This study was conducted to assess the ultrasound's (US's) ability to identify pertinent landmarks for lumbar puncture (LP) in patients of various body mass indices (BMIs) and establish spatial relationships of pertinent LP landmarks across BMIs.

In this institutional review board–approved cross-sectional study, we calculated the BMIs of eligible patients and then categorized them as normal (BMI ≤24.9), overweight (BMI 24.9-30), or obese (BMI ≥30). We recorded the difficulty in palpating traditional LP landmarks. Identification and measurement of the spatial relationships of the sacrum; spinous processes of lumbar vertebrae L3, L4, and L5; ligamentum flavum; and the spinal canal by US was attempted.

Successful identification of pertinent structures (L4-L5 spinous processes and the spinal canal) occurred in 100% of patients with normal BMI, 95% of those who were overweight, and 74% of those who were obese (P = .011). Difficulty in palpating landmarks was noted in 5% of patients with normal BMI, 33% of those who were overweight, and 68% of those who were obese (P < .0001). In subjects with difficult-to-palpate landmarks, US identified pertinent structures in 16 of 21 (76%; 95% confidence interval, 53-92). The average distance from skin to ligamentum flavum was 44 mm in those with normal BMI, 51 mm in those who were overweight, and 64 mm in those who were obese (P < .00001); measurements between spinous processes did not vary by BMI. Overall, there was a moderate correlation (0.62) between BMI and the distance from skin to ligamentum flavum.

The usefulness of US in identifying structures for LP is inversely related to BMI. Even with this limitation, US is still able to identify obese patients' pertinent landmarks almost 75% of the time. In addition, US may be helpful in identifying pertinent structures for LP in those patients with difficult-to-palpate landmarks. In patients who were obese with structures not palpable by hand or identifiable by US, other modalities should be considered.