Lamotrigine (LTG) is a commonly used anticonvulsant and mood stabilizer. While generally well-tolerated, lamotrigine toxicity can occur. Presentation of lamotrigine toxicity can be diverse and include seizure, ocular abnormalities, ataxia, and encephalopathy, mimicking other neurological conditions, causing diagnostic challenges. Ocular findings are varied and can include diplopia, downbeat nystagmus, and gaze palsy.
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Case 1: A 52-year-old female with epilepsy and migraine, taking lamotrigine 300mg BID and brivaracetam, presented with breakthrough seizure, encephalopathy, nausea/vomiting, ataxia, and tonic upward gaze. Lamotrigine toxicity was suspected (LTG level: 31.2 ug/mL). Symptoms progressed slowly over nearly 10 months, initially manifesting as worsening balance, dizziness, and falls attributed to subclinical seizures and later diagnosed as vestibular migraine. One month prior to admission, downbeat nystagmus and diplopia developed, prompting a workup for autoimmune encephalitis or structural lesions, all negative.
Case 2: A 56-year-old female with a history of alcohol use disorder, right MCA stroke, and epilepsy, taking LTG 150mg BID, levetiracetam, and gabapentin, presented with breakthrough seizure and encephalopathy two days after an ED visit for an altercation and intoxication. Her examination revealed mydriasis, roving eye movements and a vesicular ear lesion. Initially, viral encephalitis or alcohol-related seizure were suspected. Lamotrigine toxicity was only considered after the family reported finding empty medication bottles (LTG level: 32.5 ug/mL), and eventually admitted to intentional overdose. Inability to ambulate, abnormal eye movements, confusion, and visual hallucinations resolved.
These two cases underscore the importance of considering lamotrigine toxicity in patients presenting with breakthrough seizures, encephalopathy, and ocular abnormalities, even with alternative plausible diagnoses. Lamotrigine toxicity can present acutely or slowly progressive, leading to diagnostic challenges. A thorough medication history and early measurement of lamotrigine levels are crucial for timely diagnosis and management. In the presence of unexplained neurological symptoms and lamotrigine use, toxicity should remain a consideration.