AN42.1-3 | Back Region — Gate Quiz

Correct. In adults, the conus medullaris ends at L1–L2 vertebral level. Below this, the lumbar cistern contains the cauda equina (L2–S5 nerve roots) and the filum terminale in CSF. This is why lumbar puncture is performed at L3–L4 or L4–L5 — safely below the cord.

Conus medullaris: L1–L2 in adults (L3 in neonates). Lumbar puncture: L3–L4 or L4–L5. The dural sac (containing cauda equina + CSF) extends to S2. Below S2, only the filum terminale continues to the coccyx.

Correct. The epidural (extradural) space is between the dura mater and the periosteum/ligamentum flavum of the vertebral canal. It contains epidural fat, the internal vertebral venous plexus (Batson's plexus), and spinal blood vessels. This is where epidural anaesthetics are deposited (to block nerve roots as they pass through the epidural space).

Epidural space = between dura and vertebral canal wall; contains fat + Batson's venous plexus. Subarachnoid space = between arachnoid and pia; contains CSF. Subdural space = potential space between dura and arachnoid. The central canal is within the spinal cord itself.

Correct. Occlusion of the artery of Adamkiewicz (the great anterior radicular artery, T8–L2) deprives the anterior spinal artery of its major reinforcing supply → anterior spinal artery syndrome. Motor loss (corticospinal + anterior horn) and pain/temperature loss (spinothalamic — anterior white commissure) occur below the lesion. Vibration and proprioception (posterior columns — supplied by posterior spinal arteries) are preserved.

Anterior spinal artery syndrome: motor paralysis + loss of pain/temperature + autonomic dysfunction below lesion; PRESERVED vibration and proprioception. Caused by occlusion of the anterior spinal artery or its main feeder (artery of Adamkiewicz) — most commonly during thoracoabdominal aortic surgery.

Correct. The suboccipital triangle is bounded by: (1) Rectus capitis posterior major (superomedial — spinous process of C2 → occipital bone), (2) Obliquus capitis superior (superolateral — transverse process of atlas → occipital bone), (3) Obliquus capitis inferior (inferior — spinous process of C2 → transverse process of atlas). Together these form the triangular space containing the vertebral artery (V3), suboccipital nerve (C1), and greater occipital nerve (C2).

Suboccipital triangle = rectus capitis posterior MAJOR (superomedial) + obliquus capitis SUPERIOR (superolateral) + obliquus capitis INFERIOR (inferior boundary). The floor is the posterior arch of the atlas. The roof is semispinalis capitis.

Correct. The greater occipital nerve (dorsal ramus of C2) emerges below the inferior oblique, pierces the semispinalis capitis, and then the trapezius at the superior nuchal line. It supplies the posterior scalp from the occiput to the vertex. Entrapment or irritation → occipital neuralgia: throbbing pain radiating from occiput to vertex ± forehead (via the supraorbital nerve territory of CN V1, via trigemino-cervical interneurons).

Greater occipital nerve (C2 dorsal ramus) = the nerve of occipital neuralgia. It is the sensory nerve of the posterior scalp, pierces semispinalis capitis + trapezius, and is palpable/tender at the superior nuchal line. Suboccipital nerve (C1) is purely motor — it does not cause sensory pain. Lesser occipital nerve (C2 ventral ramus) supplies the upper posterior lateral scalp/ear — different distribution.

Correct. The correct sequence for a midline lumbar puncture: skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum (resistance then give) → epidural space (fat, veins) → dura mater (firm resistance) → subdural space (very thin, potential) → arachnoid mater → subarachnoid space (CSF flows). The "give" felt when the needle penetrates the ligamentum flavum, then the dura, is used to confirm entry into the subarachnoid space.

Lumbar puncture needle path: supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid space. Two distinct "pops" may be felt — one through ligamentum flavum, one through dura-arachnoid. The posterior longitudinal ligament lines the inside of the vertebral body — not the interlaminar space.

Correct. The vertebral artery in the suboccipital triangle (V3 segment — from the foramen transversarium of C1 to the foramen magnum) carries a significant volume of blood from the subclavian artery toward the basilar artery. Inadvertent injury during posterior atlas dissection causes torrential bleeding, which is notoriously difficult to control in the confined space of the suboccipital triangle. Embolism from the injured artery can also cause cerebellar or brainstem infarction.

The V3 segment of the vertebral artery is the most exposed and vulnerable portion of the vertebral artery — it lies directly in the floor and superior wall of the suboccipital triangle. Injury causes massive haemorrhage (high-flow artery) and/or vertebrobasilar ischaemia (posterior circulation stroke — cerebellar/brainstem infarction).

Correct. Splenius capitis (from midline cervical/thoracic spinous processes to the mastoid process and superior nuchal line) pulls the mastoid process toward the origin — rotating and laterally flexing the head to the SAME side. Bilateral contraction = extension of the head and neck. This is in contrast to the semispinalis group (transversospinales), which rotates the head to the OPPOSITE side.

Splenius capitis: bilateral = head extension; unilateral = ipsilateral rotation and lateral flexion. Semispinalis capitis: bilateral = head extension; unilateral = CONTRALATERAL rotation (transversospinales rotate to the opposite side). Splenius = same side; semispinalis = opposite side. This difference is commonly tested.

Correct. Batson's vertebral venous plexus is a valveless, low-pressure internal vertebral venous network that surrounds the spinal cord in the epidural space and interconnects with the external vertebral plexus, intercostal/lumbar veins, and pelvic veins. Increased intrathoracic pressure (from coughing, Valsalva) can reverse blood flow, seeding tumour cells retrogradely into the vertebral bone marrow. Breast, prostate, and lung cancers most commonly use this route for vertebral metastasis.

Batson's plexus is the key mechanism for vertebral metastasis from breast, prostate, and lung tumours. Its valveless nature means blood (and tumour cells) can flow bidirectionally. During Valsalva manoeuvre or increased thoracic pressure, tumour emboli are directed into the vertebral venous system → vertebral bone marrow → bone metastases (mixed osteolytic/osteoblastic for breast cancer).

Correct. Tuffier's (intercristal) line drawn between the highest points of the iliac crests crosses at the level of the L4 spinous process (or the L4–L5 intervertebral disc). This reliable surface landmark guides the safe insertion of the lumbar puncture needle at L3–L4 (one space above Tuffier's line) or L4–L5 (at or one space below) — both are safely below the conus medullaris at L1–L2.

Tuffier's line (intercrestal line) = L4 spinous process. LP is performed at L3–L4 or L4–L5 (both below the L1–L2 conus). In obese patients or those with poor landmarks, ultrasound-guided LP is performed. In neonates, the conus is at L3 — use L4–L5.