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The Neurological System: What Every Clinical Pharmacist Must Know

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1. Foundational Neuropharmacology

Neurotransmission and Drug Targets

Key Neurotransmitter Systems to Know

Key Presynaptic Transporters (Drug Targets)

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2. Blood-Brain Barrier (BBB) - Pharmacokinetic Implications

• The BBB consists of tight junctions between capillary endothelial cells, supported by astrocyte foot processes and pericytes
• Lipophilicity is the primary determinant of CNS penetration - more lipid-soluble drugs cross more readily
• P-glycoprotein (P-gp) and other efflux transporters actively pump many drugs out of the CNS - this is why some drugs with good lipophilicity still have poor CNS concentrations
• Molecular size matters: small molecules (<400-500 Da) penetrate better
• Drugs can use carrier-mediated transport (e.g., levodopa uses the large neutral amino acid transporter)
• The BBB is disrupted in meningitis, encephalitis, brain tumors, and trauma - this can paradoxically allow CNS penetration of drugs that normally would not enter
• Clinical implication: When treating CNS infections, choose agents with proven CNS penetration (e.g., ceftriaxone, meropenem, linezolid, rifampicin for TB meningitis)

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3. Epilepsy and Antiseizure Drugs

Seizure Classification (ILAE)

• Focal onset (previously "partial") - arise from one hemisphere
  • Focal aware (previously simple partial)
  • Focal impaired awareness (previously complex partial)
  • Focal to bilateral tonic-clonic (previously secondary generalized)
• Generalized onset - involve both hemispheres simultaneously
  • Absence (petit mal)
  • Tonic-clonic (grand mal)
  • Myoclonic, tonic, atonic
• Unknown onset
• Status epilepticus: seizure lasting >5 minutes or repeated seizures without recovery = medical emergency

Mechanisms of Antiseizure Drugs

Drug Selection by Seizure Type

Critical Pharmacist Monitoring Points

Antiepileptics in Pregnancy

• ALL antiseizure drugs cross the placenta
• Valproate has the highest teratogenicity risk - avoid in women of childbearing potential unless no alternative
• Folic acid supplementation (4-5 mg/day) should be given before conception and throughout first trimester
• Enzyme-inducing antiseizure drugs reduce efficacy of oral contraceptives - use alternative contraception

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4. Parkinson's Disease and Movement Disorders

Pathophysiology

• Tremor (resting, pill-rolling, 4-6 Hz)
• Rigidity (cogwheel or lead-pipe)
• Akinesia/Bradykinesia
• Postural instability (late feature)

Drug Therapy - Stepwise Approach

• Levodopa/carbidopa: Gold standard. Most effective symptomatic treatment. Carbidopa prevents peripheral conversion of levodopa to dopamine (prevents nausea, vomiting, peripheral side effects)
• Dopamine agonists (pramipexole, ropinirole, rotigotine): Preferred in younger patients to delay levodopa initiation and reduce dyskinesias; avoid in >70 years or impulse control disorder risk
• MAO-B inhibitors: Rasagiline, selegiline - neuroprotective potential (unproven), avoid with serotonergic drugs (serotonin syndrome risk)
• COMT inhibitors: Entacapone, tolcapone - prevent levodopa breakdown peripherally; used for motor fluctuations; tolcapone requires LFT monitoring (hepatotoxicity)
• Amantadine: Weak NMDA antagonist; also used for levodopa-induced dyskinesias

Long-term Levodopa Complications

Drug-Induced Parkinsonism

• Antipsychotics (dopamine D2 blockers): Haloperidol, phenothiazines, metoclopramide
• Vesicular monoamine depletors: Reserpine, tetrabenazine, deutetrabenazine, valbenazine
• Mechanism: Appears within 3-4 months, resolves weeks-months after drug withdrawal
• Key point: Do NOT give levodopa to drug-induced parkinsonism while the causative drug continues - it won't work and may worsen psychiatric symptoms
• Use antimuscarinic agents if treatment needed

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5. Alzheimer's Disease and Cognitive Disorders

Pathophysiology

• Loss of cholinergic neurons from the nucleus basalis of Meynert to the cortex and hippocampus
• Amyloid plaques (Aβ42) and neurofibrillary tangles (tau protein) are hallmarks
• "Cholinergic hypothesis": cognitive decline correlates with cholinergic deficiency

Drug Treatment

Pharmacist Monitoring

• AChE inhibitors cause cholinergic side effects: nausea, vomiting, diarrhea, bradycardia (use with caution in sick sinus syndrome, bradycardia, active peptic ulcer)
• Rivastigmine patch has better GI tolerability than oral
• Memantine: generally well-tolerated; caution with other NMDA antagonists (dextromethorphan, amantadine)

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6. Pain Management

Pain Classification

• Nociceptive: Tissue damage activating nociceptors (somatic or visceral)
• Neuropathic: Aberrant somatosensory processing (diabetic neuropathy, postherpetic neuralgia, central pain)
• Mixed: Cancer pain, low back pain

WHO Analgesic Ladder

1. Non-opioid: NSAIDs, paracetamol
2. Mild opioid + non-opioid: Codeine, tramadol
3. Strong opioid + non-opioid: Morphine, oxycodone, fentanyl, hydromorphone

Opioid Pharmacology - Must Know

• All opioids act on μ (mu), κ (kappa), and δ (delta) receptors
• μ receptors: analgesia, euphoria, respiratory depression, constipation, physical dependence
• Respiratory depression is the critical adverse effect - use naloxone (μ antagonist) for reversal
• Constipation does NOT develop tolerance - always prescribe a laxative with chronic opioids
• Key drug interactions: MAOIs + meperidine = potentially fatal serotonergic/excitatory reaction; avoid
• Epidural/intrathecal opioids allow much lower doses with better analgesia and fewer systemic effects
• Tramadol: weak opioid agonist + SNRI mechanism; seizure risk; serotonin syndrome risk with serotonergic drugs

Neuropathic Pain Drugs

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7. Migraine

Pharmacotherapy

• Triptans (sumatriptan, rizatriptan, etc.): 5-HT1B/1D agonists; cause cerebral vasoconstriction and inhibit trigeminal nociception; contraindicated in ischemic heart disease, uncontrolled hypertension, basilar/hemiplegic migraine
• Ergotamines: Older, less selective; 5-HT1 agonists + vasoconstriction; contraindicated in pregnancy, vascular disease
• CGRP antagonists (gepants: ubrogepant, rimegepant): Newer; no vasoconstriction; can be used where triptans are contraindicated
• NSAIDs (aspirin + metoclopramide is effective), paracetamol

• Beta-blockers (propranolol, metoprolol)
• Topiramate, valproate
• Amitriptyline
• Anti-CGRP monoclonal antibodies (erenumab, fremanezumab, galcanezumab) - for chronic migraine

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8. Stroke

Ischemic Stroke

• tPA (alteplase): IV thrombolysis within 4.5 hours of symptom onset; check contraindications (recent surgery, hemorrhage, anticoagulants, BP >185/110 not controllable); Tenecteplase increasingly used
• Mechanical thrombectomy: For large vessel occlusion, up to 24 hours
• Aspirin: 300 mg within 24-48 hours (not if thrombolysis given within 24 hours)
• Secondary prevention: Antiplatelet therapy (aspirin, clopidogrel, aspirin/dipyridamole), anticoagulation for cardioembolic stroke (AF), statin therapy, BP control
• BP management: In acute stroke, BP should generally not be aggressively lowered unless >220/120 mmHg (or >185/110 before thrombolysis) - autoregulation is impaired

Hemorrhagic Stroke

• Reverse anticoagulation immediately: Vitamin K + PCC for warfarin; specific reversal agents for NOACs (idarucizumab for dabigatran, andexanet alfa for factor Xa inhibitors)
• No routine antihypertensive indicated unless BP >150 mmHg sustained

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9. Drug-Drug Interactions in CNS Pharmacotherapy

Serotonin Syndrome

CYP Enzyme Interactions

Anticholinergic Burden

• TCAs, first-generation antihistamines, antipsychotics, oxybutynin, some anti-Parkinson drugs, diphenhydramine
• Use Anticholinergic Cognitive Burden (ACB) scale in practice

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10. CNS Drug Toxicity and Monitoring Summary

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11. Special Populations

Elderly Patients

• Reduced hepatic CYP activity and renal clearance → drug accumulation
• Increased sensitivity to CNS depressants (benzodiazepines, opioids)
• Beers Criteria: Avoid benzodiazepines, anticholinergics, first-generation antihistamines, TCA, meperidine
• Falls risk from sedatives, antiepileptics, antipsychotics, antidepressants

Pregnancy

• Avoid valproate in first trimester (neural tube defects risk)
• Phenytoin: fetal hydantoin syndrome
• All AEDs: supplement with folate; maintain seizure control (uncontrolled seizures also harm fetus)
• Magnesium sulfate: drug of choice for eclamptic seizures

Renal Impairment

• Reduce dose of renally cleared drugs: levetiracetam (dose by CrCl), gabapentin, pregabalin, memantine, topiramate, lithium
• Phenytoin and valproate: hepatically metabolized, generally safe in renal failure

Hepatic Impairment

• Use caution with valproate (hepatotoxicity risk), benzodiazepines, opioids (reduced first-pass)
• Prefer renally cleared drugs like levetiracetam

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12. Neuromuscular Diseases

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13. Key Clinical Pharmacist Counseling Points

1. Never stop antiepileptics abruptly - withdrawal seizures risk
2. Medication overuse headache: Counsel patients on analgesic frequency limits
3. Driving restrictions for patients with active seizures or those starting sedating medications
4. Hormonal contraception failures with enzyme-inducing AEDs - recommend barrier methods or LARC
5. Opioid laxative co-prescription - always prescribe stimulant laxative with regular opioids
6. MAOI washout periods: 14 days before starting serotonergic drugs; 5 weeks after fluoxetine before starting MAOI
7. Naloxone prescribing: Co-prescribe with patients on high-dose opioids or at overdose risk
8. Drug-induced movement disorders: Screen for EPS/tardive dyskinesia in patients on antipsychotics or metoclopramide
9. HLA-B*1502 testing before carbamazepine in patients of South/Southeast Asian ancestry (Stevens-Johnson syndrome risk)
10. Free phenytoin levels in hypoalbuminemia or renal failure patients (phenytoin is highly protein-bound)

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• Katzung's Basic and Clinical Pharmacology, 16th Edition - Chapter 24 (Antiseizure) and Chapter 28 (Parkinson's)
• Goodman & Gilman's The Pharmacological Basis of Therapeutics - Blocks 6 and 7
• Stahl's Essential Psychopharmacology - Chapters 1-2 (neurotransmitters and drug targets)
• Adams & Victor's Principles of Neurology, 12th Edition
• Bradley & Daroff's Neurology in Clinical Practice