# Clinical Cases: Neurons and Synaptic Transmission ## Case 1: Myasthenia Gravis - Neuromuscular Junction Disorder ### Patient Presentation **Demographics:** 32-year-old female **Chief Complaint:** Progressive double vision and difficulty swallowing over 3 weeks **History of Present Illness:** A 32-year-old woman presents with intermittent double vision that worsens in the evening and after reading for extended periods. She has also noticed difficulty swallowing solid foods and occasional choking on liquids. Over the past week, she has developed drooping of her eyelids, particularly at the end of the day. She reports that symptoms improve after rest. She has also noticed her voice becoming more nasal-sounding by dinnertime. She denies weakness in her limbs, numbness, or sensory changes. **Physical Examination:** - Vital signs: BP 118/72, HR 78, RR 14, T 36.9C - General: Appears comfortable at rest - Neurological: - Mental status: Alert, oriented, normal cognition - Cranial nerves: Bilateral ptosis worse on sustained upgaze (fatigable ptosis); diplopia on lateral gaze; nasal voice quality; weak palate elevation bilaterally; facial weakness with flattened nasolabial folds - Motor: Normal strength proximally and distally at rest; mild proximal weakness after repeated testing - Sensory: Intact throughout - Reflexes: 2+ and symmetric; no pathological reflexes **Workup:** - **Acetylcholine receptor (AChR) antibodies:** Positive (elevated at 15.2 nmol/L; normal <0.4) - **CT chest:** 2.5 cm anterior mediastinal mass consistent with thymoma - **Repetitive nerve stimulation:** Decremental response (>10% decrement at 3 Hz stimulation) in facial muscles - **Edrophonium (Tensilon) test:** Transient improvement in ptosis and diplopia following administration **Diagnosis:** Myasthenia gravis with thymoma **Treatment:** - Pyridostigmine 60 mg three times daily (acetylcholinesterase inhibitor) - Prednisone 60 mg daily initiated with slow taper - Referral to cardiothoracic surgery for thymectomy - Pre-operative plasmapheresis to reduce antibody burden - Education regarding myasthenic crisis precautions **Clinical Pearl:** Myasthenia gravis exemplifies the critical importance of acetylcholine (ACh) in neuromuscular transmission. Autoantibodies against nicotinic acetylcholine receptors at the neuromuscular junction reduce the number of functional receptors, causing fatigable weakness. The characteristic fatigability reflects progressive depletion of ACh quanta with repeated stimulation when fewer receptors are available. Treatment with acetylcholinesterase inhibitors prolongs ACh action at the synapse, partially compensating for receptor loss. --- ## Case 2: Lambert-Eaton Myasthenic Syndrome ### Patient Presentation **Demographics:** 58-year-old male **Chief Complaint:** Progressive difficulty rising from a chair and climbing stairs over 2 months **History of Present Illness:** A 58-year-old man with a 40-pack-year smoking history presents with gradually worsening proximal leg weakness. He has trouble getting out of low chairs and climbing stairs. Interestingly, he notes that his strength temporarily improves after a few seconds of sustained effort. He also reports dry mouth, constipation, and erectile dysfunction. He has lost 15 pounds unintentionally over 3 months. **Physical Examination:** - Vital signs: BP 142/88, HR 68, RR 16 - General: Thin, appearing older than stated age - Neurological: - Cranial nerves: Minimal ptosis; extraocular movements full - Motor: 4/5 strength hip flexors and shoulder abduction initially, improving to 4+/5 with sustained effort (facilitation); distal strength preserved - Sensory: Intact - Reflexes: Absent initially; present after brief sustained muscle contraction (post-tetanic potentiation) - Other: Dry oral mucosa **Workup:** - **Voltage-gated calcium channel (VGCC) antibodies:** Positive (P/Q-type) - **Electromyography/nerve conduction:** Low compound muscle action potential (CMAP) amplitude at rest; >100% increment in CMAP amplitude after brief exercise - **CT chest:** 3 cm spiculated mass in right upper lobe - **PET-CT:** Hypermetabolic lung mass with mediastinal lymphadenopathy - **Lung biopsy:** Small cell lung carcinoma **Diagnosis:** Lambert-Eaton myasthenic syndrome (LEMS) as paraneoplastic syndrome secondary to small cell lung carcinoma **Treatment:** - Oncology referral for chemotherapy and radiation of small cell lung cancer - 3,4-diaminopyridine (amifampridine) to enhance presynaptic acetylcholine release - Pyridostigmine added for additional symptomatic benefit - Surveillance for tumor response with expectation of LEMS improvement **Clinical Pearl:** LEMS demonstrates the critical role of presynaptic calcium channels in neurotransmitter release. Antibodies against P/Q-type voltage-gated calcium channels impair calcium influx at the presynaptic terminal, reducing vesicle fusion and ACh release. The characteristic "facilitation" with sustained effort occurs because repeated depolarization allows calcium accumulation, temporarily overcoming the channel blockade. This distinguishes LEMS from myasthenia gravis, where weakness worsens with repeated effort. --- ## Case 3: Botulism - Presynaptic Neuromuscular Blockade ### Patient Presentation **Demographics:** 26-year-old male **Chief Complaint:** Blurred vision, difficulty swallowing, and weakness for 12 hours **History of Present Illness:** A 26-year-old man presents to the emergency department with rapidly progressive symptoms that began yesterday evening. He first noticed blurred vision and drooping eyelids, followed by difficulty swallowing and speaking clearly. Over the past several hours, he has developed arm and leg weakness. He ate home-canned green beans at a family gathering 36 hours ago. Several other family members who ate the same dish are also ill with similar symptoms. **Physical Examination:** - Vital signs: BP 110/70, HR 92, RR 22, T 37.0C, SpO2 94% on room air - General: Alert, anxious-appearing, weak voice - Neurological: - Mental status: Alert, oriented (cognition preserved) - Cranial nerves: Bilateral ptosis; sluggish pupillary responses; complete ophthalmoplegia; facial weakness; absent gag reflex; weak tongue protrusion - Motor: 3/5 proximal, 4/5 distal strength in all extremities; symmetric - Sensory: Intact to all modalities - Reflexes: Diminished throughout (1+) - Respiratory: Shallow breathing, weak cough, negative inspiratory force -25 cmH2O (concerning) **Workup:** - **Serum botulinum toxin assay:** Positive for Clostridium botulinum toxin type A (results returned 48 hours later) - **EMG/nerve conduction:** Low CMAP amplitudes with incremental response to rapid repetitive stimulation - **Stool culture:** Positive for Clostridium botulinum - **Analysis of remaining canned goods:** Positive for botulinum toxin - **CSF analysis:** Normal (excluding Guillain-Barre syndrome) **Diagnosis:** Foodborne botulism **Treatment:** - Immediate intubation and mechanical ventilation for respiratory failure - Heptavalent botulism antitoxin administered within 24 hours of presentation - ICU admission with continuous monitoring - Notification of public health authorities - Supportive care through prolonged recovery (weeks to months) - Other affected family members treated similarly **Clinical Pearl:** Botulinum toxin blocks neuromuscular transmission by cleaving SNARE proteins (SNAP-25, synaptobrevin, syntaxin) essential for synaptic vesicle fusion with the presynaptic membrane. Without vesicle fusion, acetylcholine cannot be released, causing flaccid paralysis. Unlike myasthenia gravis (postsynaptic) or LEMS (calcium channel), botulism targets the vesicle release machinery directly. The toxin's extraordinary potency explains its use as both a bioweapon concern and a therapeutic agent (at minuscule doses) for dystonia and cosmetic applications. --- ## Clinical Image ![Neuromuscular junction electron microscopy](case_01_image.jpg) **Image Description:** Electron micrograph of the neuromuscular junction showing the presynaptic terminal with synaptic vesicles containing acetylcholine, the synaptic cleft, and the postsynaptic membrane with junctional folds containing nicotinic acetylcholine receptors. **Attribution:** Image from Wikimedia Commons (https://commons.wikimedia.org/), Creative Commons license.