side effects of epidurals - subdural block

Subdural block

Inadvertent subdural block is rare complication of epidural. It is suspected in cases of inadequate pain relief, asymmetrical and unexpectedly high level of the block. In vast majority of patients the symptoms resolve without special treatment.

Subdural, also sometimes called subdural-extra-arachniod, block is an uncommon complication of epidural anesthesia. It is characterized by inadequate distribution of local anesthetic and is caused by inadvertent placement of epidural catheter between the dura and arachnoid layers.

Dura mater is a tough membrane that surrounds the spinal cord and cerebro-spinal fluid. From practical point of view, local anesthetic is placed outside the dura during epidural and under it during spinal block. However, there is another anatomical layer under the dura, arachnoid mater. Translated from Latin the name means “spider-(web)-like”, due to its resemblance to the spider web. Delicate arachnoid and pia mater are sometimes considered the same anatomical structure given the collective name leptomeninx.

Arachnoid mater lines up the dura from the inside, and the tiny space between them is filled with miniscule amounts of serous fluid. If a fluid is injected between these layers the space will expand. The potential space is the widest in the cervical region and narrowest at lumbar level. The width of subdural space and its potential to expand varies greatly between patients.

During placement of labor epidural it is possible to inadvertently to place the catheter and subsequently inject local anesthetic mixture between the dura and arachnoid, in the subdural space. It can happen either by piercing the dura by the epidural needle, or the catheter may migrate through the dura after effective epidural has been established.

Signs and symptoms associated with subarachnoid block vary between patients and depend largely on the extent and the pattern of spread of local anesthetic between the layers. This, in turn, depends on individual anatomy. One of the first signs of subarachnoid block is slow onset of analgesia, between 15 and 20 minutes. The level of sensory block is often disproportionately high, though in some cases pain relief may be inadequate or completely absent. On the contrary, motor nerve function is often unaffected, due anatomical distribution of the subdural space. However, if motor weakness does occur, it develops gradually. That helps distinguish this complication from inadvertent intrathecal block, where muscle weakness occurs rapidly, over several minutes.

Other described signs of subdural block are weakness of intercostals muscles (making it difficult to breathe or cough), asymmetrical distribution of analgesia and skin numbness, patchy block, unexpectedly prolonged sensory blockade. Rarely, subdural block may lead to permanent nerve damage due to local anesthetic distribution and compression around the nerve roots. Even rarer complications include Horner’s syndrome and trigeminal nerve palsy. The former is characterized by drooping of the upper lid, the impression that the eye is sunken and decreased sweating on the affected side of the face. In trigeminal palsy the patient complains of loss of sensation in the face, usually on one side. Trigeminal palsy is potentially more dangerous as it means that local anesthetic has spread to the level of the medulla, important anatomical structure that contains many vitally important neural structures, such as respiratory center. The last two presentations of the subdural block are very rare.

Though subdural block is most commonly observed in obstetric patients who receive labor epidurals, the incidence of this complication is low, though it varies greatly on the way how it is diagnosed. One of the largest studies that observed 145,550 patients who were given epidurals over the period of 17 years found the incidence to be one in 4200 patients, or 0.024%. In this particular study inadvertent subdural injection was diagnosed by unexpectedly high block, often asymmetrical and involving the arms and the face. The diagnosis was not confirmed by x-ray. On the other hand, several studies that checked the position of epidural needles during placement with x-ray imaging found that the epidural needle is placed at least partially between the dura and arachnoid more frequently, between 1% and 13%. The latter findings are likely to have academic rather than practical significance: if the quality of epidural block is satisfactory and subdural injection of local anesthetic is not accompanied by symptoms it is probably not clinically significant.

There are several risk factors that may increase the chance of inadvertent subdural block. As many other complications, subdural block is more likely to happen in difficult epidural placement. In addition, rough handling of the epidural needle may cause laceration of the dura and subsequent placement of the catheter in the subdural space. In some cases the placement of the epidural catheter is difficult, and anesthetists often rotate the needle slightly. Such rotation may also lead to dural laceration and increase the chances of epidural block. Finally, previous back surgery and recent lumbar puncture may also increase the risk of this complication.

Fortunately, vast majority of patients with inadvertent subarachnoid block do not develop serious symptoms, and treatment is mostly supportive. Local anesthetic eventually wears off and the symptoms resolve. Epidural catheter must be removed and re-inserted at different level, and further injection of local anesthetic must be done with care, constantly monitoring the patient. In rare cases severe weakness of respiratory muscles of the chest may develop; this may require respiratory support until the block resolves.

The most important measure of prevention inadvertent subdural block is to place the epidural needle and the catheter with care. Rotation of the needle once the epidural space is identified should be avoided when possible. Early detection of subdural block helps prevent its possible complication and worsening of symptoms. For this reason in cases of inadequate, asymmetrical, patchy, unexpectedly high or slow onset of the block the possibility of subdural block should be suspected immediately.

References:
1. D Agarwal et al. Subdural Block and the Anaesthetist. Anaesthesia and Intensive Care, 2010,Vol 38, No. 1.

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Dr. Eugene Smetannikov is a practicing anesthesiologist with the interest in obstetric anesthesia. He is the author of the most comprehensive book on the subject, The Truth About Labor Epidural