Preexisting medical conditions can affect your vision and the safety and efficacy of LASIK and other vision correction surgery. Some may rule out any type of refractive surgery, others will affect which type of surgery you should choose, and some which may seem as if they would impact refractive surgery actually make no difference at all.
Acoustic Neuroma
Acoustic neuroma is a non-cancerous growth on the eighth cranial nerve, a nerve which runs from the brain to the inner ear. Balance and hearing problems develop with this condition, and it can result in loss of sensation in the face and reduced ability to make facial expressions. In most cases it does not affect a person’s candidacy for LASIK vision correction surgery.
Asthma
Asthma does not interfere with LASIK surgery, but some of the medications prescribed for asthma may cause complications. The most common problem that asthma suffers encounter with LASIK is dry eye, due to taking antihistamines. Some asthma patients take steroids or use steroid inhalers which should not cause a problem, but may be in conflict with medications that your ophthalmologist would normally prescribe. You should inform your ophthalmologist of any prescription, over-the-counter, and herbal remedies you use for asthma well in advance of surgery.
Diabetes
Having diabetes does not mean that you cannot have LASIK or other vision correction surgery. However, if you have diabetic retinopathy, refractive surgery should be avoided. Diabetic retinopathy is the leading cause of blindness in Americans under the age of 65. It damages the blood vessels that supply the retina. Most refractive surgeries involve applying suction to the eye, which is dangerous if the blood vessels are already weakened. PRK and LASEK do not involve suction and may be safe in some cases.
Keloid
There is some disagreement within the medical community as to whether vision correction surgery is safe for keloid formers. It is generally believed that internal procedures such as LASIK are safe, but procedures such as PRK which scrape the outer surface of the eye may pose a risk, and CK should be avoided altogether.
Lyme disease
Lyme disease is transmitted by deer ticks. Among many symptoms, including fever, fatigue, muscle and joint aches, and rashes, Lyme disease can cause inflammation of the cornea (keratitis). Of course, vision correction surgery should be avoided when there is a likelihood of keratitis, due to increased risk of post-operative complications, and the inability to take accurate pre-operative measurements.
Pregnancy and Nursing
Women who are pregnant, nursing, or intend to become pregnant within the next six months should postpone vision correction surgery until at least two months after they have finished nursing.
Hormonal changes during pregnancy, and while nursing, cause drastic fluctuations in vision. It is impossible to take accurate refractive measurements during this time and, of course, inappropriate to perform the surgery while these fluctuations are occurring even if the measurements were taken beforehand.
Becoming pregnant during recovery can make it impossible to tell if fluctuations in vision are due to the surgery or to hormonal changes. In the rare case that complications arise from the surgery, it may be necessary to take medications which are not safe for the baby during pregnancy or nursing.
Source: http://www.the-lasik-directory.com/article_medical.html