Clinical Presentation of Muscle Weakness
What is a key symptom distinguishing upper motor neuron from lower motor neuron muscle weakness?
UMN weakness often presents with spasticity and hyperreflexia, while LMN weakness shows flaccidity and hyporeflexia.
In myasthenia gravis, what is the main pathophysiologic mechanism causing muscle weakness?
Autoantibodies block or destroy acetylcholine receptors at the neuromuscular junction, impairing synaptic transmission.
What is the first-line diagnostic test for myasthenia gravis?
Acetylcholine receptor antibody testing
Another acceptable answer: electromyography (EMG) to assess neuromuscular transmission
What is the mainstay of treatment for myasthenia gravis?
Acetylcholinesterase inhibitors (e.g., pyridostigmine) and immunosuppressive therapy
What is the normal role of acetylcholine in muscle contraction?
It binds to receptors at the neuromuscular junction, triggering depolarization and muscle contraction.
What are the hallmark signs of muscle weakness in myasthenia gravis?
Fluctuating weakness that worsens with activity and improves with rest, often affecting ocular and bulbar muscles.
What causes muscle weakness in Guillain-Barré syndrome?
An autoimmune attack on peripheral nerves, leading to demyelination or axonal damage.
What laboratory test can help diagnose Guillain-Barré syndrome?
Lumbar puncture showing elevated protein in CSF without an increase in white blood cells (aka "albuminocytologic dissociation").
How is acute muscle weakness in Guillain-Barré syndrome treated?
With intravenous immunoglobulin (IVIG) or plasmapheresis
(To reduce the autoimmune attack on nerves)
What happens to the nerve conduction in demyelinating diseases?
There is a slowing of nerve conduction velocity due to loss of the myelin sheath that insulates axons.
What clinical feature is typical of muscle weakness in Guillain-Barré syndrome?
Symmetrical, ascending muscle weakness starting in the legs and progressing upwards.
How does hypokalemia lead to muscle weakness?
Low potassium levels impair muscle cell membrane excitability.
How is Duchenne muscular dystrophy typically diagnosed?
By genetic testing to identify dystrophin gene mutations
Additionally, DMD is often diagnosed by measuring creatine kinase levels, which are markedly elevated
What is the treatment approach for polymyositis?
Corticosteroids are the first-line treatment, often combined with immunosuppressive agents like methotrexate or azathioprine.
How do muscle fiber types (I/II) contribute to normal muscle function?
Type I fibers are used for endurance (slow-twitch), while Type II fibers provide strength and power (fast-twitch).
Describe the muscle weakness associated with polymyositis.
Proximal muscle weakness, particularly of the shoulders and hips, without sensory involvement.
What genetic mutation is responsible for muscle weakness in Duchenne muscular dystrophy?
Mutations in the dystrophin gene, leading to loss of dystrophin, a protein crucial for muscle fiber integrity.
What electrodiagnostic study is used to differentiate between myopathy and neuropathy?
Electromyography (EMG) and nerve conduction studies (NCS), with myopathy showing small amplitude motor potentials and neuropathy showing prolonged conduction times.
What supportive treatment is often needed for patients with ALS as their disease progresses?
Non-invasive ventilation (e.g., BiPAP)
This will aid with respiratory muscle weakness
How does denervation affect (1) muscle structure and (2) electromyography findings?
(1) Denervated muscles undergo atrophy
(2) Denervated muscles develop fibrillations that can be seen on EMG
How does muscle weakness in amyotrophic lateral sclerosis (ALS) present?
ALS presents with BOTH upper and lower motor neuron signs -- including muscle weakness, fasciculations, spasticity, and eventual atrophy.
What role does mitochondrial dysfunction play in causing muscle weakness in mitochondrial myopathies?
Defective mitochondria impair ATP production, leading to energy deficiency in muscle cells and resultant weakness.
What advanced imaging technique is used to assess muscle involvement in inflammatory myopathies?
Magnetic resonance imaging (MRI)
(To visualize inflammation, edema, or fatty replacement within muscles)
Which drug is commonly used to reduce the risk of progression in multiple sclerosis and related muscle weakness?
Disease-modifying therapies (DMTs) like interferons or glatiramer acetate.
Glatiramer acetate is a mixture of synthetic polypeptides that binds to MHC molecules and competes with myelin antigens for presentation to T cells. It also induces suppressor cells that migrate to the brain and suppress inflammation.
How does Botulinum toxin impair muscle function?
It blocks the release of acetylcholine at the neuromuscular junction, leading to paralysis of the affected muscles.