The crystal structure of lacosamide form I has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques (density functional theory). Lacosamide form I crystallizes in space group P21 (#4) with a = 10.677 73(5), b = 4.799 68(2), c = 13.639 16(9) Å, β = 91.6331(10)̊, V = 698.719(6) Å3, and Z = 2. Van der Waals interactions are important in the crystal structure. Two N–H···O hydrogen bonds form C1,1(4) chains along the b-axis. Several weaker C–H···O hydrogen bonds to the ketone oxygens also contribute to the packing energy. These C–H···O extend both along the b-axis and in the ac-plane, and help link the molecules in three dimensions. The powder pattern has been submitted to International Centre for Diffraction Data for inclusion in the Powder Diffraction File™.

Antiepileptic medications have been reported to cause disturbances in cardiac conduction. Lacosamide decreases seizure burden by modulating sodium channels. Although it has been demonstrated to have few side effects, there have been reports of clinically significant cardiac conduction disturbances. We report the case of a child with hypoplastic left-heart syndrome and well-controlled multifocal atrial tachycardia who developed haemodynamically significant atrial tachycardia after receiving two doses of lacosamide.