Multiple sclerosis is a disease of the central nervous system consisting of brain, spinal cord and optic nerves. The nerve is surrounded by a fatty tissue known as myelin sheath that protects the nerve from external damage and helps in electrical impulse conduction. When, this myelin sheath is damaged at multiple locations, leaving behind scar tissue known as sclerosis, the conduction of electrical impulses in nerves is often disrupted and abnormal. This abnormal and disrupted conduction of impulses leads to various symptoms of MS.
Although, the cause of multiple sclerosis is still unclear, it is believed to be due to attack of the immune system against the myelin sheath. This autoimmunity is supposed to be triggered by various factors, including genetics, infections, smoking, female sex and geographical location away from equator.
The symptoms experienced in MS are variable in each patient. These may range from fatigue, pain, weakness, tingling, visual changes, speech abnormality, bowel changes, bladder incontinence, ataxia, abnormal sensations, spasms, hearing problems and depression.
Can I Still Have MS If My MRI Is Normal?
The diagnosis of MS is not based on any particular imaging or laboratory test. The diagnosis of MS is a combination of clinical symptoms and signs, MRI, evoked potential and CSF examination for the presence of oligoclonal bands.
Magnetic Resonance Imaging (MRI)
MRI is a highly sensitive and specific imaging test for MS. MRI is used to detect the scarring or sclerosis caused by MS in the central nervous system. MRI is superior to CT scan in identifying areas of damage and white matter lesions with added advantage of zero radiation. However, it is avoided in people with metal implants, aneurysm clips and cardiac pacemakers.
About 95% patients with clinically definitive MS have an abnormal MRI, but MRI is not a definitive investigation as up to 4% normal healthy individuals can have periventricular lesions that cannot be distinguished from MS. White matter lesions can also be found in other conditions, including ischemic and age related changes. A person with an abnormal MRI has about 83% risk of developing clinically definite MS in 10 years. When the number of lesions is more in an MRI, the disability will be worse and time of conversion to a clinically definite diagnosis of MS will be relatively earlier to one who has less number of lesions.
On the other hand, a normal MRI does not rule out the diagnosis of MS. About 5% people, who have confirmed MS based on other diagnostic criteria, do not show any brain lesions on MRI. These people may have lesions elsewhere, such as spinal cord or lesions that are undetectable by MRI. A person with normal MRI has about 11% risk of developing clinically definite MS in 10 years. Although, the chances are less, a patient can still be positive for MS, even if their MRI is normal. Therefore, MRI alone is not used to diagnose MS, especially in patients with atypical or non-specific histories. In addition, other diagnostic tests such as CSF exam and evoked potential should also accompany MRI before ruling out MS and to reach a definitive diagnosis.
Cerebrospinal Fluid (CSF) Exam
CSF examination also has high sensitivity and specificity for MS. CSF examination is done to look for specific staining pattern of antibodies known as oligoclonal bands (OCBs). About 95% of patients with clinically definitive MS are positive for CSF oligoclonal bands. Although, OCBs are present in many inflammatory diseases too, it can also be found in patients with non-inflammatory neurological diseases.
Evoked Potential (EP) Test
It is the diagnostic study of electrical conduction through nerve that studies the rate of conduction and amplitude of nerve impulse. Visual evoked potential (VEPs) is most contributory for the diagnosis of MS. A delay in impulse conduction means demyelination and reduction in amplitude means damage to axons or conduction block. Although, EP is helpful in identifying the initial block, it is limited in finding out further damage. Therefore, it is the least efficacious of all three diagnostics.
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