10.2 Pediatric Epilepsy Syndromes & Maintenance Anticonvulsants

10.2 Pediatric Epilepsy Syndromes & Maintenance Anticonvulsants

Pediatric epilepsy represents an extraordinarily heterogeneous collection of distinct neurodevelopmental syndromes. Tailoring pharmacotherapy requires aligning specific electroclinical syndromes with drug mechanisms of action, navigating therapeutic drug monitoring ranges, mitigating idiosyncratic and black-box toxicities, and maintaining strict excipient oversight in specialized dietary therapies.

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Major Pediatric Epilepsy Syndromes

Clinical Spectrum of Pediatric Epilepsy Syndromes:

┌────────────────────────┐      ┌────────────────────────┐      ┌────────────────────────┐
│   INFANTILE SPASMS     │      │    ABSENCE EPILEPSY    │      │  LENNOX-GAUSTAUT (LGS) │
│    (West Syndrome)     │      │   (Childhood Absence)  │      │  (Epileptic Encephal.) │
├────────────────────────┤      ├────────────────────────┤      ├────────────────────────┤
│ • Age: 3 - 12 months   │      │ • Age: 4 - 10 years    │      │ • Age: 2 - 8 years     │
│ • Spasms in clusters   │      │ • 5-15s vacant stares  │      │ • Multiple seizure tx: │
│ • EEG: Hypsarrhythmia  │      │ • EEG: 3-Hz spike/wave │      │   atonic drops, tonic  │
│ • Psychomotor arrest   │      │ • Provoked by hypervent│      │ • EEG: Slow spike-wave │
│ • Rx: ACTH, Vigabatrin │      │ • Rx: Ethosuximide     │      │ • Rx: Clobazam, CBD,   │
│   (Vigabatrin for TSC) │      │   (Valproate alt.)     │      │   Fenfluramine, Rufin. │
└────────────────────────┘      └────────────────────────┘      └────────────────────────┘

Infantile Spasms (West Syndrome)

Infantile spasms is a severe epileptic encephalopathy of early infancy characterized by an electroclinical triad:

1. Epileptic Spasms: Brief (1 to 2 second) axial contractions occurring in clusters (often 20 to 100+ spasms upon waking), presenting as flexor, extensor, or mixed "jackknife" spasms.
2. Hypsarrhythmia: Interictal EEG demonstrating a chaotic, disorganized, high-voltage background with continuous multifocal spike and slow-wave discharges.
3. Psychomotor Regression: Loss of previously acquired developmental milestones (e.g., visual tracking, rolling, social smile).

First-Line Pharmacotherapeutic Regimens

• Adrenocorticotropic Hormone (ACTH): High-dose natural or synthetic repository corticotropin injection (150 IU/m2/day IM divided BID for 2 weeks, followed by a gradual taper over 2 to 3 weeks). Mechanism involves suppression of endogenous corticotropin-releasing hormone (CRH), an excitatory neuropeptide that triggers neuronal hyperexcitability. Clinical response (cessation of spasms and resolution of hypsarrhythmia) is monitored via 24-hour video-EEG at day 14. Adverse effects are severe: profound secondary hypertension (requires baseline and twice-weekly blood pressure checks), hypertrophic cardiomyopathy (mandates baseline and follow-up echocardiograms), severe immunosuppression, hypokalemia, hyperglycemia, and gastrointestinal ulceration.
• Vigabatrin (Sabril): Synthetic structural analog of GABA that acts as an irreversible, suicide inhibitor of GABA-transaminase (GABA-T), the enzyme responsible for GABA catabolism. This irreversibly elevates intracellular and synaptosomal GABA concentrations in cerebral tissue.
  • Dosing: Initiate at 50 mg/kg/day orally divided BID; titrate over 7 to 10 days to 100 to 150 mg/kg/day.
  • Tuberous Sclerosis Complex (TSC): In infantile spasms secondary to TSC (caused by TSC1 or TSC2 mutations hyperactivating the mTOR pathway), vigabatrin is the definitive first-line agent of choice, achieving spasm cessation and hypsarrhythmia resolution in >90% of patients (significantly superior to ACTH).
  • Black Box Warning & SHARE REMS Program: Vigabatrin causes permanent, bilateral, concentric peripheral visual field constriction in 30% to 40% of patients, resulting in severe "tunnel vision." Central visual acuity is preserved. All patients must be registered in the Vigabatrin REMS Program. Baseline visual assessment (optical coherence tomography [OCT], electroretinography [ERG], or visual evoked potentials) is required within 4 weeks of initiation, repeated every 3 months during therapy, and performed 3 to 6 months post-discontinuation. If clinical spasm cessation is not documented within 12 to 14 weeks of therapy, vigabatrin must be discontinued.

Childhood Absence Epilepsy (CAE)

• Clinical Presentation: Onset typically between ages 4 and 10 years in structurally normal children. Manifests as sudden, brief (5 to 15 seconds) behavioral arrest, unresponsiveness, blank facial staring, and subtle eyelid fluttering, occurring dozens to hundreds of times per day. Spells can be provoked in the clinic via hyperventilation (blowing on a pinwheel for 3 minutes). Postictal confusion is completely absent; the child immediately resumes prior activity.
• Pathognomonic EEG: Synchronous, symmetrical, generalized 3-Hz (cycles per second) spike-and-wave discharges originating from thalamocortical oscillatory circuits.
• First-Line Pharmacotherapy:
  • Ethosuximide (Zarontin): The undisputed drug of choice. Selectively blocks low-threshold T-type voltage-gated calcium channels in thalamic relay neurons, breaking the synchronizing burst pacemaker responsible for the 3-Hz spike-and-wave discharge.
  • Dosing & TDM: Initiate at 15 to 20 mg/kg/day orally divided BID; titrate by 5 mg/kg/day weekly to a target dose of 20 to 40 mg/kg/day (max 1500 mg/day). Therapeutic serum trough concentration: 40 to 100 mcg/mL.
  • Ethosuximide vs. Valproate: The NIH-funded landmark Childhood Absence Epilepsy trial established that while ethosuximide and valproate demonstrated equivalent seizure freedom rates (~55% vs 54% at 16 weeks), ethosuximide had significantly lower rates of attentional and cognitive adverse effects (33% vs 49%), establishing ethosuximide as the preferred first-line agent. Valproic acid is reserved for absence epilepsy associated with concurrent generalized tonic-clonic convulsions.

Lennox-Gastaut Syndrome (LGS)

LGS is a catastrophic epileptic encephalopathy accounting for 1% to 4% of childhood epilepsies, defined by an invariant triad:

1. Multiple Seizure Types: Atonic drop attacks ("astatic seizures" causing facial trauma), tonic seizures (particularly nocturnal), and atypical absence.
2. Slow Spike-and-Wave EEG: Interictal discharges slower than <2.5 Hz on waking EEG, with paroxysmal fast activity (10–20 Hz) during non-REM sleep.
3. Severe Cognitive Impairment: Progressive neurodevelopmental stagnation and intellectual disability.

Specialized Add-On Pharmacotherapy for LGS

• Clobazam (Onfi): A 1,5-benzodiazepine that functions as a positive allosteric modulator of GABA-A receptors. Unlike traditional 1,4-benzodiazepines (diazepam, clonazepam), clobazam displays preferential binding to α2-subunit-containing GABA-A complexes, resulting in potent anticonvulsant activity with markedly reduced sedation and psychomotor impairment. Metabolized by CYP3A4 to N-desmethylclobazam, which is cleared by CYP2C19. In CYP2C19 poor metabolizers, active metabolite levels surge up to 5-fold.
• Cannabidiol (Epidiolex): Plant-derived, purified oral cannabidiol (100 mg/mL oral solution in sesame oil). Exerts anticonvulsant activity independently of cannabinoid CB1/CB2 receptors, acting via antagonism of G-protein-coupled receptor 55 (GPR55) and desensitization of TRPV1 channels.
  • Dosing: Initiate at 5 mg/kg/day divided BID; titrate weekly to 10 to 20 mg/kg/day.
  • Drug-Drug Interactions & Hepatic Toxicity: Co-administration of cannabidiol with valproate sodium exponentially increases the incidence of severe serum transaminase elevations (ALT/AST >3× ULN in up to 30% of patients). Baseline serum transaminases and total bilirubin must be obtained prior to initiation, and monitored at 1, 3, and 6 months. Furthermore, cannabidiol inhibits CYP2C19, increasing active N-desmethylclobazam plasma concentrations three-fold, causing severe somnolence.
• Fenfluramine (Fintepla): A serotonin releasing agent and 5-HT2 receptor agonist that enhances GABAergic transmission and inhibits sigma-1 receptors. Approved for LGS and Dravet syndrome.
  • Dosing: 0.2 to 0.7 mg/kg/day divided BID (maximum 26 mg/day; maximum 17 mg/day if co-administered with stiripentol).
  • REMS Program: Due to historical associations with fenfluramine-phentermine ("Fen-Phen") causing cardiac valvulopathy and pulmonary arterial hypertension, fenfluramine is dispensed exclusively under a restricted REMS program. Comprehensive baseline transthoracic echocardiograms are mandatory, repeated every 6 months during therapy, and performed 3 to 6 months post-discontinuation.
• Rufinamide (Banzel): Prolongs the inactive state of voltage-gated sodium channels. Specifically FDA-approved for atonic drop attacks in LGS. Causes shortening of the QTc interval; contraindicated in patients with congenital short QT syndrome.

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Maintenance Antiseizure Medications: TDM & Toxicities

Key Maintenance Antiseizure Medications (ASMs) Summary:

┌─────────────────┬──────────────────┬───────────────────────────────────────────┐
│ Drug & Target   │ Target Trough    │ High-Yield Clinical Pearls & Toxicities   │
├─────────────────┼──────────────────┼───────────────────────────────────────────┤
│ Valproic Acid   │ 50 - 100 mcg/mL  │ • Fatal hepatotoxicity (<2 yr with POLG)  │
│                 │                  │ • Acute pancreatitis; Teratogenicity      │
│                 │                  │ • Hyperammonemic encephalopathy (Carnit.) │
├─────────────────┼──────────────────┼───────────────────────────────────────────┤
│ Carbamazepine   │ 4 - 12 mcg/mL    │ • CYP3A4 autoinduction (t1/2 drops at 3wk)│
│                 │                  │ • HLA-B*1502 screening for SJS/TEN        │
│                 │                  │ • SIADH / Hyponatremia; Epoxide toxicity  │
├─────────────────┼──────────────────┼───────────────────────────────────────────┤
│ Lamotrigine     │ 3 - 14 mcg/mL    │ • Black box SJS/TEN: Slow 6-8 wk titration│
│                 │                  │ • Valproate cuts clearance 50% (half dose)│
│                 │                  │ • Inducers double clearance (double dose) │
├─────────────────┼──────────────────┼───────────────────────────────────────────┤
│ Topiramate      │ 5 - 20 mcg/mL    │ • Carbonic anhydrase inhibition:          │
│                 │                  │   Oligohidrosis, hyperthermia, stones     │
│                 │                  │ • Metabolic acidosis; Cognitive slowing   │
├─────────────────┼──────────────────┼───────────────────────────────────────────┤
│ Levetiracetam   │ 12 - 46 mcg/mL   │ • 66% renal clearance; no CYP interactions│
│                 │                  │ • Behavioral agitation (Give Pyridoxine)  │
└─────────────────┴──────────────────┴───────────────────────────────────────────┘

Valproic Acid / Divalproex Sodium

• Mechanisms: Increases cerebral GABA by inhibiting GABA-transaminase and succinic semialdehyde dehydrogenase; prolongs sodium channel inactivation; inhibits T-type calcium currents.
• Therapeutic Serum Concentration: 50 to 100 mcg/mL (up to 125 mcg/mL in refractory epilepsy).
• Black Box Warnings:
  1. Fatal Hepatotoxicity: Highest risk in children under 2 years of age receiving polytherapy, particularly those with underlying inborn errors of metabolism or mutations in the nuclear gene encoding mitochondrial DNA polymerase gamma (POLG, causing Alpers-Huttenlocher syndrome). POLG genetic testing is mandatory before prescribing valproate in suspected infants.
  2. Pancreatitis: Life-threatening hemorrhagic or necrotizing pancreatitis occurring unpredictably at any point in therapy, independent of dose or serum concentration.
  3. Teratogenicity: Major congenital malformations, specifically open neural tube defects (spina bifida, 1–2% risk) and permanent reductions in cognitive IQ scores.
• Hyperammonemic Encephalopathy: Valproate inhibits carbamoyl phosphate synthetase I (CPS I), the rate-limiting enzyme of the urea cycle, while depleting carnitine stores. Patients present with acute lethargy, vomiting, cognitive worsening, and asterixis in the setting of completely normal serum AST/ALT and bilirubin levels. Treat with intravenous or oral L-carnitine (50 to 100 mg/kg/day).
• Other Toxicities: Dose-dependent thrombocytopenia (troughs >100 mcg/mL), platelet dysfunction, alopecia, significant weight gain, insulin resistance, and tremor.

Carbamazepine & Oxcarbazepine

• Mechanisms: Voltage-gated sodium channel blockers, stabilizing the inactive channel conformation.
• Therapeutic Serum Concentration (Carbamazepine): 4 to 12 mcg/mL.
• Hepatic Autoinduction: Carbamazepine is a potent substrate and inducer of CYP3A4. Upon therapy initiation, it induces its own hepatic metabolism. Its elimination half-life drops precipitously from 30 to 40 hours initially down to 10 to 20 hours after 2 to 4 weeks of continuous therapy. Maintenance doses must be proactively uptitrated at weeks 2 to 3 to prevent subtherapeutic breakthrough seizures.
• Pharmacogenomics:
  • Patients of Asian ancestry must be screened for the HLA-B1502* allele prior to starting carbamazepine. Presence of this allele confers an odds ratio >100 for developing fatal Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN).
  • The HLA-A3101* allele is prevalent in European and Japanese populations and predicts carbamazepine-induced DRESS syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms).
• Water Retention & SIADH: Carbamazepine and oxcarbazepine sensitize the renal collecting tubule to vasopressin (ADH), causing hyponatremia. Hyponatremia is significantly more frequent with oxcarbazepine (up to 25% of patients) than carbamazepine.

Lamotrigine

• Mechanism: Voltage-gated sodium channel inactivation and inhibition of presynaptic glutamate and aspartate exocytosis.
• Black Box Warning & Strict Titration: Carries a boxed warning for severe cutaneous adverse reactions, including SJS/TEN. The risk of toxic rash is directly correlated with a rapid initial titration rate or exceeding recommended starting doses. Titration must proceed slowly over 6 to 8 weeks.
• Critical Drug-Drug Interactions:
  • Concurrent Valproic Acid: Valproate is a potent inhibitor of UGT1A4-mediated glucuronidation. Co-administration doubles the elimination half-life of lamotrigine (from 25–30 hours to 60–70 hours) and reduces clearance by 50%. The initial lamotrigine starting dose must be reduced by 50% (start at 0.15 mg/kg/day for weeks 1–2), and titration increments must be halved.
  • Concurrent Enzyme Inducers (Carbamazepine, Phenytoin, Phenobarbital): Inducers double lamotrigine clearance. The starting dose must be doubled (start at 0.6 mg/kg/day for weeks 1–2).

Topiramate

• Mechanisms: Multi-modal action: blocks voltage-gated sodium channels, potentiates GABA-A receptor activity, antagonizes AMPA/kainate glutamate receptors, and functions as a weak carbonic anhydrase inhibitor (isozymes II and IV).
• Therapeutic Serum Concentration: 5 to 20 mcg/mL.
• Carbonic Anhydrase Inhibition Toxicities:
  1. Hypohidrosis / Oligohidrosis & Hyperthermia: Reduced sweat production prevents thermoregulation, causing severe hyperthermia, heat stroke, and hospitalization during warm weather or exercise. Pediatric patients and caregivers must be counseled to avoid hot environments and maintain vigorous hydration.
  2. Non-Anion Gap Metabolic Acidosis: Renal proximal tubular carbonic anhydrase inhibition causes bicarbonate wasting and metabolic acidosis (decreased serum HCO3- down to 14–18 mEq/L).
  3. Nephrolithiasis: Alkaline urine combined with hypocitraturia promotes calcium phosphate renal stone formation (1.5% incidence).
• Cognitive & Weight Effects: Pronounced psychomotor slowing, word-finding difficulty ("Dopamax"), memory impairment, anorexia, and weight loss.

Levetiracetam

• Clinical Profile: Exceptional broad-spectrum efficacy across focal and generalized epilepsies, zero CYP450 metabolism, and 66% unchanged renal clearance.
• Behavioral Toxicity: Induces severe neuropsychiatric adverse effects in 10% to 15% of pediatric patients, manifested by irritability, emotional lability, aggressive outbursts, oppositionality, and depression.
• Clinical Practice Pearl: Co-administration of oral pyridoxine (vitamin B6) at 1 to 5 mg/kg/day (or 50 to 100 mg daily) successfully reverses or prevents levetiracetam-induced behavioral agitation and irritability in approximately 50% to 60% of affected children, avoiding the need to discontinue an otherwise efficacious anticonvulsant.

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The Ketogenic Diet in Pediatric Epilepsy

Ketogenic Diet Macronutrient Ratio (4:1 Diet):

┌────────────────────────────────────────────────────────┬─────────────┐
│                     FAT: 80% (4 grams)                 │  CHO + PRO  │
│               (Heavy cream, butter, oils)              │ 20% (1 gram)│
└────────────────────────────────────────────────────────┴─────────────┘

The ketogenic diet (KD) is a non-pharmacologic, high-fat, low-carbohydrate, adequate-protein dietary therapy utilized for refractory pediatric epilepsy, achieving a >50% seizure reduction in over half of patients and complete seizure freedom in 10% to 15%. It is the treatment of choice for Glucose Transporter Type 1 (GLUT1) Deficiency Syndrome and Pyruvate Dehydrogenase Deficiency (PDHD).

• Mechanisms: The diet forces hepatic β-oxidation of fatty acids into ketone bodies (acetoacetate, β-hydroxybutyrate, acetone). Ketones cross the blood-brain barrier via monocarboxylate transporters, providing an alternative fuel source that stimulates mitochondrial biogenesis, enhances GABA synthesis via altered glutamate transamination, and directly inhibits presynaptic vesicular glutamate transporter (VGLUT).
• Dietary Ratios: The classic ketogenic diet utilizes a 4:1 or 3:1 ratio of fat grams to combined non-fat grams (carbohydrate grams + protein grams). Target ketosis requires maintaining serum β-hydroxybutyrate levels between 3.0 and 5.0 mmol/L.
• The Critical Clinical Pharmacist Mandate:
  • Zero-Tolerance Carbohydrate Rule: The consumption of even small quantities of extraneous carbohydrates triggers an immediate insulin surge, halting hepatic ketogenesis. Serum β-hydroxybutyrate levels plummet within minutes, precipitating catastrophic breakthrough status epilepticus.
  • Medication Formulation Audits: Clinical pharmacists must systematically audit every medication, nutritional supplement, and intravenous fluid. Liquid oral dosage forms (syrups, suspensions, elixirs) are strictly prohibited because commercial vehicles are heavily formulated with sucrose, sorbitol, glycerin, maltitol, or propylene glycol.
  • Management Strategies: Convert all medications to non-coated whole tablets (crushed in water or mixed with butter/heavy cream), hard gelatin capsule contents, or extemporaneously compounded carbohydrate-free suspensions using pure microcrystalline cellulose or Ora-Sweet SF / SyrSpend.
  • Intravenous Fluids: Never administer dextrose-containing intravenous maintenance fluids (D5W, D5 0.45% NS); utilize Normal Saline (0.9% NaCl) or Lactated Ringer's.
• Dietary Complications: Nephrolithiasis (mitigated by oral potassium citrate to alkalinize urine), dyslipidemia, prolonged QTc interval, gastroesophageal reflux, constipation, osteopenia, and impaired linear growth.

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Practice Pearls & BCPPS Exam Traps

• Exam Trap 1: In a child receiving lamotrigine who is started on valproic acid, you must cut the lamotrigine dose in half immediately; failing to do so precipitates Stevens-Johnson Syndrome.
• Exam Trap 2: An asymptomatic child on topiramate who presents with a serum bicarbonate of 17 mEq/L does not have an acute infection or lactic acidosis; this is expected, drug-induced type 2 proximal renal tubular bicarbonate wasting via carbonic anhydrase inhibition.
• Board Rule: Never prescribe carbamazepine to a patient of Asian descent without documenting a negative HLA-B1502* test result.
• Exam Trap 3: Never give liquid acetaminophen or ibuprofen elixir to a patient on the ketogenic diet; even a single 5 mL dose containing 2 grams of sorbitol will completely terminate neuroprotective ketosis and provoke breakthrough seizures.