Sorry, but pneumocephalus does not mean filling the entire cranial vault with air. 1cc of air can easily travel subarachnoid during a noted or unrecognized dural puncture, and cause a whopping headache for many days.
Below is part of a monograph I wrote on the subject:
While it is difficult to appreciate the incidence of unintended subarachnoid injection due to a paucity of studies on the subject, there is some evidence the incidence of unrecognized subarachnoid/subdural placement up to 7% (635). The dural puncture rate is as high as 3.6% in the obstetrical population undergoing epidural labor analgesia. (838). One study using epidural access for placement of steroids had a 4% recognized dural puncture rate and a 2% unrecognized dural puncture rate for a total of 6% rate (600).
The other issues regarding inadvertent subarachnoid puncture relates to the injection of undesired substances into the cerebrospinal fluid, and the complications of the dural puncture itself. The loss of resistance technique is the most common method of locating the epidural space when a blind, non-fluoroscopic guided technique is employed. Frequently, air is used as the injectate, although by 2006 in the UK, the percentage of anesthesiologists using air had fallen to 25% (820). In Spain, the use of air loss of resistance in 2005 was 59%. (821). In the US, air loss of resistance remains commonly employed. The use of air instead of liquid for determining the loss of resistance causes a significant increase in dural punctures (647). If air is inadvertently injected subarachnoid or subdural, the results may range from mild side effects to catastrophic. Since interlaminar epidural injections are frequently performed without the benefit of diagnostic fluoroscopy with contrast study, the actual location of any air injection is speculative. However, a strictly epidural injection would be very unlikely to cause a pneumocephalus due to the termination of the epidural space at the foramen magnum. Certainly a subarachnoid injection of air could rapidly ascend to the brain, causing severe headache with radiological evidence of pneumocephalus. This complication has been reported on primary epidural access using the air loss of resistance technique for labor epidural analgesia (822, 836). In the first case, after 4ml air injected, it was noted CSF effluxing from the needle, and there was an immediate onset of a severe occipital headache that required seven days before spontaneous resolution. Pneumocephalus has also been reported with air loss of resistance during an attempted epidural blood patch as a treatment for a post lumbar dural puncture headache. The onset of symptoms was immediate and was associated with a positive brain CT for subarachnoid air. There was no clinical evidence of a new dural puncture during the air loss of resistance location of the epidural space. due to a previous dural puncture. (823). There are other reports of pneumocephalus after attempted epidural access (837). A large study (n=3,730) demonstrated a 7 times higher incidence of post dural puncture headaches when using air for the loss of resistance compared with saline.(839).
Other more severe complications from air injection during air loss of resistance location of the epidural space include generalized convulsions and loss of consciousness associated with pneumocephalus (824), paraplegia due to nerve root displacement from injected peridural space air (825), and in a review by Saberski spinal cord compression, retroperitoneal air, subcutaneous emphysema, and venous air embolism. (826)
Saberski and others (827) have called for the end of air loss of resistance usage with the substitution of saline loss of resistance that is devoid of these complications, yet the former technique inexplicably persists with significant numbers of uses today.