In this case, a progressive neurological deficit was probably provoked by a new herniated disc but only occurring after the injection since the neurological examination at the emergency department was normal and a new herniated disc not visible on the first MRI 4 days earlier occurred. Therefore, the TFESI given to this patient obscured the causal relation with the new neurological deficit that became more obvious the day after the intervention.

Previously reported major neurological complications after epidural corticosteroids include direct needle trauma to the nerve or spinal cord, epidural hematoma and ischemia, infection, the neurotoxic effect of the injected corticosteroid, and accidental intravascular injection of particulate corticosteroids, creating an embolus causing infarction of the conus medullaris at the lumbar level [8].

Wibowo reported a case of CES after caudal epidural sacral injection to treat severe radiating pain in both legs due to lumbar spinal stenosis [10]. Although the authors claim that caudal injections are safer than epidural injections, they found two potential causes for the occurrence of CES: a mechanical problem causing neurological damage and the increased epidural pressure due to the high volume of injected fluid (up to 20ml).

These authors report four other cases of neurological deficit after epidural steroid injection in patients with spinal stenosis [11,12].

The contribution of disc herniation to the occurrence of CES was illustrated in a case report where the TFESI was given without previous imaging because the patient could not change into a supine position due to severe pain. The neurological examination showed motor weakness. The patient had no urinary incontinence or saddle anesthesia, and an anal sphincter tone was retained. After the TFESI, the pain was significantly reduced, and the patient underwent an MRI. The patient reported acute paraplegia and complete loss of motor and sensory functions in both legs during the MRI, including sensation around the anus [13]. The MRI showed that the patient's conus medullaris was above the L1 body and the disc herniation at L1-L2 caused the CES. This case report illustrates the window of time during which symptoms progress.

Also, in our case, the contribution of the TFESI to the isolated L5 lesion combined with the CES was initially unclear.

Iatrogenic?

Theoretically, direct needle trauma may worsen the L5 lesion. However, this is unlikely because we positioned the needle correctly using the safe triangle approach. It is also plausible that an acute increase in mass effect attributed to the volume of injectate resulted in ischemia by exacerbating the underlying neuroforaminal stenosis.  Despite using fluoroscopy with contrast application, an intravascular injection cannot be entirely ruled out. But this would then more likely give a spinal cord injury with paraplegia rather than a single nerve root injury. However, if the pathophysiology were essentially iatrogenic, this likely would have resulted in acute pain during and after the procedure. This is very unlikely since the patient was transferred to the ward after the procedure without any additional pain.

Natural progression of a herniated disc/cauda equina?

The most probable cause of the nerve injury in combination with the CES seems to be the natural evolution of radiculopathy. The patient was initially hospitalized with progressive neurogenic claudication. No gas-containing disc extrusion was noted on the first MRI but only became apparent at the urgent CT scan after the transforaminal procedure four days after the initial MRI, when the neurological symptoms deteriorated. So, the new disc extrusion probably critically compromised the L5 nerve root and the epidural space on an underlying moderate to severe central spinal canal stenosis L3-4 and anterolisthesis L4-5, leading to a CES. The estimated incidence of CES is up to 1.9 per 100.000 inhabitants [14]. This contrasts with the prevalence of neurological complications after epidural corticosteroid injections, which can even not be estimated because of the very low incidence [15]. The recently published study on serious adverse events after epidural corticosteroid injections in the Medicare population shows an incidence of 8.8 in 1 000 000 patients after lumbar epidural injection. None of these patients had received a transforaminal injection [16].

CES occurs, therefore, more frequently than a neurological complication secondary to an epidural corticosteroid infiltration.

Contributing factors to cauda equine syndrome

In our case, the new herniated disc contained gas, described in the literature as the vacuum phenomenon (VP). Spinal VP within intervertebral disks has a high prevalence in the elderly (>60 years age). It was shown to be exacerbated by extension of the spine and to resolve with flexion. Spinal VP can evolve into disc rupture and sciatica. Moreover, spread of gas through ruptured annulus fibrosis allows gas to expand into the epidural space and impinge upon surrounding structures [17]. Gas containing disc extrusions have been described in the literature in patients with acute radiculopathy or exacerbation of chronic pain [18-22]. One case report describes an acute cauda equina syndrome due to a gas-containing prolapsed herniated disc requiring urgent surgery [23].

Further, patients with sciatica and a CES due to a lumbar herniated disc have a significantly smaller epidural space than sciatica patients without CES [24]. Since 1–10% of patients with a known lumbar herniated disc develop CES [24], the chances that an interventional procedure is performed during the time-frame of this natural neurological progression is realistic, thereby obscuring the causal relation with the epidural corticosteroid injection. In patients with spinal stenosis, injection of a small volume is recommended to minimize a possible pressure increase in the epidural space.