• Yuewei Wu-Fienberg, Hossein Ansari, Shawn Zardouz, Samer Narouze, Taryn Blaha, Marco Swanson, and Ali Totonchi.
• Department of Neurology, University of California, San Diego, San Diego, CA.
• Reg Anesth Pain Med. 2018 Nov 1; 43 (8): 869-874.

Background And ObjectivesWhile existing studies about onabotulinumtoxinA for chronic migraines have focused on injection location and appropriate dosing, little consideration has been given to patient body habitus and its potential impact on efficacy. We hypothesized that with increasing patient body mass index (BMI) there would be more subcutaneous fat separating targeted muscle groups from the skin surface, such that standard 0.5-inch needles used in existing protocols may not allow intramuscular injection. This may have implications for treatment planning.MethodsAnatomically normal computed tomography scans of the head, neck, and face were randomly selected. Subjects were stratified into 4 groups based on BMI, with 30 patients in each group. Four standardized locations were chosen to obtain measurements from the skin surface to the underlying muscle fascia, including (1) frontalis, (2) temporalis, (3) semispinalis capitis, and (4) trapezius.ResultsMedian depth for the temporalis was 12.65 mm (Q1 = 9.32 mm, Q3 = 15.08 mm) for the BMI greater than 35 kg/m group. Median depth for the semispinalis capitis was 13.77 mm (Q1 = 10.3 mm, Q3 = 15.7 mm) for the BMI 30 to 35 kg/m group, and 14.75 mm (Q1 = 11.00, Q3 = 17.00 mm) for the BMI greater than 35 kg/m group. Median depth for the trapezius was 13.95 mm (Q1 = 10.18 mm, Q3 = 19.00 mm) for the BMI greater than 35 kg/m group. These medians exceeded the length of the standard 0.5-inch (12.-mm) needle used in existing protocols.ConclusionsOur study demonstrates that with increasing BMI there is a greater distance between the skin surface and the muscle fascia of muscles that are targeted for injection in standard chronic migraine botulinum toxin injection protocols. Because of this, patient body habitus may be an important factor in injection technique.

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