Dictionary

mydriasis Dilation of the pupil. The opposite of mydriasis is miosis.

myelin The fatty substance that covers and protects nerves. Myelin is a layered tissue that surrounds the nerve fibers (axons). This sheath around the axons acts like a conduit in an electrical system, ensuring that messages sent by axons are not lost en route.

myelination The formation of the myelin sheath around a nerve fiber. Also known as myelinization. See also myelin.

myelitis Inflammation of the spinal cord, such as from infection or immune inflammation.

myelodysplastic syndrome One of a group of disorders characterized by abnormal development of one or more of the cell lines that are normally found in the bone marrow. Patients can develop a variety of symptoms related to anemia, low or high white blood cell count, infections, and bleeding problems. Myelodysplastic syndrome may progress and become…

myeloencephalitis See encephalomyelitis.

myelofibrosis Spontaneous scarring (fibrosis) of the bone marrow that disrupts the normal production of blood cells, leading to severe anemia and enlargement of the spleen and liver. Myelofibrosis usually begins slowly and worsens over time. It can be associated with a variety of diseases, primarily myeloproliferative (preleukemic) disorders. Also known as agnogenic myeloid metaplasia.

myelogenous See myeloid.

myelogram An X-ray test of the spinal cord and the bones of the spine. A myelogram is used to detect impingement of the spinal cord by bone, disc, or other tissues.

myeloid Referring to myelocytes, a type of white blood cell. Also known as myelogenous.

myeloproliferative disorder One of the malignant diseases leading to overproduction of certain bone marrow cells, including those that give rise to the red blood cells, the granulocytes, and the blood platelets. The myeloproliferative disorders include chronic myelogenous leukemia, chronic idiopathic myelofibrosis, essential thrombocythemia, chronic neutrophilic leukemia, chronic eosinophilic leukemia, and polycythemia vera.

myocardial infarction See heart attack.