Dalley, 10th Edition

Chapter 2 ■ Back

140

Lumbar Spinal Puncture

transecting the highest points of the iliac crests—the suprac ristal plane —usually passes through the L4 spinous process. At these levels, there is no danger of damaging the spinal cord. After passing 4–6 cm in adults (more in obese persons), the needle “pops” through the ligamentum avum, then punctures the dura and arachnoid, and enters the lumbar cistern. When the stylet is removed, CSF escapes at the rate of approximately one drop per second. If subarachnoid pres sure is high, CSF ows out or escapes as a jet. Lumbar puncture is not performed in the presence of in creased intracranial pressure (within the cranial cavity). The intracranial pressure is generally previously determined by CT scanning but may also be determined by examination of the fundus (back) of the interior of the eyeball with an oph thalmoscope (see Clinical Box “Papilledema” in Chapter 9, Head ). Spinal Anesthesia An anesthetic agent is injected into the subarach noid space. Anesthesia usually occurs within 1 minute. A headache may follow spinal anesthesia , which likely results from the leakage of CSF through the lumbar puncture (see Clinical Box “Anesthesia for Child birth” in Chapter 6, Pelvis and Perineum). Epidural Anesthesia (Blocks) An anesthetic agent is injected into the epidural space using the position described for lumbar spi nal puncture, or through the sacral hiatus (caudal epidural anesthesia/block) (see Clinical Box “Anes thesia for Childbirth” in Chapter 6, Pelvis and Perineum). Ischemia of Spinal Cord The segmental reinforcements of the blood supply to the spinal cord from the segmental medullary arteries are important in supplying blood to the an terior and posterior spinal arteries. Fractures, dis locations, and fracture–dislocations may interfere with the blood supply to the spinal cord from the spinal and medul lary arteries. De cient blood supply (ischemia) of the spinal cord af fects its function and can lead to muscle weakness and paral ysis. The spinal cord may also suffer circulatory impairment if the segmental medullary arteries, particularly the great anterior segmental medullary artery (of Adamkiewicz), are narrowed by obstructive arterial disease . Sometimes the aorta is purposely occluded (cross clamped) during surgery. Patients undergoing such sur geries, and those with ruptured aneurysms of the aorta or occlusion of the great anterior segmental medullary ar tery, may lose all sensation and voluntary movement infe rior to the level of impaired blood supply to the spinal cord ( paraplegia ) secondary to death of neurons in the part of

Lumbar puncture (LP, spinal tap), the withdrawal of CSF from the lumbar cistern, is an important diagnostic tool for evaluating a variety of central nervous system (CNS) disorders. Meningitis and diseases of the CNS may alter the cells in the CSF or change the concentration of its chemical constituents. Examination of CSF can also determine if blood is present. LP is performed with the patient lying on the side with the back and hips exed (knee–chest position) (Fig. B2.24). Flexion of the vertebral column facilitates insertion of the needle by spreading apart the vertebral laminae and spinous processes, stretching the ligamenta ava. The skin covering the lower lumbar vertebrae is anesthe tized, and a lumbar puncture needle , tted with a stylet , is inserted in the midline between the spinous processes of the L3 and L4 (or L4 and L5) vertebrae. Recall that a plane

Subarachnoid space

Ligamentum flavum

Skin

Supraspinous ligament Interspinous ligament

Conus medullaris

L2

CSF in lumbar cistern

Epidural space

Lumbar spinal puncture for spinal anesthesia Lumbar injection for epidural anesthesia

Spinous process of L4 Filum terminale internum Spinal dural sac

Copyright © 2027 Wolters Kluwer. Unauthorized reproduction of the article is prohibited.

S2

Sacrum

Epidural space in sacral canal

Filum terminale externum

Sacral hiatus

Median section

FIGURE B2.24. Lumbar spinal puncture.