Describe alternative approaches to the subarachnoid space

Describe alternative approaches to the subarachnoid space.

In the United States, the most popular approach to the subarachnoid space is the median approach. With the lumbar spine flexed, the spinal needle is inserted through a point approximately midway between the upper and lower verte-bral spines. The spinal needle should be maintained at a 90° angle to the back at the point of entry. It is frequently necessary to reinsert the spinal needle through a point some-what closer to the inferior spine. The median approached is the most commonly taught in medical school and, therefore, is most comfortable to many physicians.

The lateral or paramedian approach is especially useful in patients with calcified supraspinous and interspinous liga-ments, which might prevent passage of a fine spinal needle. This technique is, also, particularly useful in patients who are unable to flex their spines because of arthritis or ankylosing spondylitis. Flexion of the lumbar spine is useful but not necessary. The spinal needle is inserted approximately 2 cm lateral to the supraspinous ligament, which lies in the mid-line, and level with the upper quarter of the inferior verte-bral spine. The needle is passed in a superomedial direction to pierce the ligamentum flavum in the middle of the inter-laminar space. In the event that bone is encountered, the spinal needle is “walked off” the lamina until it encounters the interlaminar space. The distance between the skin and subarachnoid space varies considerably from patient to patient, but is usually between 3.5 and 5 cm.

The major advantages of the lateral or paramedian approach are that the calcified interspinous ligament is avoided and the needle traverses the widest portion of the interlaminar space.

The Taylor approach involves placing a spinal needle through the skin 1 cm medial and 1 cm cephalad to the posterior superior iliac spine and directing the needle in a mediocephalad direction. This places the spinal needle through the L₅–S₁ interlaminar space. The Taylor approach provides the patient with maximal comfort by using the largest interlaminar space. The overriding L₅ spinous process is avoided. The modified Taylor approach involves placing the needle 1.5 cm lateral to the midpoint of the L₅–S₁ space and directing it toward the midline at a 25° angle. The caudal approach uses a 10-cm spinal needle, which is introduced into the caudal canal and advanced superiorly until it pierces the dura at approximately the S₂ level.