MRI images show that with full flexion of the hips and back (A), cauda equine fluid moves to the non-dependent side compared to when hips and spine are straightened (B)

MRI images show that with full flexion of the hips and back (A), cauda equine fluid moves to the non-dependent side compared to when hips and spine are straightened (B)

Let’s say we want a unilateral spinal block. In the lateral position with the back straight, the cauda equina sinks towards the floor. If the back is flexed (i.e., the knee is brought up to the chest), then the spinal nerves are tightened, and the cauda equina is pulled up to the center of the intrathecal sac. Does the change in position of the cauda equina with flexion affect laterality of the spinal block performed in the lateral position?

In the manuscript “The Influence of Spinal Flexion in the Lateral Decubitus Position on the Unilaterality of Spinal Anesthesia” published in the current edition of Anesthesia & Analgesia, Dr. Jae-Hyon Bahk, Department of Anesthesiology and Pain Medicine, Seoul National University Hospital, Seoul, Korea, and coauthors randomized 36 patients scheduled for elective knee arthroscopy under spinal anesthesia to either the lateral decubitus position with the hips and back fully flexed for 15 minutes, or the lateral position with the body straight (i.e., with the spinal column in the normal lordotic curvature). A 25-gauge Quincke needle was placed in the L3-4 interspace and 8 mg of 0.5% hyperbaric bupivacaine was injected over 80 seconds. Sensory level was assessed by pin prick and motor level was assessed by movement of the legs and feet.

Here is the good news: if the surgeon wants to operate on the dependent lower extremity with the patient’s knees flexed to the chest, then a unilateral block can be achieved! In the lateral position, only 1 of 16 patients in the flexed posture developed a bilateral block. Without flexion 14 of 16 patients had bilateral blocks while in the lateral position.

Unfortunately, there are no operations in which the surgeon wants to operate on the dependent lower extremity with the patient’s knees brought up to the chest. Operations are usually performed with the patient turned supine. All patients developed bilateral spinal blocks when turned supine, regardless of the patient’s position when the block was placed.

Curvature of the spine does not affect the ability to obtain unilateral spinal blockade with hyperbaric bupivacaine in the lateral position because the differences between the dependent and non-dependent limbs vanish when the patients are turned supine.

Though the authors here discuss hyperbaric blocks, others describe the use of hypobaric blocks for unilateral spinal anesthesia. In a study published in Anesthesia & Analgesia earlier this year, for example, patients who received hypobaric ropivacaine combined with 5 µg sufentanil remained in a lateral position for 15 minutes after spinal injection. When their dosing regimen was used, there was a 95% success in spinal anesthesia for traumatic femoral neck surgery.

More study is needed.