A small piece of your muscle sample may be put in a preservative so it can be examined at very high magnification using an electron microscope (see Electron microscopy below).
Some of the muscle sample may also be used for other types of studies, which also help to explain what is happening inside your muscle.
If you agree, any parts of the sample which have not been used can be stored so they can be studied for research purposes. The pathologist who analyses your muscle sample will examine it in different ways depending on which muscle-wasting condition your doctor suspects you may have.
Histology
This involves looking at the overall appearance and structure of your muscle cells so the pathologist can look for characteristics that are specific to certain conditions. The muscle is dyed using various chemicals so the different structures show up under the microscope.
Histochemistry
This technique also uses chemical dyes, but looks at the activity of chemicals within your muscle fibres, which is important when diagnosing metabolic disorders. Histochemistry also reveals the characteristics of your muscle fibres. Certain changes can help to identify particular conditions.
Immunohistochemistry
This procedure uses antibodies that bind to a specific protein and can show the presence or absence of important proteins within the muscle.
When antibodies are tagged with a marker, they can be seen under the microscope and can show if a protein is in the wrong place, or is absent or reduced in amount.
This is important for several muscular dystrophies, such as Duchenne muscular dystrophy, which is caused by the absence of the protein dystrophin.
Electron microscopy
Electron microscopy allows high magnification of each muscle cell, making it easier to see structural abnormalities. This is relevant for conditions such as nemaline myopathy, where the diagnosis is based on the presence of rod structures in the muscle. Some abnormal features are visible only with the high magnification provided by electron microscopy.
What are pathologists looking for?
Appearance of fibres
Healthy muscle has a characteristic appearance, and is made up of closely-packed fibres, which are more or less evenly sized (Figure 1).
Muscle affected by a muscle-wasting condition looks different from normal muscle (Figure 2). These differences vary between conditions.
Figure 1: Healthy adult muscle biopsy
Figure 2: Muscle biopsy from a patient with Inclusion body myositis
There are two types of muscle fibre: type 1 and type 2.
In some conditions, the muscle fibres are smaller or larger than they should be. They may be damaged, or the proportion of type 1 to type 2 fibres may be unbalanced. In some cases, only one type of fibre may be affected.
Important proteins
Muscle fibres are built from a number of different, essential proteins. If some of these proteins are missing, in the wrong place, or there are too many or too few of them, this may cause problems with the muscle.
Buildup of substances in your muscle
There are many different chemical pathways within muscle tissue that can be affected in muscle-wasting conditions. These result in changes in the amounts of key substances in your muscle.
One example of this is glycogen, which is an important energy storage molecule. In healthy muscle, glycogen is broken down by a pathway involving several proteins called enzymes.
Enzymes control the speed of chemical reactions in your body, but in some muscle-wasting conditions, one of these enzymes is missing or abnormal.
This results in a build-up of glycogen in your muscle, which can be seen under the microscope with certain histochemical dyes. Special biochemical studies may be needed to identify the exact problem. A build-up of certain proteins in muscle fibres can occur in some conditions, and these can be identified by certain antibodies.
Structural changes
Some conditions are diagnosed by the presence of structural abnormalities within the muscle.
For example, muscle affected by core myopathy (including central core disease and multiminicore disease) has characteristic core structures.
In mitochondrial myopathies, structures called mitochondria, which convert food into energy, contain faulty proteins that disrupt their function. These can sometimes be seen under a microscope, or revealed with special biochemical studies.
Distribution
It is important to look at the distribution of any abnormalities within the muscle. This can sometimes, but not always, give an indication of the severity, and the rate of progression of a condition.
How long does it take to get the results?
How long you will have to wait for your results will depend on the number of tests to be done and the date of your next appointment with your consultant. Results can be made available more quickly, if necessary.
When your consultant has received the results of your muscle biopsy, they will invite you to an appointment to discuss the results and the follow-up. What happens next depends on the results of your tests. Your consultant will discuss this with you.