The subjects covered in this section are:
The brain 
Breathing and swallowing muscle weakness 
Head, neck and face muscle weakness 
Heart difficulties 
Insulin resistance 
Internal organs 
Limb and hand muscle weakness 
Myotonia and muscle pain 
Research suggests that, in MMD1, there may be abnormalities in the parts of the brain that determine the rhythm of sleeping and waking, making excessive daytime sleepiness a barrier to full participation in work, school or social life for many adults with the disorder. In some people, there is a kind of overall "apathy" that may be due to changes in the brain related to MMD1.
Although not as much is known about the effects of MMD2 on personality, cognition and sleepiness in MMD2 as in MMD1, it appears that people with MMD2 can have some of the same difficulties in these areas as people with MMD1, but to a lesser degree.
To learn more, read The Brain in MMD  (cognitive and emotional aspects of MMD1) and Excessive Daytime Sleepiness Can Be 'Debilitating' in MMD1 and MMD2  (complex effects of MMD on the brain's sleep-wake cycles and respiratory muscles).
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Respiratory muscle weakness does not appear to be a common feature of MMD2.
However, in MMD1, respiratory muscles weakness can affect lung function and deprive the body of needed oxygen. Weakness of the diaphragm and other breathing muscles can lead to problems getting enough oxygen when a person is asleep, even if they don’t have any symptoms of breathing difficulty while awake.
Respiratory problems in MMD1 are further aggravated, many experts believe, by an abnormality in the brain’s breathing control center. This abnormality can lead to a condition known as sleep apnea , in which people stop breathing for several seconds or longer many times a night while asleep.
Swallowing muscles, if weakened, can lead to choking  or “swallowing the wrong way,” (called aspiration) with food or liquid going down the trachea (windpipe) to the lungs instead of down the esophagus to the stomach. Swallowing is partly voluntary and partly involuntary, and both sets of muscles can be affected.
To learn more, read Excessive Daytime Sleepiness Can Be 'Debilitating' in MMD1 and MMD2  (complex effects of MMD on the brain's sleep-wake cycles and respiratory muscles).
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Cataracts — cloudy areas of the lens of the eye that eventually can interfere with vision — are extremely common in both MMD1 and MMD2. They generally occur earlier than the common, age-associated cataracts seen in people without MMD.
Cataracts are caused by a chemical change in the lens, which gradually goes from clear to cloudy the way the clear part of an egg changes to white when cooked. Exactly why cataracts occur in MMD isn’t known.
The person with a cataract may notice that things start to look blurry, hazy or dim, and that this worsens gradually over time. It often happens in both eyes, but not necessarily at the same time or at the same rate.
Read Keeping Your Focus: Eye Care , particularly the section called Other vision problems: Not common, sometimes treatable , for additional information about eye care in neuromuscular disorders.
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|A long, thin face with hollow temples, drooping eyelids and, in men, balding in the front, is typical in myotonic dystrophy.|
The muscles of the neck, jaw and parts of the head and face may weaken, especially in MMD1. Facial weakness is less common in MMD2.
Weakness and loss of bulk in these muscles leads to a characteristic appearance doctors and experienced family members recognize as MMD.
In men, early balding in the front part of the scalp is very common, adding to the distinct appearance of MMD.
Eyelids may droop (called ptosis; the “p” is silent). The chewing muscles can be affected, which makes the temples appear hollow and the face look thin.
Weak neck muscles, common in both types of MMD, can make it hard to sit up quickly or lift one’s head straight up off a bed or couch. The stronger trunk muscles have to be used for these actions.
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The heart  can be affected in MMD1 or MMD2. Oddly, since MMD is mostly a muscle disease, it isn’t the muscle part of the heart (which pumps blood) that’s most affected, but rather the part that sets the rate and rhythm of the heartbeat — the heart’s conduction system.
It’s common in MMD1, especially after many years, to develop a conduction block, which is a block in the electricity-like signal that keeps the heart beating at a safe rate. This appears to occur in MMD2 as well, although there aren’t as many studies in this form of the disease.
Fainting, near fainting or dizzy spells are the usual symptoms of conduction block, and these should never be ignored. Such problems can be fatal.
In both forms of MMD, cardiac muscle impairment also can occur, although it isn’t as common as conduction abnormalities.
To learn more, read Cardiac Care in MMD: Lack of Symptoms May Mask Deadly Problems  and Revising Cardiac Care in Muscular Dystrophies  (covers different types of heart problems that occur in these disorders and how to monitor and treat them).
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Fortunately, most people with MMD1 and MMD2 don’t have diabetes, but they may develop a diabetes-like condition that is sometimes referred to as insulin resistance. This means the body makes insulin (a hormone needed for the cells to take up and use sugars), but for some reason, it takes more insulin to do the job because the muscle tissues don’t respond normally to the usual amounts. High blood sugar may result from insulin resistance.
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|The digestive tract and uterus (womb) often are affected in type 1 myotonic dystrophy. These organs contain involuntary muscles, which can weaken or develop myotonia (trouble relaxing). Abnormalities in the brain can lead to excessive sleepiness or apathy. The heart (especially the “electrical” part) also can be affected.|
Most of the internal organs in the body are hollow tubes (such as the intestines) or sacs (such as the stomach). The walls of these tubes and sacs contain involuntary muscles that squeeze the organs and move things (food, liquids, a baby during childbirth and so forth) through them.
In MMD1, many of the involuntary muscles that surround the hollow organs can weaken. These include the muscles of the digestive tract, uterus and blood vessels.
Such problems appear to be absent or mild in MMD2.
Abnormal action of the upper digestive tract can impair swallowing. Once food is swallowed, the involuntary muscles of the esophagus should take over and move food into the stomach. However, in MMD1, these muscles can have spasms and weakness, causing a feeling of food getting stuck and sometimes leading to inhaling food into the lungs.
The lower digestive tract — large intestine (colon), rectum and anus — also can be affected by weakness and spasm in MMD1. Crampy pain, constipation and diarrhea can occur.
The gallbladder — a sac under the liver that squeezes bile into the intestines after meals — can weaken in MMD1. People with MMD probably are more likely than the general population to develop gallstones. Symptoms are difficulty digesting fatty foods and pain in the upper right part of the abdomen.
Fortunately, most people do not experience incontinence or urination problems in MMD.
Because of weakness and uncoordinated action of the muscle wall of the uterus, women with MMD1 may experience difficulties in childbirth that can be serious for both mother and baby. These may involve excessive bleeding or ineffective labor. Sometimes a Caesarean operation (C-section) is advised, but surgery also can be a problem in MMD (see Medical Management ).
Blood pressure in MMD1 tends to be low. This is probably due to low tone of the smooth muscles in the blood vessels. It usually poses no problem and may even be one beneficial effect of MMD1.
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Weakness of the voluntary muscles usually is the most noticeable symptom for people with adult-onset MMD.
The distal muscles (those farthest from the center of the body) usually are the first — and sometimes the only — limb muscles affected in MMD1. Areas of the limbs affected may include the forearms, hands, lower legs and feet. Over time, these muscles get smaller, so the lower legs and arms may appear thinner than the upper legs and arms.
In MMD2, proximal muscles — those closer to the center of the body — tend to show more weakness than in MMD1. Weakness in the upper part of the leg (thigh) occurs early in MMD2. In MMD1, thigh weakness, if it occurs at all, comes later in the disease.
People with both types of MMD often notice that their grip is weak and that they have trouble using their wrist or hand muscles. The muscles that pick up the foot when walking may weaken, allowing the foot to flop down and cause tripping and falling (foot drop).
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Myotonia of voluntary muscles can make it hard for someone with MMD1 or MMD2 to relax their grip, especially in cold temperatures. Door handles, cups, handwriting and using hand tools may pose a problem, although some people never notice it. Myotonia also can affect the muscles of the tongue and jaw, causing difficulty with speech and chewing.
Myotonia can be uncomfortable and even cause pain, although people with MMD1 and MMD2 also can have muscle pain that isn’t connected to the myotonia.
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