Deductible. The amount you pay for covered healthcare services before your insurance plan starts to pay. With a $2,000 deductible for example, you pay the first $2,000 of covered services yourself. Once you exceed $2,000, your insurance company will pay for additional medical costs.

Degenerative. Deteriorating, getting worse.

Depolarizing neuromuscular blocking agents. (Example: suxamethonium) Drugs causing skeletal muscle paralysis.

Distal. Situated away from the trunk of the body, at the end of the limbs toward the feet and hands.

DM1. The abbreviation for the Latin name for myotonic dystrophy (dystrophia myotonica) type 1, the more severe form of myotonic dystrophy with the mutation found on chromosome 19.

DM2. The abbreviation for the Latin name for myotonic dystrophy (dystrophia myotonica) type 2, with the mutation found on chromosome 3.

DMPK. The abbreviation for the myotonic dystrophy gene, the Myotonic Dystrophy Protein Kinase gene that causes DM1; it is located on chromosome 19.

Double helix. Two strands of DNA held together by hydrogen bonds; when enlarged it resembles a tiny ladder (with many rungs) that has been gracefully twisted; it is along these rungs that the chemical repeats stutter beyond their normal number and cause the mutation, or change, in the gene that causes myotonic dystrophy.

Dominant inheritance. As in myotonic dystrophy, the expression of a gene where if one parent carries the mutated gene, the children have a 50% chance of getting it.

Dysphagia. Difficulty swallowing.

Dysphasia. Difficulty speaking.

Dyspnea. Shortness of breath.

Dystrophy. An inherited muscle disorder in which the muscles become weaker.