The popularity of Botulinum Toxin Type A (BTX-A) injections for facial rejuvenation is undeniable. According to the latest statistics from the International Society of Aesthetic Plastic Surgery (ISAPS), over 8.8 million botulinum toxin procedures are performed worldwide every year.

Although its primary neuromuscular mechanism—temporarily relaxing target muscle fibers to smooth wrinkles—is extensively documented by pioneers such as Carruthers & Carruthers, its direct influence on local blood vessels and regional microcirculation has traditionally taken a backseat. Understanding these local tissue reactions is crucial, as improper placement can lead to highly distressing complications for the patient, such as severe facial asymmetry, diffusion anomalies, or localized persistent edema.

An innovative longitudinal study published by Kytko et al. (2026) in the journal Life shifts this paradigm. Utilizing dynamic infrared thermography (dIRT) as a functional imaging modality, the researchers successfully mapped the exact temporal evolution of the skin temperature regime following aesthetic botulinum toxin therapy on the forehead.

The Methodological Approach: Standardization of Facial ROIs

To objectively track microcirculatory variations without relying on complex and cumbersome diagnostic tools such as lasers, the research team conducted a highly structured prospective study involving 126 female patients (mean age: 34.4 ± 1.2 years).

Under strictly controlled environmental conditions (ambient temperature of 22 °C to 24 °C and 40–60% humidity), absolute skin temperature (Tsk) values were recorded using high-precision thermal imaging across 9 standardized Regions of Interest (ROIs), or control points (P1 to P9):

• Points P1–P4 (Upper Frontalis Muscle Row): Located 2 cm above the inferior border of the frontal region.
• Points P5–P6 (Lower Frontalis Muscle Row): Positioned 1 cm above the superciliary arches.
• Points P7–P9 (Glabellar Region): Target zones overlying the procerus muscle (P7) and the corrugator supercilii muscle (P8, P9).

Thermal profiles were captured longitudinally across five distinct stages to monitor both immediate vascular reflexes and long-term homeostatic changes: baseline (T0), immediately post-procedure (T1), 30 minutes post-procedure (T2), 14 days (T14), and 30 days (T30).

Results: Unveiling a Novel Triphasic Vasomotor Response

The most significant finding of the study by Kytko et al. was the discovery of a distinctive triphasic thermal response pattern on the facial skin surface following injections:

• Phase 1: Immediate Microvascular Spasm (T1): Immediately following needle insertion, a statistically significant decrease in skin temperature was observed across all points (-1.7% to -4.8% relative to baseline, $p < 0.001$). This rapid cooling reflects a localized vasoconstrictive response triggered by mechanical trauma and acute autonomic sympathetic activation (Kirillova-Woytke et al., 2014).
• Phase 2: Compensatory Reactive Hyperemia (T2): At 30 minutes, the initial vasoconstriction was entirely replaced by a prominent hyperthermic reaction, demonstrating a marked increase of +2.35% to +5.8% in skin temperature (particularly visible at points P1, P2, and P4–P7). This indicates profound local vasodilation and increased tissue blood flow.
• Phase 3: Long-Term Homeostatic Stabilization (T14–T30): By days 14 and 30, as the muscle-relaxing effect became clinically established, acute inflammatory and vasomotor signals subsided, establishing a new balanced microcirculatory homeostatic baseline.

Anatomical Variability and the Glabellar Peak

Interestingly, the intensity of these phases depended heavily on the topography of the treated facial muscles. The upper frontalis muscle row (P1–P4) exhibited brief and moderate cooling followed by rapid stabilization. In contrast, the interbrow points (P7–P9) experienced the most pronounced and sustained thermographic alterations.

This behavior is explained by key physiological and anatomical characteristics:

• Angioarchitecture: The glabellar region features a highly dense capillary network where the supratrochlear, supraorbital, and angular arteries intersect, making it highly reactive to trauma and vasoactive substances.
• Tissue Modulators: The lower third of the forehead possesses a thicker layer of Subcutaneous Adipose Tissue (SAT) than the upper zones. As confirmed by previous findings from the pilot study (Kytko et al., 2025), a pronounced SAT layer effectively acts as a thermal insulator, "retaining" the heat generated by deeper tissue layers and accentuating the thermographic signal on the skin surface.
• Muscle Topography: Needle penetration into the depressor muscles depends heavily on individual muscle fiber patterns. According to the classification by Raveendran & Anthony (2020), a proportion of individuals exhibit aponeurotic tissue instead of muscle mass in the central forehead region. Therefore, the absence of the expected hyperthermic "peak" at points P7–P9 can serve as an objective warning of inadequate injection depth or an anatomical variation lacking target muscle fibers.

Practical Applications for the Advanced Aesthetic Clinician

The integration of infrared thermography into the clinical routine of aesthetic medicine provides multilevel benefits that extend far beyond standard visual documentation:

• Objective Quality Control: The hyperthermic peak acts as an indirect physiological marker of a successful injection into the deep depressor muscles.
• Patient Safety and Personalization: Pre-procedural analysis of facial blood flow profiles allows specialists to anticipate individual vascular traits and adjust dosages, thereby minimizing the incidence of adverse effects such as asymmetric muscle relaxation or diffusion complications (Freixo & Camões-Barbosa, 2026).
• Holistic Patient Monitoring: Combining objective functional data (thermal profiles) with subjective improvements in quality of life—measured in this study using the SF-36 questionnaire (Ware & Sherbourne, 1992)—demonstrates that skin temperature changes accurately reflect a comprehensive and healthy tissue response to treatment.

Conclusions

Dynamic infrared thermography successfully visualizes the intricate vascular variations induced by aesthetic botulinum toxin injections, which are driven by individual anatomy. Although further clinical trials are necessary to determine whether dIRT can directly predict severe adverse events before they manifest clinically, this study establishes thermal imaging as an indispensable, non-invasive tool for objective guidance and treatment personalization in contemporary aesthetic medicine.

References

• Carruthers, J. D., & Carruthers, J. A. (1992). Treatment of glabellar frown lines with C. botulinum-A exotoxin. Journal of Dermatolology, Surgery and Oncology, 18(1), 17–21.
• Freixo, S., & Camões-Barbosa, A. (2026). Management of Complications Following Botulinum Toxin Facial Injections: A Narrative Review. Cureus, 18, e100675.
• Kirillova-Woytke, I., Baron, R., & Jänig, W. (2014). Reflex inhibition of cutaneous and muscle vasoconstrictor neurons during stimulation of cutaneous and muscle nociceptors. Journal of Neurophysiology, 111(9), 1833–1845.
• Kytko, O., Vasil’ev, Y., Emelyanova, E., Kutin, E., Sarmadian, R., Trofimova, S., Kondrina, I., Moiseenko, A., Dydykin, S., & Rebrova, E. (2025). Evaluation of the Effectiveness of Botulinum Therapy Based on the Anthropometric Characteristics of the Face Using Non-Invasive Thermal Imaging Data. Diagnostics, 15(18), 2519.
• Kytko, O., Emelyanova, E., Kutin, E., Enina, Y., Troitskiy, V., Dydykin, S., Moiseenko, A., Popova, A., & Vasil’ev, Y. (2026). Skin Temperature as a Marker of Vasomotor Response and Restoration of Blood Flow After Aesthetic Botulinum Toxin Therapy in the Forehead. Life, 16(5), 803.
• Raveendran, S. S., & Anthony, D. J. (2020). Classification and Morphological Variation of the Frontalis Muscle and Implications on the Clinical Practice. Aesthetic Plastic Surgery, 45(1), 164–170.
• Triana, L., Palacios Huatuco, R. M., Campilgio, G., & Liscano, E. (2024). Trends in Surgical and Nonsurgical Aesthetic Procedures: A 14-Year Analysis of the International Society of Aesthetic Plastic Surgery-ISAPS. Aesthetic Plastic Surgery, 48, 4217–4227.
• Ware, J. E., & Sherbourne, C. D. (1992). The MOS 36-item short-form health survey (SF-36). I. Conceptual framework and item selection. Medical Care, 30(6), 473–483