Intramuscular innervation patterns of the splenius capitis and splenius cervicis and their clinical implications for botulinum toxin injections.

INTRODUCTION: The purpose of this study was to clearly characterize the intramuscular nerve distributions of the splenius capitis and splenius cervicis muscles that are both responsible for the onset of a chronic tension type headache and to use this information to identify the effective botulinum toxin (BoNT) injection sites. MATERIALS AND METHODS: Ten splenius capitis and splenius cervicis specimens were subjected to Sihler's staining to reveal intramuscular nerve arborization patterns and determined the optimal location for BoNT injection. RESULTS: Nerve distribution patterns in the splenius capitis were identified as nerve pathways that travel down toward the origin point and others that travel up toward the insertion point. This neuromuscular innervation from the central (50%) point was distributed evenly in these two directions. The neural pathways of splenius cervicis traveled vertically from the insertion point to the origin point. If the length from the muscle origin point to the insertion point is normalized to 100%, motor neurons innervate the muscle from around the 30% to the 70% point. CONCLUSIONS: The safest and most-effective BoNT injection sites for the splenius capitis and splenius cervicis were found at around the 50% point and the 30% to the 70% point, respectively.