stop the lens from becoming cloudy, or reduce the cloudiness, this might improve people's vision and quality of life.
What are the main results of the review?
The Cochrane researchers found two potentially relevant studies. The studies compared NAC eye drops with placebo or no treatment. These studies were from Russia and the United States and were conducted by the same research group.The Cochrane researchers were unable to find out enough information about these studies to include in the review. These studies are assigned as ‘awaiting classification’ in the review until sufficient information can be obtained from the authors.
How up-to-date is this review?
The review authors searched for studies that had been published up to 28 June 2016..
There is currently no convincing evidence that NAC reverses cataract, nor prevents progression of cataract (defined as a change in cataract appearance either for the better or for the worse). Future studies should be randomized, double-masked, placebo-controlled trials with standardised quality of life outcomes and validated outcome measures in terms of visual acuity, contrast sensitivity and glare, and large enough to detect adverse effects.
Cataract is the leading cause of world blindness. The only available treatment for cataract is surgery. Surgery requires highly-trained individuals with expensive operating facilities. Where these are not available, patients go untreated. A form of treatment that did not involve surgery would be a useful alternative for people with symptomatic cataract who are unable or unwilling to undergo surgery. If an eye drop existed that could reverse or even prevent progression of cataract, then this would be a useful additional treatment option.
Cataract tends to result from oxidative stress. The protein, L-carnosine, is known to have an antioxidant effect on the cataractous lens, so biochemically there is sound logic for exploring L-carnosine as an agent to reverse or even prevent progression of cataract. When applied as an eye drop, L-carnosine cannot penetrate the eye. However, when applied to the surface of the eye, N-acetylcarnosine (NAC) penetrates the cornea into the front chamber of the eye (near to where the cataract is), where it is metabolised into L-carnosine. Hence, it is possible that use of NAC eye drops may reverse or even prevent progression of cataract, thereby improving vision and quality of life.
To assess the effectiveness of NAC drops to prevent or reverse the progression of cataract.
We searched CENTRAL (which contains the Cochrane Eyes and Vision Trials Register) (2016, Issue 6), Ovid MEDLINE, Ovid MEDLINE In-Process and Other Non-Indexed Citations, Ovid MEDLINE Daily, Ovid OLDMEDLINE (January 1946 to June 2016), Embase (January 1980 to June 2016), Allied and Complementary Medicine Database (AMED) (January 1985 to June 2016), Cumulative Index to Nursing and Allied Health Literature (CINAHL) (1982 to June 2016), the ISRCTN registry (www.isrctn.com/editAdvancedSearch), ClinicalTrials.gov (www.clinicaltrials.gov) and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) (www.who.int/ictrp/search/en). We did not use any date or language restrictions in the electronic searches for trials. We last searched the electronic databases on 28 June 2016. We handsearched the American Society of Cataract and Refractive Surgery (ASCRS) and the European Society of Cataract and Refractive Surgeons (ESCRS) meetings from 2005 until September 2015.
We planned to include randomized or quasi-randomised controlled trials where NAC was compared to control in people with age-related cataract.
We used standard methodological procedures expected by Cochrane.
We identified two potentially eligible studies from Russia and the United States. One study was split into two arms: the first arm ran for six months, with two-monthly follow-up; the second arm ran for two years with six-monthly follow-up. The other study ran for four months with a data collection point at the start and end of the study only. A total of 114 people were enrolled in these studies. The ages ranged from 55 to 80 years.
We were unable to obtain sufficient information to reliably determine how both these studies were designed and conducted. We have contacted the author of these studies, but have not yet received a reply. Therefore, these studies are assigned as ‘awaiting classification’ in the review until sufficient information can be obtained from the authors.