# Clinical Cases: Action Potential and Nerve Conduction ## Case 1: Multiple Sclerosis - Central Demyelination ### Clinical Image ![Multiple Sclerosis Demyelination](case_01_image.jpg) *Source: [Wikimedia Commons - MS Demyelination](https://commons.wikimedia.org/wiki/File:MS_Demyelinisation_CD68_10xv2.jpg) - CC BY-SA 3.0* ### Patient Presentation A 28-year-old female presents to the neurology clinic with a 3-week history of blurred vision in her right eye that has progressively worsened. She describes pain with eye movement. She also reports an episode 6 months ago of numbness and tingling in her legs that resolved after several weeks without treatment. She noticed that her symptoms seemed worse after taking a hot shower or exercising (Uhthoff phenomenon). ### Demographics - Age: 28 years - Sex: Female - Ethnicity: Northern European descent ### Chief Complaint Progressive blurred vision in right eye with pain on eye movement ### Physical Examination - Visual acuity: Right eye 20/100, Left eye 20/20 - Fundoscopy: Right optic disc pallor, normal left eye - Pupillary exam: Right relative afferent pupillary defect (RAPD/Marcus Gunn pupil) - Color vision: Impaired red desaturation in right eye - Neurological: Brisk reflexes in lower extremities, positive Babinski sign bilaterally - Sensory: Mild decreased vibration sense in feet - Lhermitte sign: Positive (electric shock sensation down spine with neck flexion) ### Workup - MRI brain with contrast: Multiple periventricular white matter lesions, some enhancing with gadolinium (indicating active inflammation), Dawson fingers pattern - MRI spine: Two demyelinating lesions in cervical spinal cord - Visual evoked potentials (VEP): Prolonged P100 latency in right eye (delayed conduction) - Lumbar puncture: Oligoclonal bands present in CSF (not in serum), elevated IgG index - Serum studies: Negative for NMO-IgG antibodies ### Diagnosis Relapsing-remitting Multiple Sclerosis (RRMS) with optic neuritis, meeting McDonald criteria for dissemination in space and time ### Treatment 1. Acute episode: IV methylprednisolone 1g daily for 3-5 days (hastens recovery, does not change long-term outcome) 2. Disease-modifying therapy: Initiate high-efficacy DMT (e.g., ocrelizumab, natalizumab, or fumarates) 3. Symptomatic treatment for spasticity, fatigue, and neuropathic pain as needed 4. Physical therapy and rehabilitation 5. Regular MRI monitoring for new lesion activity 6. Vitamin D supplementation 7. Counseling regarding prognosis and disease management ### Physiological Principles Demonstrated - **Saltatory conduction**: In myelinated axons, action potentials "jump" from one node of Ranvier to the next, dramatically increasing conduction velocity (up to 120 m/s vs. 0.5-2 m/s in unmyelinated fibers). - **Myelin function**: Myelin acts as an electrical insulator, increasing membrane resistance and decreasing capacitance, allowing current to flow efficiently to distant nodes. - **Demyelination effects**: Loss of myelin exposes the axon membrane, causing current leakage, slowed conduction velocity, and eventually conduction block when the current reaching the next node is insufficient to reach threshold. - **Temperature sensitivity (Uhthoff phenomenon)**: Heat worsens conduction in demyelinated fibers. Elevated temperature increases potassium channel activity and decreases sodium channel availability, widening the safety factor gap in already compromised axons. - **Visual evoked potentials**: The prolonged P100 latency directly measures slowed conduction velocity in the optic nerve due to demyelination. - **Length constant**: Demyelination decreases the length constant (the distance over which a signal decays to 37% of its original value), impairing the ability of local currents to depolarize distant membrane segments. --- ## Case 2: Guillain-Barre Syndrome - Peripheral Demyelination ### Clinical Image ![Guillain-Barre Syndrome](case_02_image.jpg) *Source: [Wikimedia Commons - Guillain-Barre Syndrome](https://commons.wikimedia.org/wiki/File:Guillain-Barr%C3%A9_Syndrome.png) - CC BY-SA 4.0* ### Patient Presentation A 45-year-old male presents to the emergency department with progressive weakness in both legs that began 3 days ago and has now spread to involve his arms. He reports tingling and numbness in his hands and feet. Two weeks ago, he had a gastrointestinal illness with diarrhea that resolved after several days. He now has difficulty walking and climbing stairs. ### Demographics - Age: 45 years - Sex: Male - Recent history: Gastroenteritis 2 weeks prior (likely Campylobacter jejuni infection) ### Chief Complaint Progressive ascending weakness and paresthesias ### Physical Examination - Vital signs: BP 142/88, HR 98, RR 18, SpO2 96% on room air - General: Anxious, mildly dyspneic when supine - Motor: 3/5 strength in proximal lower extremities, 4/5 in distal lower extremities, 4/5 throughout upper extremities - Reflexes: Absent in ankles and knees, diminished in upper extremities (areflexia) - Sensory: Decreased light touch and vibration in stocking-glove distribution - Gait: Unable to walk without assistance - Respiratory: Negative inspiratory force (NIF) -35 cmH2O (borderline; normal > -60) - Cranial nerves: Intact at this time ### Workup - Lumbar puncture: Protein 125 mg/dL (elevated), WBC 3 cells/uL (albuminocytologic dissociation - elevated protein without pleocytosis) - Nerve conduction studies (NCS): Prolonged distal motor latencies, reduced conduction velocities (32 m/s; normal >50 m/s), temporal dispersion, conduction block - consistent with demyelinating polyneuropathy - EMG: No active denervation (early stage) - Anti-ganglioside antibodies: Positive for anti-GM1 antibodies - Stool culture: Campylobacter jejuni (retrospective confirmation) - Serial pulmonary function: NIF and FVC monitored q4-6 hours ### Diagnosis Guillain-Barre Syndrome (Acute Inflammatory Demyelinating Polyneuropathy - AIDP) with impending respiratory compromise ### Treatment 1. ICU admission for respiratory monitoring 2. Serial measurement of NIF and FVC (intubate if NIF < -25 or FVC < 20 mL/kg) 3. Intravenous immunoglobulin (IVIG) 0.4 g/kg/day for 5 days OR plasmapheresis (5 exchanges over 2 weeks) 4. DVT prophylaxis (high risk for immobility-related thrombosis) 5. Pain management for neuropathic pain 6. Physical and occupational therapy 7. Monitor for autonomic dysfunction (blood pressure lability, arrhythmias) 8. Nutritional support 9. Psychological support and counseling regarding recovery timeline (weeks to months) ### Physiological Principles Demonstrated - **Compound action potential**: The nerve conduction study measures the summated electrical activity of all axons in a peripheral nerve. In GBS, demyelination causes prolonged distal latencies and slowed conduction velocities. - **Temporal dispersion**: Different axons conduct at different velocities when variably demyelinated, causing the compound action potential to spread out over time and decrease in amplitude. - **Conduction block**: When demyelination is severe enough, the local current generated by an action potential cannot reach threshold at the next segment, causing complete conduction failure. This manifests as a >50% amplitude drop between proximal and distal stimulation sites. - **Fiber classification**: Motor fibers (A-alpha, largest and fastest) and large sensory fibers are affected early. The ascending pattern reflects the greater vulnerability of longer axons. - **Schwann cell attack**: Autoimmune attack on Schwann cell myelin (triggered by molecular mimicry with Campylobacter lipooligosaccharides resembling gangliosides) disrupts the myelin sheath, exposing the axon membrane. - **Areflexia mechanism**: Deep tendon reflexes depend on intact afferent (Ia sensory) and efferent (alpha motor neuron) conduction. Demyelination of either component abolishes the reflex. --- ## Case 3: Local Anesthetic Toxicity - Sodium Channel Blockade ### Clinical Image ![Local Anesthetic](case_02_image.jpg) *Source: Clinical illustration of sodium channel blockade mechanism* ### Patient Presentation A 35-year-old female undergoes an interscalene brachial plexus block for shoulder surgery. Approximately 2 minutes after injection of 25 mL of 0.5% bupivacaine, she develops perioral numbness, tinnitus, metallic taste, and becomes increasingly agitated. She then has a generalized tonic-clonic seizure followed by cardiac arrest with pulseless electrical activity (PEA). ### Demographics - Age: 35 years - Sex: Female - Procedure: Interscalene nerve block with bupivacaine ### Chief Complaint Post-procedural seizure and cardiac arrest ### Physical Examination (Pre-arrest) - Mental status: Initially alert, then agitated, then unresponsive - Neurological: Perioral and tongue numbness, tinnitus - Cardiovascular: Tachycardia progressing to bradycardia, then PEA arrest - Musculoskeletal: Generalized tonic-clonic seizure activity ### Workup - Clinical diagnosis based on timing and symptom progression - ECG: Widened QRS complex, prolonged PR interval preceding arrest - Arterial blood gas: Metabolic and respiratory acidosis during arrest ### Diagnosis Local Anesthetic Systemic Toxicity (LAST) from inadvertent intravascular injection or rapid systemic absorption of bupivacaine ### Treatment 1. Stop injection immediately 2. Call for help and initiate LAST protocol 3. Airway management with 100% oxygen (hyperventilation to raise pH) 4. Benzodiazepines for seizures (avoid propofol in cardiovascular compromise) 5. Intravenous lipid emulsion (Intralipid 20%): 1.5 mL/kg bolus followed by 0.25 mL/kg/min infusion - this is the specific antidote 6. ACLS protocol if cardiac arrest occurs (avoid vasopressin, avoid large epinephrine doses) 7. Consider cardiopulmonary bypass or ECMO if refractory 8. Prolonged resuscitation may be required (patients can recover after prolonged CPR) ### Physiological Principles Demonstrated - **Mechanism of local anesthetics**: Local anesthetics block voltage-gated sodium channels from the intracellular side. They bind preferentially to the inactivated state (use-dependent block), which is why rapidly firing neurons are more susceptible. - **Action potential blockade**: By preventing sodium channel opening, local anesthetics block the depolarization phase of the action potential, preventing nerve impulse propagation. - **CNS toxicity**: The CNS is exquisitely sensitive to sodium channel blockade. Initial excitatory symptoms (seizures) occur because inhibitory interneurons are blocked first, releasing excitatory circuits from suppression. Higher doses block all neurons, causing CNS depression. - **Cardiac toxicity**: Bupivacaine has high affinity for cardiac sodium channels and dissociates slowly ("fast-in, slow-out"). This prolongs QRS duration, decreases contractility, and can cause refractory arrhythmias and cardiac arrest. - **Lipid emulsion mechanism**: Lipid emulsion creates a "lipid sink" that sequesters lipophilic bupivacaine from tissue binding sites. It also provides metabolic substrate for the myocardium and may enhance drug redistribution. - **Differential nerve block**: Smaller, unmyelinated C fibers (pain) are blocked before larger myelinated A fibers (motor, proprioception) because they have higher surface-to-volume ratios and firing frequencies. This is why analgesia precedes motor block.