neurologic disorders
Amyotrophic Lateral Sclerosis (ALS) and Other Motor Neuron Diseases (MNDs)
Diagnosis of ALS and Other MNDs
Electrodiagnostic tests MRI of the brain and, if no cranial nerve involvement, cervical spine Laboratory tests to check for other, treatable causes Diagnosis of motor neuron diseases is suggested by progressive, generalized motor weakness without significant sensory abnormalities. Other disorders that cause pure muscle weakness should be ruled out: Disorders of Neuromuscular Transmission None Various myopathies (including noninflammatory and drug-induced) Spinal Muscular Atrophies (SMAs) (mostly in children) Autoimmune Myositis None Autoimmune Myositis None Overview of Thyroid Function and Electrolyte abnormalities (eg, Hypokalemia , Various infections (eg, Syphilis , Autoimmune-mediated motor neuropathies Cervical Spondylosis and Spondylotic Cervical Myelopathy None Behavioral variant of Behavioral (frontal) variant FTD (15 to 20% develop motor neuron disease) Heavy metal toxicity (eg, lead, mercury) When cranial nerves are affected, a treatable cause is less likely. Upper and lower motor neuron signs plus weakness in facial muscles strongly suggest amyotrophic lateral sclerosis. If cranial nerves are affected and findings are compatible with ALS, a treatable alternative cause is less likely. Electrodiagnostic testsshould be done to check for evidence of disorders of neuromuscular transmission or demyelination. Such evidence is not present in MNDs; nerve conduction velocities are usually normal until late in the disease. Needle electromyography (EMG) is the most useful test, showing fibrillations, positive waves, fasciculations, and sometimes giant motor units, even in unaffected limbs. Brain MRIis required. When there is no clinical or EMG evidence of cranial nerve motor weakness, MRI of the cervical spine is indicated to exclude structural lesions that may be compressing the spinal cord. Laboratory testsare done to check for treatable causes. Tests include complete blood count, electrolytes, creatine kinase, and thyroid function tests. Serum and urine protein electrophoresis with immunofixation is done to check for a paraprotein that is rarely associated with MNDs. Discovering an underlying paraproteinemia may indicate that the MND is paraneoplastic, and treatment of the paraproteinemia may ameliorate the MND. Antimyelin-associated glycoprotein (MAG) antibodies are associated with a demyelinating motor neuropathy, which may mimic ALS. A 24-hour urine collection is done to check for heavy metals in patients who may have been exposed to them. Lumbar Puncture (Spinal Tap) may be done to exclude other clinically suspected disorders; if white blood cells or the protein level is elevated, an alternative diagnosis is likely. The serum Venereal Disease Research Laboratories (VDRL) test, erythrocyte sedimentation rate, and measurement of certain antibodies (rheumatoid factor, Lyme titer, HIV, hepatitis C virus, antinuclear [ANA], anti-Hu [to check for anti-Hu paraneoplastic syndrome]) are indicated only if suggested by risk factors or history. Genetic testing (eg, for superoxide dismutase gene mutation or genetic abnormalities that cause spinal muscular atrophies) and enzyme measurements (eg, hexosaminidase A for Tay-Sachs disease) should not be done unless patients are interested in genetic counseling; disorders detected by these tests have no known specific treatments. 