Tuesday, December 16, 2008

Type 1.5 Diabetes

Type 1.5 diabetes, as the name implies, falls between type 1 and type 2 diabetes. It manifests some of the symptoms of both types, and it is important because it affects about 15% of those diagnosed with diabetes.

Type 1 diabetes is characterized by the presence of autoantibodies against insulin or against certain components of the insulin-producing system such as glutamic acid decarboxylase (GAD), tyrosine phosphatase or the islet cells themselves. These autoantibodies cause the patient's own immune system to kill the beta cells of the pancreas, making the patient unable to produce any endogenous insulin.

Type 2 diabetes is characterized by insulin resistance and diminished production of insulin by the pancreas. If a patient is well-managed, this kind of diabetes can be controlled for many years by diet, excercise and oral medication.

Type 1.5 diabetes has characteristics of type 1 and type 2. Like type 2, its onset is in adulthood, and the pancreas is able to produce insulin for several years. Like type 1, it often occurs in thin people, and it often involves autoantibodies to GAD or islet cells. Unlike type 1, in type 1.5 diabetes, the autoantibodies work much more more slowly. However, their destruction of beta cells is relentless, and within 5-10 years of diagnosis, patients with type 1.5 diabetes will require insulin.

Type 1.5 diabetes is sometimes called Mature Onset Diabetes of the Young (MODY), Latent Autoimmune Diabetes in Adults (LADA), slow onset type 1 diabetes, or double diabetes. As might be expected, each of these terms involves specific criteria and there is disagreement about who falls into which category. Leaving the questions about specific terminology aside, why should it matter that about 15% of the diabetic population is neither type 1 nor type 2?

It matters because type 1.5 diabetics are often misdiagnosed as type 2 diabetics. It matters because type 1.5 diabetics will initially respond to dietary modifications and oral medication, but because their condition stems from the death of beta cells rather than insulin resistance, eventually they will not. Physicians who are not familiar with type 1.5 diabetes may not understand why a misdiagnosed type 1.5 diabetic has stopped responding to standard treatments and may assume that the patient is no longer complying with their instructions. (This happens often enough that physicians have good reason to suspect noncompliance as an explanation for poor diabetic control.) If the patient knows that he or she is following the doctor's guidelines, it might be a good idea to ask for autoantibody tests to see if destruction of the pancreatic beta cells is taking place. As in every other aspect of health care, it becomes important for the patient to become an active participant in the monitoring and management of his condition.

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