Case Study
A 23 year-old gravida 1, para 0 woman at 39 weeks’ gestation with a body mass index (BMI) of 28 kg/m2 was admitted to the labor and delivery suite for a scheduled elective cesarean delivery due to a breech presentation. The patient was healthy with no past medical, surgical, or obstetric history and reported an uncomplicated pregnancy. A preoperative assessment revealed no concerning features for difficult airway management or neuraxial placement; informed consent was obtained for a single-shot spinal anesthetic technique.
Following aseptic preparation and patient positioning in the seated position, a spinal technique with a 25-gauge Whitacre needle was attempted at the presumed L3–L4 interspace, as determined by palpation. During the first attempt, the spinal needle hit bone. Despite repeated attempts with needle angle readjustment, passage of the spinal needle into the subarachnoid space was not successful. The anesthesiologist subsequently moved to the next higher interspace and reinserted the spinal needle to a depth of 4–5 cm when a sudden change in resistance was felt. At this point, the patient screamed and indicated the presence of a sharp pain radiating from her right hip to her foot. The pain gradually subsided without moving the needle; with removal of the spinal needle stylet, the anesthesiologist observed clear CSF return and administered an admixture containing hyperbaric 0.5% bupivacaine 12 mg, fentanyl 10 µg, and preservative-free morphine 100 µg. The injection occurred without patient discomfort and resulted in a bilateral T4 sensory level block to pinprick with a Bromage grade 3 motor block of the lower limbs. The operation proceeded uneventfully, and a healthy baby girl was born.
At 24 hours postoperatively, the patient complained of throbbing pain and paresthesias in her right thigh and buttock radiating down her right leg. She also reported right-sided numbness over her lateral calf and the dorsum of her foot. She was able to stand and walk, but not without dragging her right foot and toes. An examination revealed preserved bilateral hip, knee, and ankle reflexes, with weak right-sided ankle dorsiflexion and plantar flexion, manifesting as a foot drop. The patient also reported difficulty passing urine, which required intermittent catheterization. A MRI scan of the lumbar/sacral spine revealed a high T2-weighted signal in the conus at the L1 vertebral level.
Key Points
This patient experienced neurologic deficits after a routine single-shot spinal anesthetic technique.
The anesthesiologist selected a lumbar interspace that was inadvertently one or more segments higher than intended and made direct needle contact with the spinal cord/conus.
Although rare, lower extremity neurologic complications after neuraxial anesthesia techniques can occur, and this patient most likely suffered a conus medullaris injury.
Discussion
Neurologic Complication Following Central Neuraxial Anesthesia
Although the incidence of permanent neurologic dysfunction is extremely low, the consequences can be devastating.1 The Royal College of Anaesthetists’ Third National Audit Project (NAP3) estimated the incidence of permanent injury after central neuraxial blockade to be 4.2 per 100,000 (95% confidence interval [CI] 2.9–6.1) and the incidence of paraplegia or death to be 1.8 per 100,000 (95% CI 1.0–3.1).2 Among the obstetric population, the risk for complications was greatest for the combined spinal-epidural technique, intermediate for the spinal technique, and lowest for the conventional epidural technique.2
Anatomic and Clinical Considerations of the Spinal Cord
Knowledge of normal and abnormal spinal cord anatomy and the limitations of external landmark palpation is essential for the safe administration of lumbar neuraxial techniques. The spinal cord terminates as the conus medullaris before dividing into nerve fibers known as the cauda equina, which includes the filum terminale and the lumbosacral nerve roots3 (Figure 41.1). Functionally, the conus medullaris represents a major site of transition between the central and peripheral nervous systems. As a consequence, injuries distal to the conus medullaris (e.g., the nerve roots and cauda equina) result in symptoms and signs consistent with cauda equina syndrome, lumbosacral radiculopathies, or peripheral nerve injuries.
The spinal cord typically terminates at the interspace between the first and second lumbar vertebral level (L1–L2), but this can vary widely (range T12 to L3–L4).4 In 43 percent of women, the conus medullaris reaches the upper part of the L2 vertebral body, and in up to 10 percent of the population, the spinal cord may terminate caudal to L2.5
A spinal needle inserted at the level of the L2–L3 interspace may contact the conus medullaris in 4–20 percent of patients6 (Figure 41.2). As a consequence, instruction for neuraxial techniques emphasizes insertion below Tuffier’s line, an imaginary line between the posterosuperior iliac crests that is believed to intersect the L4 vertebral body or the L4–L5 interspace.4 Radiologic studies indicate that the anatomic location identified by Tuffier’s line can range from L1 to L5.7, 8 Even experienced anesthesiologists identify lumbar intervertebral spaces correctly in only 29 percent of patients.9
Figure 41.2 Normal distribution of the lower border of the conus medullaris, illustrating the inherent variability in the healthy adult population without spinal deformity.
Conus Medullaris Injury
Injury to the conus medullaris has been reported in parturients receiving neuraxial anesthesia using 25- and 27-gauge pencil-point spinal needles; these needles have a tip that is at least 1 mm beyond the opening and require deeper insertion into the subarachnoid space to obtain CSF return than a beveled, open-tip needle.6 Most patients who have experienced this outcome recall having pain on needle insertion but not on medication injection; CSF flow through the needle and the resulting spinal anesthesia block pattern or spread characteristics were described as being normal.6
The conus medullaris gives origin to the lumbar sympathetic, sacral parasympathetic, and sacral nerve roots; provides important lower extremity sensory, motor, and autonomic functions3; and when injured, results in disabilities related to sacral outflow (S2–S4). In the largest case series of seven conus medullaris injury patients following spinal anesthesia (six obstetric, one surgical; all placed at the L2–L3 level), unilateral sensory and motor deficits occurred, and a syrinx was demonstrated on MRI (T2 weighted).6 Symptoms experienced by these patients included numbness in the lower extremities and saddle area along L4–S2, weakness manifesting as foot drop, and urinary dysfunction.6
Bladder dysfunction related to conus medullaris injuries is variable depending on the neuroanatomic structures involved, but typically weak or flaccid detrusor activity results in urinary retention and overflow incontinence.3 Treatment consists of intermittent catheterization to prevent bladder overdistension and kidney retrograde flow injuries and infections.3
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