The classic anatomical description of the subdural space is of "a potential cavity between the dura and arachnoid mater, containing a small volume of serous fluid" (1). The space runs up from the lower border of the second sacral vertebra into the cranial cavity as high as the diaphragma sellae in the floor of the third ventricle (2) but does not communicate with the subarachnoid space. It is continued onto the cranial and spinal nerves for a short distance. The space appears to be widest in the cervical region (3).

Contemporary anatomists have suggested, however, that the spinal subdural space is not a "potential" space at all, but that it occurs as a result of tissue damage which creates a cleft in this area of the meninges (4). These anatomists contend that there is no natural space between the arachnoid barrier cell layer and the dural border cell layer. When tissue damage occurs, it results in a cleaving open of the dural border cell layer.

The force required to enter the subdural space or produce this cleavage is usually small. Blomberg (5) reported little difficulty in inserting an endoscope and viewing the cavity in the majority of his autopsy studies.

Entry into the subdural space in the course of attempted epidural block appears to result from:
1. the bevel of an epidural needle perforating the dura, particularly if the bevel has been rotated in the epidural space (6),
2. invasion by an epidural catheter at the time of its insertion, or
3. subsequent migration of the catheter.

The subdural space is well known to radiologists as a place to go astray when attempting subarachnoid contrast injection (6, 9). This is particularly likely to occur following previous subarachnoid block, or lumbar puncture, and is recognized on fluoroscopic screening by the sluggish flow of contrast away from the injecting needle, which is little improved by tilting the patient.

Injection of local anaesthetic into the subdural space usually results in an unexpectedly high level of sensory block - as little as 3.5 ml bupivacaine 0.5% has produced a block as high as the C5 level (7). The high sensory block is usually evident between 10 and 35 minutes following injection (Chapter 8). It may spread intracranially producing apnoea and unconsciousness. Accompanying hypotension is gradual in onset and systolic blood pressure is rarely below 60 mm Hg. (8).

References:

1. GRAY'S ANATOMY. Neurology pp 1044-1102 33rd edition, Editors: Davies DV, Davies F. London,Longmans, 1964

2. Jones MD, Newton TH. Inadvertent extra-arachnoid injections in myelography. Radiology 1963 80:818-821.

3. Mehta M, Maher RM. Injection into the extra-arachnoid subdural space. Anaesthesia 1977 32:760-766.

4. Haines DE. On the question of a subdural space. The Anatomical Record 1991 230:3-21.

5. Blomberg RG. The lumbar subdural extra-arachnoid space of humans; an anatomical study using spinaloscopy in autopsy cases. Anesthesia and Analgesia 1987 66:177-180.

6. Schultz EH, Brogdon BG. The problem of subdural placement in myelography. Radiology 1962 79:91-95.

7. Brindle-Smith G, Barton FL, Watt JH. Extensive spread of local anaesthetic solution following subdural insertion of an epidural catheter during labour. Anaesthesia 1984 39:355-358.

8. Collier CB. Accidental subdural block: four more cases and a radiographic review. Anaesthesia and Intensive Care 1992 20:215-232.

9. Reynolds F and Speedy HM. The subdural space: The third place to go astray. Anaesthesia 1990: 45:120-123