Evaluation of Skin-to-Epidural Distance at Different Vertebral Levels by Ultrasonography in Infants: A Prospective Observational Study

Epidural analgesia is an effective method for perioperative pain management in pediatric patients and is widely used to reduce postoperative pain and opioid requirements. In infants, however, epidural procedures may be technically challenging because spinal anatomical structures are smaller and continue to change during early development. Variability in vertebral anatomy, ligament thickness, and spinal cord termination level may increase the difficulty of predicting epidural depth during neuraxial procedures. Even small differences in needle depth may be clinically important in this age group.

Ultrasonography has become an increasingly important tool in pediatric regional anesthesia because it provides real-time visualization of spinal anatomy without radiation exposure or harmful effects on the body. Preprocedural ultrasonographic assessment may help identify anatomical landmarks and estimate epidural depth before needle insertion. Previous studies have demonstrated that ultrasound measurements may correlate with actual epidural needle depth in pediatric patients. However, available data regarding normative skin-to-epidural distance measurements in infants remain limited, particularly at different vertebral levels and with different ultrasound imaging planes.

In infants, identification of the epidural space using conventional loss-of-resistance techniques may be more difficult than in older children and adults because tissue resistance changes may be less distinct. In addition, spinal anatomy changes rapidly during infancy, and measurements obtained at one vertebral level may not reflect measurements at other levels. Evaluation of multiple vertebral levels and imaging planes may therefore provide more comprehensive anatomical information for pediatric epidural procedures.

This study is designed to evaluate ultrasonographic skin-to-epidural distance measurements at multiple lumbar and thoracolumbar vertebral levels in infants undergoing elective surgery under general anesthesia. Measurements will be obtained using two commonly used ultrasound imaging planes to better characterize epidural depth in this population. The study will also evaluate the relationship between ultrasonographic measurements and demographic characteristics such as age, body weight, height, and gestational age.

Ultrasonographic evaluations will be performed during routine perioperative preparation under general anesthesia without additional invasive intervention for study purposes. In participants who undergo epidural catheter placement as part of routine clinical management, ultrasonographic measurements will additionally be compared with clinically measured epidural needle depth. Findings from this study may contribute to the development of more reliable reference data for infant epidural procedures and may support safer and more individualized pediatric regional anesthesia practices.

• Ultrasonographic Assessment of Skin-to-Epidural Distance (Diagnostic test): Infants aged 0 to 12 months undergoing elective surgery under general anesthesia will undergo ultrasonographic assessment of the distance between the skin and the epidural space. Measurements will be performed in the lateral decubitus position using a high-frequency linear ultrasound transducer with minimal probe pressure to avoid soft tissue compression. The distance will be evaluated at the intervertebral levels between the twelfth thoracic vertebra and the first lumbar vertebra, the first and second lumbar vertebrae, the second and third lumbar vertebrae, and the third and fourth lumbar vertebrae. Measurements will be obtained using both the paramedian sagittal oblique and transverse median ultrasound views. Two experienced anesthesiologists will independently perform the measurements. The probe will be removed and repositioned before repeat measurements. No needle insertion or epidural catheter placement will be performed as part of this intervention.
• Epidural Catheter Placement for Clinical Indications (Procedure): A subgroup of infants requiring epidural analgesia as part of routine perioperative care will undergo epidural catheter placement. Epidural catheterization will not be performed for research purposes and will only be undertaken when clinically indicated. Following ultrasonographic assessment and induction of general anesthesia, infants will be positioned in the lateral decubitus position. Epidural catheter placement will be performed according to standard institutional practice. The epidural space will be identified using the loss-of-resistance technique at the vertebral level selected by the attending anesthesiologist, and the depth from the skin to the epidural space will be recorded. Catheter insertion, confirmation of placement, and analgesic management will follow routine clinical protocols. Data from these procedures will be used to compare actual epidural depth with ultrasonographic measurements. No additional invasive procedures will be performed for study purposes.

Primary outcomes

• Ultrasonographic Skin-to-Epidural Distance [During perioperative ultrasonographic assessment immediately after induction of general anesthesia]: Distance between the skin and the epidural space measured by ultrasonography at the intervertebral levels between the twelfth thoracic vertebra and the first lumbar vertebra, between the first and second lumbar vertebrae, between the second and third lumbar vertebrae, and between the third and fourth lumbar vertebrae in infants.

Secondary outcomes

• Comparison of Parasagittal Oblique and Transverse Median Ultrasound Measurements [During perioperative ultrasonographic assessment immediately after induction of general anesthesia]: Comparison of skin-to-epidural distance measurements obtained in the paramedian sagittal oblique and transverse median ultrasound planes.
• Intraobserver and Interobserver Measurement Reliability [During perioperative ultrasonographic assessment immediately after induction of general anesthesia]: Assessment of consistency of repeated ultrasonographic skin-to-epidural distance measurements performed by experienced anesthesiologists.
• Agreement Between Ultrasonographic Measurements and Epidural Needle Depth [During epidural analgesia procedure performed as part of routine perioperative care]: Agreement between ultrasonographically measured skin-to-epidural distance and clinically measured epidural needle depth in participants undergoing epidural analgesia for routine clinical indications.

Sex: All, ages: any to 1 Year

Inclusion Criteria:

* Infants aged 0 to 12 months
* Scheduled for elective surgery under general anesthesia
* American Society of Anesthesiologists physical status I to III
* Written informed consent obtained from parents or legal guardians

Exclusion Criteria:

* Known vertebral or spinal cord anomalies, including scoliosis or spina bifida
* Previous spinal surgery
* Neuromuscular disease or known spinal cord pathology
* Congenital syndromes associated with systemic malformations
* Severe pulmonary, cardiac, hemodynamic, or neurological instability
* Emergency surgery
* Unreliable or incomplete ultrasonographic measurement data due to technical difficulties
* Absence of written informed consent from parents or legal guardians

• Sharapi M, Mahfouz A, Philip K, Mektebi A, Albakri K. Conventional anatomical landmark versus preprocedural ultrasound for thoracic epidural analgesia: A systematic review and meta-analysis. J Perioper Pract. 2024 Oct;34(10):315-325. doi: 10.1177/17504589231181974. Epub 2023 Sep 13. PMID 37702201
• Shaikh F, Brzezinski J, Alexander S, Arzola C, Carvalho JC, Beyene J, Sung L. Ultrasound imaging for lumbar punctures and epidural catheterisations: systematic review and meta-analysis. BMJ. 2013 Mar 26;346:f1720. doi: 10.1136/bmj.f1720. PMID 23532866
• Perlas A, Chaparro LE, Chin KJ. Lumbar Neuraxial Ultrasound for Spinal and Epidural Anesthesia: A Systematic Review and Meta-Analysis. Reg Anesth Pain Med. 2016 Mar-Apr;41(2):251-60. doi: 10.1097/AAP.0000000000000184. PMID 25493689

• Ayşe Çiğdem Tütüncü, MD, Professor, Study chair, Istanbul University Cerrahpasa- Cerrahpasa Medical Faculty
• Pınar Kendigelen, MD, Professor, Study director, Istanbul University Cerrahpasa- Cerrahpasa Medical Faculty
• Münevver Kayhan, MD, Study chair, Istanbul University Cerrahpasa- Cerrahpasa Medical Faculty
• Bahar İnan, MD, Principal investigator, Istanbul University Cerrahpasa- Cerrahpasa Medical Faculty