CMD is a medical abbreviation used for two very different conditions: congenital muscular dystrophy, a group of inherited muscle diseases present from birth, and coronary microvascular disease, a heart condition affecting the smallest blood vessels that supply the heart. Which one applies depends on context, so here’s what you need to know about both.
Congenital Muscular Dystrophy (CMD)
Congenital muscular dystrophy refers to a family of genetic disorders that cause muscle weakness and wasting starting at or shortly after birth. Unlike other forms of muscular dystrophy that appear later in childhood or adulthood, CMD is present from the very beginning of life. The condition is caused by mutations in genes responsible for producing proteins that muscles need to function and maintain their structure.
There are several subtypes, each tied to a different gene. The most well-known include:
- MDC1A (merosin-deficient CMD): Caused by mutations in the LAMA2 gene on chromosome 6, which produces a protein called merosin that helps anchor muscle fibers to surrounding tissue. This is one of the more common and severe forms.
- Collagen VI-related CMD: This exists on a spectrum. At the severe end is Ullrich CMD, which causes significant weakness and very loose joints. At the milder end is Bethlem myopathy, which may not be obvious in early childhood and tends to cause proximal muscle weakness (closer to the trunk) with variable joint tightness. Many people fall somewhere between these two extremes.
- Dystroglycanopathies: A group of subtypes caused by faulty processing of a protein that connects muscle cells to their surrounding matrix.
- SELENON-related CMD and LMNA-related CMD: Rarer forms involving different structural or regulatory proteins in muscle tissue.
Symptoms in Infants and Children
The hallmark of CMD is muscle weakness that’s noticeable at birth or within the first few months of life. In newborns, this often looks like “floppy infant syndrome,” where the baby has very low muscle tone and makes few spontaneous movements of the arms and legs. Parents may notice a weak cry, difficulty feeding due to a poor sucking reflex, and muscle or joint tightness (contractures) that limits range of motion.
As the child grows, weakness in the head and trunk muscles can become the most pressing concern because it affects breathing. Chronic respiratory failure is a relatively common complication, developing when the muscles needed for breathing become too weak to keep up with the body’s demands. The severity and progression vary widely depending on the specific subtype. Some children never walk independently, while others maintain the ability to walk into adulthood.
Diagnosis and Outlook
CMD is typically diagnosed through a combination of clinical examination, blood tests that measure markers of muscle damage, genetic testing, and sometimes muscle biopsy. Because the subtypes overlap in their symptoms, genetic testing has become especially important for pinpointing the exact mutation and guiding expectations about how the disease may progress.
There is no cure for congenital muscular dystrophy. Treatment focuses on managing symptoms and preserving function for as long as possible. This includes physical therapy to maintain flexibility, respiratory support when breathing muscles weaken, nutritional support for children with feeding difficulties, and orthopedic interventions for contractures or spinal curvature. Life expectancy varies significantly by subtype and severity, with respiratory health being the single biggest factor in long-term outcomes.
Coronary Microvascular Disease (CMD)
Coronary microvascular disease is a condition where the tiniest blood vessels supplying the heart muscle don’t work properly. Unlike traditional coronary artery disease, which involves blockages in the large arteries visible on an angiogram, CMD affects vessels smaller than 500 micrometers in diameter. These tiny arteries and capillaries are responsible for more than 70% of the resistance that regulates blood flow to the heart, making them critical for delivering oxygen when the heart is working hard.
About half of all patients who undergo cardiac catheterization for chest pain or signs of reduced blood flow to the heart turn out to have no significant blockages in their large coronary arteries. For many of these people, the problem lies in the microvascular system instead. This pattern is called ischemia with non-obstructive coronary arteries, or INOCA.
What Goes Wrong in the Small Vessels
CMD involves both structural and functional changes in the coronary microcirculation. On the structural side, the walls of tiny arteries can thicken due to smooth muscle overgrowth and excess collagen deposits, narrowing the space for blood to flow. The number of capillaries can also decrease, a process called capillary rarefaction.
Functionally, the vessels lose their ability to dilate properly when the heart needs more blood. The inner lining of these vessels (the endothelium) normally releases signals that relax the vessel walls, but in CMD this mechanism becomes impaired. In some cases, the vessels constrict excessively instead. Both problems lead to the same result: the heart doesn’t get enough oxygen during exertion or stress.
Symptoms and Who’s at Risk
The primary symptom is chest pain (angina) that can feel identical to the chest pain caused by blocked large arteries. People with CMD often experience recurrent chest pain episodes, significant physical limitations, and reduced quality of life. Studies have found that these patients report greater physical limitations and more frequent angina than people with stable coronary artery disease or even some heart attack survivors. Some patients also have heightened pain sensitivity in the heart, where even routine procedures like catheter movement or contrast dye injection trigger chest discomfort.
CMD is more prevalent among women. Traditional cardiovascular risk factors like diabetes, obesity, smoking, high blood pressure, and chronic kidney disease all contribute, but women face additional risks. The decline in estrogen after menopause plays a particularly important role. Estrogen helps maintain the health of blood vessel linings by promoting the production of nitric oxide, a molecule that keeps vessels relaxed and open. When estrogen levels drop, vascular function deteriorates. Autoimmune conditions like lupus, rheumatoid arthritis, and psoriasis, which disproportionately affect women, are also linked to higher CMD risk. Other female-predominant risk factors include migraines, hypertensive disorders of pregnancy, and psychological stress. Systemic inflammation, measured by elevated C-reactive protein levels, has been associated with reduced blood flow reserve even in patients without traditional risk factors.
How CMD Is Diagnosed
CMD can be tricky to identify because standard angiograms only visualize the large coronary arteries. The gold standard for diagnosis is invasive coronary function testing, performed during a catheterization procedure. After confirming that no large-vessel blockages exist, doctors administer a series of medications through the catheter to test how the small vessels respond. One medication forces the vessels to dilate as much as they can, measuring their maximum capacity. Another tests whether the vessel lining is releasing proper dilation signals. If blood flow doesn’t increase adequately in response to these challenges, CMD is confirmed.
Non-invasive options also exist. PET scans using radioactive tracers can measure absolute blood flow to the heart at rest and during stress, revealing regional variations that suggest microvascular problems. Cardiac MRI offers high-resolution images and can detect subtle patterns of heart muscle damage associated with chronic reduced blood flow. Ultrasound of a specific coronary artery through the chest wall is another option, though less widely available.
Treatment and Management
Because CMD was historically underrecognized, many patients went years without a clear diagnosis or appropriate treatment. The condition was once dismissed under vague labels like “cardiac syndrome X,” and patients, particularly women, were sometimes told nothing was wrong because their angiograms looked clean. That understanding has shifted significantly.
Management targets the underlying mechanisms driving reduced blood flow. Medications that improve blood vessel function, reduce spasm, and address risk factors like high blood pressure, diabetes, and high cholesterol form the backbone of treatment. Lifestyle modifications addressing smoking, weight, and physical activity also play a central role. Getting the diagnosis right matters: studies show that patients who undergo coronary function testing and receive targeted treatment have better outcomes than those managed empirically.