Vertebral Column, Spinal Cord and Back

• The spinal cord ends at the conus medullaris at roughly L1–L2 in adults (lower in children, around L3), because the vertebral column grows faster than the cord. Below that, the cauda equina floats in cerebrospinal fluid.
• Lumbar puncture is therefore performed at L3–L4 or L4–L5, identified by the supracristal line joining the iliac crests. The needle passes skin, superficial and deep fascia, supraspinous and interspinous ligaments, ligamentum flavum, epidural space (with its venous plexus), dura and arachnoid into the subarachnoid space.
• Disc herniation is usually posterolateral, because the posterior longitudinal ligament reinforces the midline. In the lumbar spine, a posterolateral herniation typically spares the exiting root and compresses the root below — an L4–L5 herniation classically affects L5. A large central herniation causes cauda equina syndrome: saddle anaesthesia, bowel and bladder dysfunction, bilateral leg weakness — a surgical emergency.
• Key reflex levels: biceps C5, brachioradialis C6, triceps C7, patellar L4, Achilles S1.
• Landmark dermatomes: nipple T4, xiphoid T7, umbilicus T10, inguinal L1.
• Vertebral levels worth knowing: hyoid C3, thyroid cartilage C4, cricoid C6 (where the oesophagus begins).

Column architecture

• Segments: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 4 fused coccygeal. Secondary (lordotic) cervical and lumbar curves develop with head-lifting and walking; the thoracic and sacral kyphoses are primary.
• Load transfer: anteriorly through vertebral bodies and intervertebral discs (compressive), posteriorly through paired facet (zygapophyseal) joints (shear and rotation control). The disc is a nucleus pulposus (notochord remnant, proteoglycan-rich, hydrostatic) inside a lamellar anulus fibrosus.
• Ligaments: anterior longitudinal ligament resists extension; posterior longitudinal ligament resists flexion but narrows caudally, leaving the posterolateral anulus the weakest point; ligamentum flavum (elastin-rich) is the last resistance felt before the epidural space.

Neural elements

• Ascensus medullae: the cord and column are the same length at ~3 months in utero, then the column outgrows the cord, so segments come to lie above their named vertebrae and roots run obliquely downward as the cauda equina. This is why the adult conus sits at L1–L2.
• Root exit rule: cervical roots C1–C7 exit above the same-numbered pedicle; C8 has no vertebra and exits between C7 and T1; from T1 down, each root exits below its same-numbered pedicle. Because the lumbar root exits high under its pedicle, a posterolateral disc misses the exiting root and strikes the traversing root below.
• Meninges and spaces: the dural sac continues to ~S2, so the lumbar cistern below the conus holds cerebrospinal fluid and floating roots — a needle there displaces rather than transects nerves. The epidural space (fat, Batson's valveless venous plexus) lies outside the dura.

Blood supply

• Arterial pattern: one anterior spinal artery perfuses the anterior two-thirds (corticospinal, spinothalamic, anterior horns); paired posterior spinal arteries supply the dorsal columns.
• Segmental reinforcement: the anterior spinal artery is fed by segmental medullary arteries, the largest being the great anterior segmental medullary artery (of Adamkiewicz), typically arising from a left-sided lower intercostal or upper lumbar segmental artery and supplying the lumbosacral enlargement.
• Watershed: the cord at roughly T4–T8 lies below the robust cervical/upper thoracic inflow and above the Adamkiewicz territory, making it the segment most vulnerable to hypoperfusion during aortic cross-clamping or profound hypotension.

Disc and degenerative disease

• Posterolateral herniation: L4–L5 disc → L5 root (weak great-toe extension, dorsum-of-foot sensory loss, normal reflexes); L5–S1 disc → S1 root (weak plantarflexion, lost Achilles reflex). A far-lateral (foraminal) herniation is the exception — it catches the exiting root.
• Lumbar spinal stenosis: hypertrophied ligamentum flavum and facets produce neurogenic claudication — pain relieved by flexion (leaning on a shopping cart), preserved pedal pulses, distinguishing it from vascular claudication.
• Imaging restraint: American College of Physicians guidance and ACR Appropriateness Criteria advise against routine imaging for acute low back pain without red flags (trauma, fever, cancer history, neurologic deficit, incontinence).

Emergencies

• Cauda equina syndrome: asymmetric flaccid, areflexic leg weakness with saddle anesthesia and late urinary retention — emergent MRI and surgical decompression.
• Conus medullaris syndrome: more symmetric, early and severe bowel/bladder dysfunction, mixed upper- and lower-motor-neuron signs.
• Metastatic epidural cord compression (breast, lung, prostate, myeloma via Batson's plexus): back pain worse at night or recumbent, then myelopathy. NCCN guidance supports immediate corticosteroids (dexamethasone) plus urgent MRI of the whole spine and radiation-oncology/surgical consultation.
• Spinal epidural abscess: fever, focal spinal tenderness, radicular pain, then deficit (the full triad is often absent). Obtain emergent contrast MRI and blood cultures, start empiric antibiotics immediately — an anti-MRSA agent (vancomycin, dosed to a 24-hour AUC/MIC of 400–600 per the 2020 IDSA/ASHP consensus) plus a gram-negative/antipseudomonal beta-lactam such as cefepime — and obtain urgent neurosurgical decompression for neurologic deficit, sepsis, or progression.
• Contrast with native vertebral osteomyelitis: in the hemodynamically stable patient without neurologic compromise, IDSA guidance favors withholding antibiotics until image-guided biopsy yields a microbiologic diagnosis.
• Spinal epidural hematoma after neuraxial puncture in an anticoagulated patient — the American Society of Regional Anesthesia interval recommendations exist precisely to prevent this.

Cord and development

• Anterior spinal artery infarction (aortic surgery, hypotension): bilateral weakness and loss of pain/temperature with spared vibration and proprioception.
• Neural tube defects: spina bifida occulta (tuft of hair, dimple) to meningomyelocele; USPSTF recommends folic acid 0.4–0.8 mg daily for all persons capable of pregnancy.
• Vertebral compression fracture in osteoporosis or steroid use presents as acute focal pain with loss of height.

• Conus at L1–L2, dural sac to S2: that gap is the entire safety margin for lumbar puncture. In infants the conus reaches ~L3, so puncture low (L4–L5) in children. The supracristal (Tuffier's) line marks roughly L4.
• The "pop" is the ligamentum flavum: the last structure before the epidural space. Distractor to avoid — the needle never pierces the posterior longitudinal ligament or the pia in a correctly placed LP.
• L4–L5 herniation compresses L5, not L4: posterolateral disc material strikes the traversing root. The exiting root is only hit by a far-lateral herniation. This is the single most repeated distractor on the topic.
• Cauda equina vs conus medullaris: cauda equina is asymmetric, radicular, areflexic, with later bladder involvement; conus is symmetric with early urinary retention and mixed UMN/LMN findings. Best next step for either is emergent MRI, then surgical decompression — do not delay for plain films.
• New back pain in a patient with cancer is a red flag: the immediate step is urgent MRI of the entire spine (skip lesions are common). Give dexamethasone right away when there are neurologic deficits or imaging-confirmed epidural compression, per NCCN guidance; back pain alone does not mandate reflexive steroids, and steroids may be deferred when untreated lymphoma is suspected and biopsy is pending, since they can obscure the histologic diagnosis.
• C8 exists as a nerve root but not as a vertebra — the cervical root-above/thoracolumbar root-below rule pivots at C7–T1.
• Anterior spinal artery syndrome spares vibration and proprioception because the posterior spinal arteries are separately supplied — the classic dissociated deficit after thoracoabdominal aortic repair.
• Dermatome anchors (nipple T4, umbilicus T10) let you localize a sensory level; a sensory level with bilateral weakness is myelopathy until proven otherwise, not peripheral neuropathy.
• Batson's valveless vertebral venous plexus explains why prostate and breast carcinoma seed the spine without passing through the lungs.