What is Ophthalmoplegic Muscular Dystrophy?
Ophthalmoplegic muscular dystrophy is a muscular dystrophy that usually presents in adulthood. Extraocular (muscles that control the movement of the human eye) muscles are involved initially and muscles used in swallowing tend to become affected. Typical facial appearance, especially drooping of the upper eyelids, resembles that found in myasthenia gravis (another muscular dystrophy).
Overall, this generally does not affect life span.
What causes Ophthalmoplegic Muscular Dystrophy?
The inheritance pattern often then follows as autosomal dominant pattern; however, occasional sporadic cases and cases with autosomal recessive inheritance have also been described.
Every tissue in the body is made up of cells. Each cell has genetic material that determines how that cell (and hence our body) functions. The basic units of genetic material which are responsible for hereditary or inheritance are called genes. Individual genes cannot be seen even with powerful microscopes but the genes are located like beads on a string, on structures called chromosomes.
Every person has two copies of each gene (or genetic message), one coming from each parent, making 23 pairs of chromosomes. The first 22 pairs chromosomes are numbered 1 – 22, and are known as autosomal chromosomes. The last pair of chromosomes, known as the sex chromosomes, are important in determining gender. A female, for example has two X chromosomes, whilst the males have one X chromosome and one Y chromosome. On each of these chromosomes, there are 1000’s of genes, each having their own role in the body. Both chromosomes have the same genes on them (although often in different forms known as alleles) and both together maintain their own, specific role in the body.
In this case the autosomal dominant form of the Ophthalmoplegic Muscular Dystrophy needs only one of these two genes to be altered for an individual to have the condition. This means, often one parent has the altered gene, but due to its late onset, is not known until much later in life that they carry that gene.
Occasionally, Ophthalmoplegic Muscular Dystrophy is passed on via a different type of inheritance. This is known as autosomal recessive inheritance. For this to occur, two altered copies of the genes are required before an individual is affected with the condition. Both parents, would each carry one copy of the altered gene, which doesn’t cause the development of the condition, and they would both have to pass on that gene onto their child. This means that child will have 2 altered copies of that gene and may then develop the condition.
Ophthalmoplegic Neuromuscular Disorder with abnormal mitochondria
It appears that this condition is frequently associated with a genetic mutation affecting a small amount of DNA that is not transmitted in the chromosomes but rather in the mitochondria. The mitochondria are subcellular organelles which are responsible for energy production and each mitochondria has a small sequence of DNA which can develop mutations similar to those seen in the chromosomes which are contained in the nucleus of the cell. It appears that an individual obtains his or her mitochondria from his or her mother’s egg and so disorders associated with abnormal mitochondria are generally considered to be “maternally” transmitted. Mitochondrial abnormalities can also be responsible for more diffuse muscle weakness, a condition known as mitochondrial myopathy.