Diagnosis of ALS
Lung Function Tests
Lung function tests are used to assess the risk of respiratory failure and the need for assisted ventilation. There are several devices and measurement parameters that are used to evaluate respiratory function in ALS patients. They include a spirometer, respiratory pressure meter, peak flow meter, and more.
A genetic test is usually only performed on individuals with a known family history of ALS. The two most common forms of ALS are sporadic ALS (SALS) and familial ALS (FALS). FALS refers to ALS cases where the individual has one or more family members with ALS, and it is known that he or she inherited the condition from a parent.
A muscle biopsy is the surgical removal of a small piece of muscle that will then be analyzed in the laboratory. The procedure can be used to diagnose ALS, but is more likely to be used to eliminate other possible diagnoses and to monitor the progression of the disease by comparing muscle biopsies taken over the lifetime of the patient.
A nerve biopsy is used to detect abnormalities in nerve cells. In ALS, the axons of upper and lower motor neurons have often degenerated. The procedure is performed under local anesthesia and involves making an incision about 2 inches long at a site where the nerve is accessible and runs below the skin. About 2 inches of the nerve is removed and sent for pathological examination, where the sample is viewed under a microscope.
Blood tests are used to look for evidence of diseases, the symptoms of which are similar to those of ALS. They can help, therefore, to exclude ALS. Some of the diseases that may be tested for include HIV, hepatitis, cancer, and autoimmune diseases. Several other tests may be performed, including thyroid function tests, parathyroid disease tests, and vitamin B12 deficiency tests.
Thyroid Function Tests
A thyroid function test can help in a search for evidence of other conditions with shared symptoms and to rule out ALS, which is not associated with impaired thyroid function. A poorly working thyroid can cause myopathy, or muscle disease, leading to the muscular weakness and cramps also experienced by ALS patients. Thyroid function can be tested in the blood or via an iodine uptake test.
A spinal tap, also called lumbar puncture, refers to the procedure of inserting a thin, hollow needle into the lower back area of the spine between two lumbar bones (vertebrae). A spinal tap is performed to exclude inflammatory nerve conditions and usually done only when the patient has unusual symptoms of ALS, such as spinal nerve abnormalities or no sign of abnormal reflexes or spasticity.
Tests for Infectious Diseases
Some infectious diseases can cause symptoms similar to ALS, and these must be ruled out before a diagnosis is confirmed. It is essential to differentiate these from ALS because they need to be managed in very different ways. In some cases, ALS-like symptoms caused by infectious diseases can be reversed.
An electromyography (EMG) is one of the key tests in diagnosing ALS. It is also referred to as “needle EMG” and is normally performed alongside nerve conduction studies. In most cases of ALS, the motor neurons that control muscle movement no longer work correctly, leading to a decrease in the patient’s ability to use certain muscles. An EMG is used to assess the health of the muscles and the motor neurons that control them.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a noninvasive tool that uses a magnetic field along with radio waves to produce detailed images of the brain, spinal cord, or other body areas. Preliminary studies suggest that MRI may help identify signs of ALS earlier, but larger studies conducted over a longer time are needed.
Nerve Conduction Study
Physicians will generally carry out a nerve conduction study — also called a nerve conduction velocity test — prior to ordering an EMG to diagnose ALS. Electrical signals are passed along nerve cells throughout the body to relay messages, for example, to tell a muscle to contract. This procedure assesses the conduction velocity, or speed of the electrical signal, through the nerve. This can determine if and where nerve damage exists.