Gastrointestinal issues can result, which can be severe, manifestations including diarrhea, constipation, and fecal incontinence. The symptoms of adult DM often manifest during adolescence. Infantile DM1 can be distinguished as another disease category, or it can be grouped with congenital DM1 or childhood-onset DM1. Childhood-onset DM1
Childhood-onset DM1 is defined as onset of symptoms between ages 1 and 10 years. Manifestations include the same intellectual and gastrointestinal symptoms seen in congenital DM1. DM2
DM2 is generally milder than DM1, with generally fewer DM2 people requiring assistive devices than DM1 people. DM2 preferentially affects muscles closer to or on the torso, including the neck flexors, hip flexors, and hip extensors. Muscle pain is prominent in DM2. Heart issues, while still potentially fatal, are less common and severe in DM2 than DM1. Symptoms onset in early to late adulthood. Severe congenital onset, which can occur in DM1, has not been observed in DM2. Genetics
Myotonic dystrophy (DM) is a genetic condition that is inherited in an autosomal dominant pattern, meaning each child of an affected individual has a 50% chance of inheriting the disease. The mutation involves satellite DNA, which is tandemly repeated sequences of DNA that do not code for a protein. The repeats implicated in myotonic dystrophy are either 3 or 4 nucleotides in length, classified as microsatellites. Disease results from an abnormally increased number of these microsatellites, termed microsatellite expansion. DM1
The microsatellite expansion responsible for DM1 is of cytosine-thymine-guanine (CTG) triplet repeats, termed trinucleotide repeat expansion and classifying DM1 as a one of several trinucleotide repeat disorders.
The normal HGD enzyme is a hexamer (it has six subunits) that are organized in two groups of three (two trimers) and contains an iron atom. Different mutations may affect the structure, function, or solubility of the enzyme. Very occasionally, the disease appears to be transmitted in an autosomal-dominant fashion, where a single abnormal copy of HGD from a single parent is associated with alkaptonuria;  other mechanisms or defects in other genes possibly are responsible in those cases. The HGD enzyme is involved in the metabolism (chemical processing) of the aromatic amino acids phenylalanine and tyrosine. Normally, these enter the bloodstream through protein-containing food and the natural turnover of protein in the body. Tyrosine is specifically required for a number of functions, such as hormones (e.g. thyroxine, the thyroid hormone), melanin (the dark pigment in the skin and hair), and certain proteins, but the vast majority (over 95%) is unused and is metabolized through a group of enzymes that eventually generate acetoacetate and malate. In alkaptonuria, the HGD enzyme cannot metabolize the homogentisic acid (generated from tyrosine) into 4-maleylacetoacetate, and homogentisic acid levels in the blood are 100-fold higher than would normally be expected, despite the fact that a substantial amount is eliminated into the urine by the kidneys.The homogentisic acid is converted to the related substance benzoquinone acetic acid  which forms polymers that resemble the skin pigment melanin. These are deposited in the collagen, a connective tissue protein, of particular tissues such as cartilage.