# Clinical Cases: Synaptic Transmission ## Case 1: Myasthenia Gravis - Postsynaptic Receptor Dysfunction ### Clinical Image ![Myasthenia Gravis Ptosis](case_01_image.jpg) *Source: [Wikimedia Commons - Myasthenia Gravis](https://commons.wikimedia.org/wiki/File:Myasthenia_Gravis.jpg) - CC BY-SA 3.0* ### Patient Presentation A 32-year-old female presents to the neurology clinic with a 4-month history of progressive fatigue, double vision (diplopia), and drooping eyelids (ptosis) that worsen throughout the day and improve with rest. She first noticed symptoms in the evening after long days at work. Recently, she has developed difficulty swallowing (dysphagia), particularly with solid foods, and her voice becomes nasal and weak after prolonged speaking. She denies muscle pain, sensory changes, or bowel/bladder dysfunction. ### Demographics - Age: 32 years - Sex: Female - Onset: Insidious over 4 months with fluctuating course ### Chief Complaint Fluctuating weakness, ptosis, diplopia, dysphagia, and dysarthria that worsen with activity ### Physical Examination - General: Alert, comfortable at rest - Cranial nerves: - Ptosis: Bilateral, worse on right, increases with sustained upgaze (fatigability test positive) - Eye movements: Limited abduction and elevation bilaterally, diplopia in multiple directions - Facial weakness: Mild bilateral facial weakness with "snarling" smile - Speech: Becomes increasingly nasal with counting to 100 - Palate: Weak palatal elevation bilaterally - Motor: 4+/5 strength in proximal upper extremities after repetitive testing (demonstrating fatigability) - Reflexes: Normal 2+ throughout - Sensory: Intact to all modalities ### Workup - Acetylcholine receptor (AChR) antibodies: Positive (highly specific for MG) - Anti-MuSK antibodies: Negative - Repetitive nerve stimulation (RNS): Decremental response >10% at 3 Hz stimulation of facial nerve (positive for NMJ disorder) - Single-fiber EMG: Increased jitter and blocking - CT chest: 3 cm anterior mediastinal mass consistent with thymoma - Pulmonary function tests: FVC 78% predicted (monitor for myasthenic crisis) - Tensilon (edrophonium) test: Rapid improvement in ptosis (positive, though rarely performed now) ### Diagnosis Myasthenia Gravis (seropositive, generalized) with thymoma ### Treatment 1. Pyridostigmine (acetylcholinesterase inhibitor) 60 mg TID for symptomatic relief 2. Prednisone taper for immunosuppression (start low, go slow to avoid initial worsening) 3. Steroid-sparing agent: Azathioprine or mycophenolate for long-term immunosuppression 4. Thymectomy for thymoma (mandatory) and potential disease modification 5. IVIg or plasmapheresis reserved for myasthenic crisis or pre-operative preparation 6. Avoid medications that worsen MG: aminoglycosides, beta-blockers, magnesium, certain antibiotics 7. MedicAlert bracelet and emergency action plan 8. Monitor for myasthenic crisis (FVC < 1L or NIF < -25 requires ICU admission) ### Physiological Principles Demonstrated - **Neuromuscular junction physiology**: Acetylcholine (ACh) is released from motor nerve terminals, crosses the synaptic cleft, and binds nicotinic ACh receptors (nAChR) on the motor end plate, causing depolarization (end-plate potential) that triggers muscle action potentials. - **Safety factor**: Normally, far more ACh is released than needed to reach threshold (safety factor). In MG, antibody-mediated destruction of nAChRs reduces receptor density, narrowing the safety factor. - **Fatigability mechanism**: With repeated stimulation, presynaptic ACh stores transiently deplete. In normal muscle, the safety factor compensates. In MG, reduced receptor numbers mean successive stimuli produce progressively smaller EPPs that fail to reach threshold. - **Decremental response**: Repetitive nerve stimulation at 2-3 Hz causes progressive amplitude decline of the compound muscle action potential (CMAP) because fewer muscle fibers are activated with each stimulus. - **Acetylcholinesterase inhibitors**: Pyridostigmine inhibits acetylcholinesterase, prolonging ACh availability in the synaptic cleft and increasing the probability of receptor binding despite reduced receptor numbers. - **Autoimmune pathophysiology**: Anti-AChR antibodies cause receptor loss through complement-mediated destruction, accelerated receptor internalization, and direct functional blockade. --- ## Case 2: Botulism - Presynaptic Release Failure ### Clinical Image ![Botulinum Toxin Mechanism](case_02_image.jpg) *Source: [Wikimedia Commons - Botulinum toxin structure](https://commons.wikimedia.org/wiki/File:Botulinumtoxin.png) - Public Domain* ### Patient Presentation A 28-year-old male presents to the emergency department with progressive weakness, blurred vision, difficulty swallowing, and dry mouth over the past 18 hours. He attended a home-canning demonstration 36 hours ago where he ate home-preserved green beans and meat. Two other attendees have been hospitalized with similar symptoms. He denies fever, sensory changes, or confusion. ### Demographics - Age: 28 years - Sex: Male - Exposure: Home-canned vegetables (green beans) 36 hours prior ### Chief Complaint Progressive descending weakness, diplopia, dysphagia, and dry mouth ### Physical Examination - Vital signs: BP 118/76, HR 92, RR 20, Temp 37.0C, SpO2 94% on room air - General: Alert but anxious, weak voice - Cranial nerves: - Pupils: Dilated bilaterally, sluggish light response - Ptosis: Bilateral - Eye movements: Ophthalmoplegia (limited movement in all directions) - Facial weakness: Bilateral, symmetric - Bulbar weakness: Absent gag reflex, weak tongue movements - Motor: Symmetric weakness, worse proximally (4/5 upper, 4/5 lower), descending pattern - Reflexes: Diminished 1+ throughout - Sensory: Intact - Autonomic: Dry mouth, constipation reported ### Workup - Mouse bioassay: Positive for botulinum toxin type A (gold standard, but takes days) - Stool culture: Clostridium botulinum isolated - Food sample analysis: Pending (from green beans) - Repetitive nerve stimulation: Incremental response at high-frequency stimulation (20-50 Hz) - characteristic of presynaptic NMJ disorder - EMG: Small amplitude, brief motor unit potentials with facilitation after exercise - Lumbar puncture: Normal (helps exclude GBS) - CT/MRI brain: Normal ### Diagnosis Foodborne Botulism (Clostridium botulinum toxin type A) ### Treatment 1. Immediate ICU admission for airway monitoring 2. Intubation and mechanical ventilation (likely required for weeks) 3. Trivalent botulinum antitoxin (equine-derived) as soon as possible - does not reverse existing paralysis but prevents progression 4. Supportive care: Nutrition (NG tube or PEG), DVT prophylaxis, prevention of secondary infections 5. Public health notification and investigation of food source 6. Contact tracing for other exposed individuals 7. No antibiotics for foodborne botulism (may increase toxin release) 8. Rehabilitation during prolonged recovery (weeks to months) ### Physiological Principles Demonstrated - **SNARE-mediated vesicle fusion**: Neurotransmitter release requires SNARE proteins (synaptobrevin/VAMP on vesicles; syntaxin and SNAP-25 on presynaptic membrane) to form a complex that pulls membranes together for fusion. - **Botulinum toxin mechanism**: Botulinum toxin is a zinc-dependent protease that cleaves specific SNARE proteins, preventing vesicle fusion and blocking ACh release. Different serotypes cleave different targets: Type A cleaves SNAP-25; Types B, D, F, G cleave synaptobrevin. - **Presynaptic vs. postsynaptic disorders**: Unlike myasthenia gravis (postsynaptic), botulism affects the presynaptic terminal. EMG shows facilitation with rapid stimulation (calcium accumulation partially overcomes the block) and incremental response to RNS. - **Descending paralysis pattern**: The toxin preferentially affects cranial nerves first, then descends to respiratory and limb muscles, opposite to the ascending pattern in GBS. - **Autonomic involvement**: Cholinergic parasympathetic neurons are also affected, causing dilated pupils, dry mouth, constipation, and urinary retention - features that help distinguish botulism from MG. - **Recovery mechanism**: New nerve terminals must sprout and form new synapses, explaining the prolonged recovery over weeks to months. --- ## Case 3: Parkinson's Disease - Dopaminergic Transmission Deficit ### Clinical Image ![Parkinson Disease](case_02_image.jpg) *Source: Clinical illustration of dopaminergic pathway dysfunction* ### Patient Presentation A 65-year-old male is brought by his wife to the neurology clinic because she has noticed progressive changes over the past 2 years. He has become slower in his movements, his handwriting has become smaller (micrographia), and he shuffles when walking. She also notes that his right hand shakes when resting but stops when he reaches for objects. He has become less expressive facially and his voice is softer. He reports constipation, loss of smell (anosmia), and vivid dreams with movement during sleep (REM sleep behavior disorder) that preceded the motor symptoms. ### Demographics - Age: 65 years - Sex: Male - Duration: Prodromal symptoms for years; motor symptoms for 2 years ### Chief Complaint Progressive slowness, resting tremor, shuffling gait, and reduced facial expression ### Physical Examination - General: Masked facies (hypomimia), reduced blinking - Speech: Hypophonic (soft), monotonous - Motor: - Tremor: 4-6 Hz resting tremor, right hand > left, "pill-rolling" quality, resolves with action - Rigidity: Cogwheel rigidity in right arm and leg - Bradykinesia: Reduced finger tapping amplitude and speed, difficulty with rapid alternating movements - Postural instability: Positive pull test with retropulsion (multiple steps backward) - Gait: Shuffling, reduced arm swing (right > left), festination, difficulty initiating movement - Writing: Micrographia demonstrated on writing sample ### Workup - Clinical diagnosis based on cardinal features (bradykinesia plus tremor or rigidity) - DaTscan (dopamine transporter SPECT): Reduced uptake in putamen bilaterally (left > right), consistent with nigrostriatal degeneration - MRI brain: No structural abnormalities, rules out vascular parkinsonism - Response to levodopa trial: Marked improvement in motor symptoms (positive) ### Diagnosis Idiopathic Parkinson's Disease (Hoehn & Yahr stage 2) ### Treatment 1. Levodopa/carbidopa (Sinemet) - most effective symptomatic treatment 2. Dopamine agonist (pramipexole or ropinirole) as adjunct or initial monotherapy in younger patients 3. MAO-B inhibitor (rasagiline or selegiline) for mild symptom control and possible neuroprotection 4. COMT inhibitor (entacapone) to extend levodopa effect 5. Physical therapy for gait training and balance 6. Speech therapy for hypophonia 7. Treatment of non-motor symptoms: Constipation (fiber, laxatives), RBD (melatonin, clonazepam), depression, cognitive changes 8. Consider deep brain stimulation (DBS) for motor fluctuations refractory to medications ### Physiological Principles Demonstrated - **Dopaminergic neurotransmission**: Dopamine is synthesized from tyrosine via tyrosine hydroxylase (rate-limiting) and DOPA decarboxylase. It is released from synaptic vesicles and acts on D1-D5 G-protein coupled receptors. - **Nigrostriatal pathway**: Dopaminergic neurons in the substantia nigra pars compacta project to the striatum (caudate and putamen). Loss of these neurons (>60-80%) produces the motor symptoms of PD. - **Direct and indirect pathways**: The basal ganglia modulate movement through two pathways. Dopamine facilitates movement by stimulating D1 receptors on the direct (go) pathway and inhibiting D2 receptors on the indirect (stop) pathway. Dopamine loss results in excessive inhibition of movement. - **Levodopa mechanism**: Levodopa (L-DOPA) is the precursor to dopamine that can cross the blood-brain barrier (dopamine itself cannot). Carbidopa inhibits peripheral decarboxylation, ensuring more levodopa reaches the brain. - **MAO-B inhibitors**: Monoamine oxidase B degrades dopamine in the brain. MAO-B inhibitors (rasagiline, selegiline) prolong dopamine action by preventing its breakdown. - **Signal termination**: Dopamine action is terminated primarily by reuptake via the dopamine transporter (DAT) and degradation by MAO and COMT. Understanding these pathways guides pharmacotherapy.