Multiple organ involvement and seemingly unrelated symptoms make the diagnosis of mitochondrial disorders extremely challenging. As a general rule of thumb, when three or more organ systems are involved without an evident cause, mitochondrial disease should be considered. On the contrary, if symptoms are not as severe then mitochondrial diseases may be overlooked and misdiagnosed.
1. Initial tests – body fluids: blood, urine and cerebrospinal fluid (CSF)
Basic biochemistry is the first test performed, with special attention given to the levels lactate and pyruvate. Often (but not always), mitochondrial disease patients suffer from a condition known as lactic acidosis which is an increase in lactic acid concentration in the body. Lactate and pyruvate are acids found in the mitochondria. Often in mitochondrial dysfunction, this ratio of lactate to pyurvate increases as a result of changes in biochemical pathways. When neurological symptoms are also present, this lactate to pyurvate ratio is tested for in the cerebrospinal fluid obtained from a lumbar puncture.
The levels of certain amino acids, proteins and organic acids are also measured in other body fluids such as urine.
Testing at the next stage is more symptomatic. Different diseases in different people present different symptoms. The commonly affected organs include (but are not restricted to) the brain, muscles, heart, eyes, the gastrointestinal tract, and the endocrine system.
2. Diagnostic tools
MRI and CT scans may show
Lesions (damage) to certain areas of the brain – symmetrical lesions to the area known as basal ganglia is common
(Left) MRI scan of a healthy brain. (Right) The basal ganglia region of the brain shows abnormal signals.
- Changes are observed in the grey matter and white matter areas in the brain
- Calcification (hardening) of certain areas, often the basal ganglia
The scan shows changes in white matter within the brain as well as symmetrical abnormalities in signals (white areas) in the area known as the putamen (bottom arrows), in both hemispheres
- Atrophy (decrease in size) of certain brain areas
MRS tests may show
- Elevated levels of lactate in the brain
Two inverted peaks that resemble a “W” at around 1.2 on the horizontal axis indicate a high lactate concentration
- EEG may reveal a slow brain wave pattern associated with diseases of the nervous system
- Nerve conduction tests are useful in diagnosing myopathy and sensory neuropathy
- Electrocardiography and echocardiography may reveal certain heart conditions such as a conduction disorder or cardiomyopathy
- Electroretinography is helpful in diagnosing eye conditions such as retinitis pigmentosa
3. Finding the root cause - the levels of analysis
After step two, doctors generally have an idea as to which group of diseases they are looking at. Now the task is to medically classify it. The ultimate goal is to find the exact genetic change, or mutation, responsible for the observable symptoms. The order of analysis is at the tissue level, then protein level and finally gene level.
Tissue level – Biopsy
A muscle biopsy is most common when obtaining a tissue sample, primarily because it is the easiest to obtain. Depending upon the symptoms and disease however, other tissue samples such as from the skin or liver may be necessary. At this level of analysis, the histology (appearance) of the tissue is tested. Often observed are ragged red fibres (abnormal mitochondria beneath the muscle membrane appear as ragged red fibres when stained with a dye) which is characteristic of mitochondrial disease.
Ragged red fibres
Chemical staining identifies diseased mitochondria, appearing as “ragged red fibres”. Left and right show lower and higher magnification, respectively.
Protein level – Enzymology
Mitochondrial disease is most frequently caused by dysfunctional proteins of the respiratory chain. Of course the respiratory chain is the pathway for energy production in mitochondria. Faulty proteins may severely reduce energy production in a cell thereby causing damage or cell death. To determine the cause of the disease, it is important to pinpoint the source of the problem. Enzymology tests measure the activity of the respiratory chain proteins. Chemical staining of these proteins is also used. These biochemical tests can exactly identify the faulty proteins in the chain.
The function of each of the five proteins of the respiratory chain is tested using biochemical reactions to identify which proteins are faulty
Gene level – DNA testing
The primary basis of any mitochondrial disease is at the level of DNA. There are numerous genes which code for proteins of the respiratory chain, found in both nuclear as well as mitochondrial DNA. Once the dysfunctional protein has been identified, the search begins to locate the genes responsible in coding for that protein complex. Using sophisticated methods such as sequencing the DNA one base at a time, we can determine the exact location of mutation. The type of mutation (deletion, duplication, point mutation etc) can also be identified. A specific type of mutation at a specific location is often the identity for a mitochondrial disease and distinguishes it from other diseases that may have similar characteristics.