A 38-year-old patient with epilepsy on long-term phenytoin reports new horizontal nystagmus on lateral gaze and an unsteady, wide-based gait. The most recent serum phenytoin level is 24 mcg/mL (therapeutic 10–20 mcg/mL). The albumin level is normal. Which is the nurse's priority interpretation and action?

A serum phenytoin level of 24 mcg/mL is above the therapeutic range of 10–20 mcg/mL. Horizontal nystagmus on lateral gaze and ataxic, wide-based gait are classic early signs of phenytoin toxicity, which appear in a predictable dose-dependent sequence: nystagmus (>20 mcg/mL) → ataxia (>30 mcg/mL) → drowsiness/confusion (>40 mcg/mL) → coma (>50 mcg/mL). Priority action is to hold the next dose and notify the provider for level confirmation and dose adjustment. Choice 1 is wrong because the level is above normal. Choice 3 misattributes drug toxicity to stroke; nystagmus and ataxia from phenytoin do not present as a stroke alert. Choice 4 confuses gingival hyperplasia (a long-term cosmetic side effect requiring dental hygiene) with the acute neurological signs of toxicity.

In-depth explanation

Phenytoin has a narrow therapeutic index and nonlinear (Michaelis-Menten) kinetics — small dose increases can produce disproportionate level jumps. Toxicity signs follow a dose-dependent ladder: nystagmus at slightly elevated levels (>20 mcg/mL), ataxia at >30, drowsiness and confusion at >40, and seizures or coma at very high levels (paradoxical because the drug is an anticonvulsant). Long-term cosmetic side effects (gingival hyperplasia, hirsutism, coarsening of facial features) and other concerns (osteoporosis, megaloblastic anemia from folate depletion, drug interactions through CYP induction) are real but separate from acute toxicity. Albumin is checked because phenytoin is highly protein-bound; in hypoalbuminemia, total level may underestimate free (active) drug — the corrected formula is: Corrected = measured / (0.2 × albumin + 0.1).

Clinical scenario

<p>A <strong>38-year-old patient</strong> with <strong>epilepsy on long-term phenytoin</strong> reports <strong>new horizontal nystagmus on lateral gaze</strong> and an <strong>unsteady, wide-based gait</strong>. The most recent <strong>serum phenytoin level is 24 mcg/mL</strong> (therapeutic 10–20 mcg/mL). The <strong>albumin level is normal</strong>.</p>

Key concepts

• Phenytoin Therapeutic Window — Total serum 10–20 mcg/mL (free 1–2 mcg/mL). Narrow therapeutic index with dose-dependent toxicity ladder: nystagmus (>20), ataxia (>30), confusion/drowsiness (>40), coma/paradoxical seizures (>50). Albumin correction is required when albumin is low.
• Phenytoin Nonlinear Kinetics — Phenytoin metabolism saturates at therapeutic doses, producing Michaelis-Menten (nonlinear) pharmacokinetics. A small dose increase at the upper end of therapeutic range can produce a disproportionately large rise in serum level. This makes individualized dose titration and serial levels essential.
• Gingival Hyperplasia — A chronic cosmetic side effect of long-term phenytoin (≈20% of patients). Mitigated by meticulous oral hygiene, regular dental cleaning, and avoiding mucosal trauma. Distinct from the acute neurological signs of toxicity addressed in this scenario.