An epidural steroid injection (ESI) is widely used to treat lumbar radiculopathy and spinal stenosis. Although generally considered safe, it may trigger unexpected neurological responses, including inadvertent subdural block, a rare and underrecognized complication of neuraxial anesthesia [1]. The subdural space, a potential space between the dura and arachnoid mater, can inadvertently be accessed during epidural injection, particularly in elderly patients with anatomical variations or spinal degenerative changes [2].

Subdural block is characterized by delayed onset of symptoms, disproportionately extensive sensory spread, relative sparing of motor function, and inconsistent hemodynamic instability [3]. Radiological confirmation is often absent, and diagnosis relies primarily on clinical features. The awareness of this possibility is crucial, as repeated injection into the subdural space may cause neurological deterioration. We present a case of a clinically suspected subdural block following lumbar ESI, with normal fluoroscopic findings, that resolved without sequelae.

The Institutional Review Board (IRB) of the Jeju National University Hospital (IRB No. 2025-06-026) approved this study.

An 87-year-old woman complained of pain in the lower back and right buttock. A lumbar magnetic resonance imaging (MRI) conducted 5 years prior revealed L2/3, indicating a large extruded disc with upward migration compressing the right L2 nerve root (central to right foraminal zone) and moderate central canal stenosis. It also revealed L3/4, indicating right paracentral protrusion with severe central stenosis and mild right L3 foraminal stenosis (Fig. 1).

For the past 4 years, she had intermittently been administered with right L2 root block (triamcinolone 20 mg) and lumbar epidural steroid injections (L-ESI) at L3-4 every 5-6 months, with effective pain control and no adverse events. She returned after 6 months and was administered the same L3-4 epidural steroid injection.

The L3/4 interspace was identified using C-arm fluoroscopy and was marked along the midline. After prepping the skin with povidone-iodine, local anesthesia was induced using 2% lidocaine (3 mL). A 20-gauge Tuohy needle was advanced using the loss-of-resistance technique with saline solution. No cerebrospinal fluid or blood was aspirated. Two milliliters of contrast medium were injected, and fluoroscopic images confirmed epidural spread in the anteroposterior and lateral views (Fig. 2). A mixture of 0.125% ropivacaine (8 mL) and dexamethasone (5 mg) was subsequently administered.

The patient was transferred to the recovery room and monitored by non-invasive blood pressure (NIBP), SpO₂, and electrocardiogram (ECG). Forty minutes after the injection, the patient reported unusual symptoms. Sensory block to the T4 dermatome was confirmed using an alcohol swab, and bilateral lower-extremity motor block was noted. Her blood pressure remained at 90/60 mmHg till this point. Suspecting a subdural block, an intravenous line was secured. Additionally, oxygen and fluid support were initiated.

Approximately 70 minutes after injection, the patient experienced significant hypotension (78/50 mmHg) and was treated with 5 mg ephedrine, subsequently raised to 10 mg. She also vomited and was administered ondansetron (4 mg intravenous). Her symptoms gradually improved, and sensory recovery began approximately 2 hours after the injection. She was observed for 6 hours in the recovery unit, after which full sensory and motor recovery was confirmed. She was discharged in stable condition.

This case illustrates the characteristic clinical presentation of an inadvertent subdural block, despite fluoroscopic imaging suggesting a typical epidural distribution. The subdural space is a narrow potential space located between the dura mater and arachnoid mater, extending cranially and caudally along the neuraxis. Anatomical studies, including those using electron microscopy, have demonstrated that fluid injection under pressure, particularly with fine or blunt needles, may dissect the interface, resulting in a transient subdural compartment [3].

A subdural block is typically characterized by delayed onset, disproportionately extensive and often bilateral sensory block, minimal motor involvement, and variable hemodynamic changes.1 Lubenow et al. [4] proposed diagnostic criteria that include two major features, negative aspiration and higher-than-expected sensory block, and one minor feature, delayed onset, asymmetrical distribution, or mild motor involvement. Subdural block tends to have a delayed onset compared to spinal or epidural anesthesia, but it may present with more profound hemodynamic changes, a pattern also observed in our case. The patient first reported discomfort 40 minutes after the injection, with the sensory block level reaching T4 and significant motor weakness. Notably, however, hemodynamic deterioration occurred approximately 70 minutes later. Thus, routine post-epidural monitoring, typically limited to 30 minutes, may be insufficient when a subdural block is suspected. This warrants extended observation in such cases.

Known supportive radiologic signs include posterior pooling of the contrast, parallel railroad track appearance in the anteroposterior view, and thin posterior linear streaking in the lateral view [5]. A particularly notable feature of this case is that the fluoroscopic images were consistent with a typical epidural injection. This underscores the importance of not relying solely on radiographic findings but also prioritizing clinical presentation when evaluating unexpected symptoms.

Although a subdural block is more commonly reported in obstetric anesthesia, it must not be overlooked in elderly patients, especially those with spinal degenerative changes. The risk factors include technically difficult procedures, prior lumbar puncture, altered spinal anatomy, and the use of blunt-tip needles. Needle rotation during insertion may increase the risk of dural compromise [2].

The management is usually conservative and supportive, involving supplemental oxygen, intravenous fluids, and vasopressors, as needed. Most patients recover completely within several hours. Preventive measures include meticulous techniques, attention to resistance and spread during injection, and vigilance regarding unexpected clinical signs.

This case highlights the importance of clinical judgment in the diagnosis of subdural blocks. Even when fluoroscopic findings appear normal, an unexpectedly high sensory block, motor impairment, and delayed onset should prompt the consideration of this rare but significant complication. Prompt recognition allows appropriate management and prevents unnecessary intervention.