Myasthenia Gravis (MG) is a chronic autoimmune neuromuscular disease characterized by fluctuating muscle weakness. It occurs when the immune system attacks the communication points between nerves and muscles. This article outlines the symptoms and diagnostic tests used to confirm Myasthenia Gravis.
Recognizing Symptoms
Individuals with Myasthenia Gravis often first notice symptoms affecting the eyes and face. These include drooping eyelids (ptosis) and double vision (diplopia), which result from weakened eye muscles. MG weakness fluctuates, worsening with activity and improving with rest. Beyond eye symptoms, individuals may experience difficulty speaking (dysarthria), causing speech to sound soft or nasal. Trouble swallowing (dysphagia) can also occur, making eating and drinking challenging, and generalized muscle weakness can extend to the limbs, making everyday tasks like lifting arms or climbing stairs difficult.
Diagnostic Tests
Diagnosing Myasthenia Gravis involves specific medical tests to confirm the condition and assess nerve-muscle communication.
Blood Tests
Blood tests are a primary tool for diagnosing MG, specifically looking for certain antibodies. Approximately 85% of individuals with generalized Myasthenia Gravis have elevated levels of acetylcholine receptor (AChR) antibodies in their blood. These antibodies interfere with the signals sent from nerves to muscles at the neuromuscular junction. For individuals with Myasthenia Gravis that primarily affects the eye muscles (ocular MG), about half will have this antibody in their blood.
Another important antibody tested is muscle-specific kinase (MuSK) antibody. While less common, about 6% of people with Myasthenia Gravis are positive for MuSK antibodies, especially those who test negative for AChR antibodies. The presence of either AChR or MuSK antibodies strongly supports an MG diagnosis when symptoms are present.
Nerve Conduction Studies and Electromyography (EMG)
Electromyography (EMG) measures the electrical activity of muscles and nerves to detect problems with nerve-muscle communication. Repetitive nerve stimulation (RNS) is a common EMG technique where nerves are stimulated repeatedly. In MG, RNS shows a progressive decline in muscle responses (a decremental response) over the first few stimuli, generally a reduction of more than 10%. This indicates a failure of nerve impulses to activate muscle fibers. RNS can show an abnormal decrement in about 75% of patients with generalized MG, and in roughly 35-50% of those with ocular MG.
Single-fiber EMG (SFEMG) is a more sensitive electrodiagnostic test. It measures “jitter,” the variation in the time interval between the firing of two muscle fibers supplied by the same nerve axon. In MG, jitter is increased, and in severe cases, “blocking” can occur. SFEMG detects abnormal jitter in a high percentage of patients, with reported sensitivities of 88-100% in ocular MG and 95-100% in generalized MG. This indicates disturbed neuromuscular transmission, even in muscles that appear strong.
Edrophonium (Tensilon) Test or Ice Pack Test
The Edrophonium (Tensilon) test involves intravenously administering a short-acting medication that temporarily inhibits the breakdown of acetylcholine, the neurotransmitter responsible for muscle contraction. In individuals with Myasthenia Gravis, this temporary increase in acetylcholine at the neuromuscular junction can lead to a noticeable, albeit brief, improvement in muscle weakness, such as a reduction in eyelid drooping. This test is performed with careful monitoring due to potential side effects like bradycardia.
The Ice Pack Test is a simpler, non-invasive alternative often used for eyelid ptosis. An ice pack is applied to the affected eyelid for a few minutes. If the drooping improves significantly after cooling, it supports a diagnosis of Myasthenia Gravis. This improvement is thought to be due to temperature affecting the breakdown of acetylcholine at the neuromuscular junction.
Imaging (CT/MRI of the Chest)
Imaging scans of the chest, specifically CT (computed tomography) or MRI (magnetic resonance imaging), are performed to examine the thymus gland, a small organ located in the upper chest. The thymus plays a role in immune system development and is associated with Myasthenia Gravis. Approximately 10-21% of individuals with MG have a tumor of the thymus gland, known as a thymoma. Imaging helps identify if a thymoma is present, which can influence treatment decisions.
The Diagnostic Process
The diagnosis of Myasthenia Gravis typically begins with a thorough evaluation by a doctor, often a neurologist. They assess symptoms, medical history, and perform a physical examination, observing muscle strength, eye movements, facial expressions, and speech patterns.
Following this, doctors order diagnostic tests. Blood tests for specific antibodies, such as AChR and MuSK, are often initial steps. Electrophysiological studies, like RNS and SFEMG, evaluate neuromuscular junction function. Imaging of the chest, such as a CT or MRI, is also included to check for thymus gland abnormalities like a thymoma.
The diagnostic process can take time because symptoms can be subtle and vary. Doctors carefully consider all test results with the patient’s symptoms to arrive at an accurate diagnosis.
Distinguishing from Other Conditions
Diagnosing Myasthenia Gravis can be challenging as its symptoms resemble other neurological or muscle disorders. This requires a careful differential diagnosis to rule out conditions with similar presentations. For example, Lambert-Eaton Myasthenic Syndrome (LEMS) can mimic MG, though LEMS shows an increase in muscle response with repetitive stimulation.
Other conditions like multiple sclerosis (MS) or stroke can also cause weakness or vision problems that might be confused with MG, and general fatigue can mask the specific weakness pattern of MG. A thorough medical evaluation by a neurologist is important to differentiate MG from these disorders and ensure appropriate treatment.