Unusual case of topiramate-induced auditory-visual synaesthesia in a non-synaesthete young adult: a case report.

Background: Synaesthesia is a perceptual phenomenon where stimulation of one sensory modality leads to automatic experiences in another. While often congenital, acquired forms-particularly pharmacologically induced synaesthesia-are rare and poorly understood. Topiramate, an antiepileptic and migraine prophylactic agent, affects neuronal excitability and sensory integration, but its association with synaesthesia has rarely been documented. Case Description: We report the case of a 28-year-old male software engineer with no prior neurological or psychiatric history who developed auditory-visual synaesthesia two weeks after initiating topiramate for migraine prophylaxis. The patient consistently visualised specific colours in response to particular environmental sounds such as phone rings and rainfall. Clinical examination, neurological evaluation, and comprehensive diagnostic workup-including brain magnetic resonance imaging (MRI), audiological, and ophthalmological assessments-were normal, excluding structural lesions and primary sensory abnormalities. The symptoms, although non-distressing, significantly intrigued the patient, prompting medical consultation. Symptoms resolved gradually over eight weeks following discontinuation of topiramate, supporting a diagnosis of drug-induced synaesthesia. The patient was subsequently started on propranolol for migraine management, which was well-tolerated, effectively controlled migraines, and did not trigger any recurrence of synaesthetic experiences. Conclusions: This case highlights a rare but clinically relevant side effect of topiramate and underscores the importance of thorough medication history in evaluating atypical sensory experiences. Primary care clinicians should consider pharmacological causes in sudden-onset synaesthesia, ensuring prompt identification, appropriate referral, and alternative treatment planning.