Neuromuscular Junction Disorders Flashcards
What limits the ability of ACh to generate an EPP normally?
Acetylcholinesterase
During repeated contractions in exercise, less ACh is released, but why do muscles not fail normally?
Although the EPP amplitudes are thereby decreased, they still remain above the threshold for generating muscle fiber action potentials, and adequate muscle contraction is still achieved.
What happens to ACh receptors in myasthenia gravis and how does this lead to the effects of the disease?
Many AChR are degraded or blocked, less ACh is able to bind, producing EPPs which are either initially below the threshold for generating muscle fiber action potentials or which become too low during repeated muscle contractions.
Myasthenia Gravis
Myasthenia gravis (MG) is an autoimmune disorder of the postsynaptic NMJ.
Lambert-Eaton Myasthenic Syndrome
Autoimmune disease with the target being the voltage-gated calcium channel at the presynaptic membrane
What is seen on microscopy of the NMJ in myasthenia gravis?
The normal architecture and folding of the postsynaptic NMJ is lost when viewed under electron microscopy.
What is a common abnormality associated with myasthenia gravis?
In most myasthenic patients, abnormalities of the thymus occur, consisting more commonly of glandular enlargement or hyperplasia, and less commonly of a thymic tumor or thymoma.
What are some common symptoms of myasthenia gravis?
- ptosis (eyelid drooping)
- diplopia (double vision from asymmetrically weak extraocular muscles)
- dysarthria (slurred speech)
- dysphagia (weakness of swallowing)
Ocular Myasthenia
A small group of MG patients who only have visual symptoms of MG
Neonatal Myasthenia
Healthy newborns of myasthenic mothers may have MG symptoms for a few days, until maternal antibodies “wash out” of their system.
Myasthenic Crisis
Profound weakness may cause quadriplegia, with the patient unable to speak, swallow or breathe. Myasthenic crisis may be suddenly triggered by a serious infection or other systemic illness in a myasthenic patient.
How is MG diagnosed?
Injection with edrophonium. If there is marked improvement, the diagnosis is highly supported.
What is the most specific test for myasthenia gravis?
Serum AChR Ab
What is the main drug used to treat MG?
Anticholinesterase Drugs
What is a surgical procedure that can possibly help MG patients?
Thymectomy
What are some immune system treatment options for MG?
- Corticosteroids
- Plasmapheresis
- IVIG
What are the symptoms of Lambert-Eaton Myasthenia Syndrome?
As another NMJ disorder, muscle fatigue and weakness occur in LEMS, but tend to affect the proximal muscles of the shoulders and hips as well as the trunk, mimicking the weakness also seen in a myopathy. Symptoms do not usually involve the eyes, swallowing, or speech.
What are some symptoms of LEMS that differs from MG?
- dry mouth
- orthostatic hypotension
- erectile dysfunction
What can the autoimmune dysfunction of LEMS be associated with?
Small Cell Carcinoma of the Lung
How is LEMS diagnosed?
Repetitive nerve stimulation and other tests in the electromyography laboratory usually show presynaptic NMJ abnormalities typical of LEMS.
