a 75-year - old man with congestive heart failure and atrial fibrillation was transferred to our hospital for the management of dyspnea and coughing . he had received posterolateral fusion with pedicle screw fixation of l4 and l5 foramina 5 years previously . however , he suffered from sustained painful dysesthesia on the right leg and received follow - up decompression surgery for l4 and l5 foramina stenosis 2 days before admission to our hospital , during which the dura was torn and dural plasty was performed . on the 18 hospital day at the department of internal medicine he complained of nuchal discomfort with headache . after 3 days , a large egg - sized swelling on the wound site was detected without any symptoms or signs of infection . opening of the skin and subcutaneous tissue to place a drain resulted in csf flushing . severe headache developed when he raised his head , which was considerably relieved in the supine position although it did not completely disappear . brain mri with enhancement showed diffuse nonnodular pachymeningeal enhancement and massive pneumocephalus involving bilateral lateral ventricles , basal cisterns , sylvian cisterns , the left ambient cistern , retrocerebellar cisterns , cerebellar folia , the anterior interhemispheric fissure , and the subarachnoid space ( fig . a 54-year - old woman visited our hospital because of headache and vomiting without fever . two days before visiting our hospital , she received an epidural block in the sitting position by a midline approach for the management of back pain with radiating pain to her right leg . the air technique was applied to confirm that the needle was located in the epidural space , after which she felt that her head was being pulled to the ground for 10 minutes . bupivacaine ( 0.5% ) mixed with triamcinolone was injected at the level of l5-s1 after checking for the proper position of the needle in the epidural space . the next day she experienced the same headache twice while performing her daily routine . on the third day after the procedure 2 ) . besides focal pneumocephalus , mild subdural hygroma in both anterior frontal areas with slightly prominent adjacent dural enhancement was observed . brain ct performed 1 month later , mild subdural hygroma remained but the air had disappeared . a 75-year - old man with congestive heart failure and atrial fibrillation was transferred to our hospital for the management of dyspnea and coughing . he had received posterolateral fusion with pedicle screw fixation of l4 and l5 foramina 5 years previously . however , he suffered from sustained painful dysesthesia on the right leg and received follow - up decompression surgery for l4 and l5 foramina stenosis 2 days before admission to our hospital , during which the dura was torn and dural plasty was performed . on the 18 hospital day at the department of internal medicine he complained of nuchal discomfort with headache . after 3 days , a large egg - sized swelling on the wound site was detected without any symptoms or signs of infection . opening of the skin and subcutaneous tissue to place a drain resulted in csf flushing . severe headache developed when he raised his head , which was considerably relieved in the supine position although it did not completely disappear . brain mri with enhancement showed diffuse nonnodular pachymeningeal enhancement and massive pneumocephalus involving bilateral lateral ventricles , basal cisterns , sylvian cisterns , the left ambient cistern , retrocerebellar cisterns , cerebellar folia , the anterior interhemispheric fissure , and the subarachnoid space ( fig . a 54-year - old woman visited our hospital because of headache and vomiting without fever . two days before visiting our hospital , she received an epidural block in the sitting position by a midline approach for the management of back pain with radiating pain to her right leg . the air technique was applied to confirm that the needle was located in the epidural space , after which she felt that her head was being pulled to the ground for 10 minutes . bupivacaine ( 0.5% ) mixed with triamcinolone was injected at the level of l5-s1 after checking for the proper position of the needle in the epidural space . the next day she experienced the same headache twice while performing her daily routine . on the third day after the procedure besides focal pneumocephalus , mild subdural hygroma in both anterior frontal areas with slightly prominent adjacent dural enhancement was observed . brain ct performed 1 month later , mild subdural hygroma remained but the air had disappeared . pneumocephalus is defined as the presence of air within the cranial cavity and can occur after trauma , spinal surgery , craniofacial surgery , lumbar puncture , epidural steroid injections , gas - forming bacterial meningitis , valsalva 's maneuver , subarachnoid - pleural fistulae , and osteoma.1,4 - 10 in our first case , the ' inverted soda - bottle mechanism ' could have been responsible for the pneumocephalus 4,5 - when a bottle containing soda is inverted , air rushes in to equilibrate the pressure differential as soda pours out . similarly , as csf flows out of the subarachnoid space through a dural arachnoid tear , a negative pressure is created within the subarachnoid space and the pressure differential results in air being drawn from the atmosphere to the lower pressure intracranial space . this air moves along csf pathway - the subarachnoid space , cisterns ( enlargements of subarachnoid space ) , and the ventricular system - via two foramina of luschka and the foramen of magendie . in our second case there was no massive csf leakage , so the injection of air while attempting to identify the epidural space via the loss - of - resistance - to - air technique might have been the cause of pneumocephalus , as reported in other cases.6,9 - 11 such a loss - of - resistance - to - air technique is used to perform epidural block to locate the epidural space . during spinal tapping , resistance to the needle advancement after skin penetration is first encountered at the ligamentum flavum , and then the needle containing air arrives at the epidural space with a negative pressure ( mean of - 7 cm of water ) , where air aspiration occurs . since the dura is closely attached to the arachnoid , dural injury would rarely happen without damage to the arachnoid , and accidental dural puncture associated with the loss - of - resistance - to - air technique could cause pneumocephalus . whilst brain ct is a well - known diagnostic tool for detecting as little as 0.55 ml of intracranial air , gradient echo images are more sensitive . in fact , in our second case , brain ct was performed 1 day before brain mri . among brain ct , t2-weighted , and gradient echo mri images in our patients , the last showed more - exaggerated pneumocephalus because of a paramagnetic effect of the air ( fig . 2 ) . therefore , in cases of orthostatic headache , it is preferable to perform gradient echo images in order to facilitate the detection of pneumocephalus . follow - up brain ct performed 1 month later revealed that the air had been absorbed but mild subdural fluid remained . intracranial air tends to resolve spontaneously with rehydration , normalization of the co2 level , n2o resorption , and replacement of csf.5 neither of our two patients showed cerebellar tonsillar descent , descent of the optic chiasm , reduction in the prepontine space , or crowding of the posterior fossa on brain images , instead showing pneumocephalus and good prognoses . if the headache was relieved entirely by lying supine , it was identified as purely postdural puncture headache ; however , if it was unchanged by position , it was diagnosed as purely resulting from intrathecal air.2 other studies have similarly found that pneumocephalus caused a headache that was aggravated by any motion and was not relieved with lying down.1,9 however , our cases showed typical orthostatic headache despite the small degree of pneumocephalus , or more severe headache if a patient intended to raise his head , which was considerably relieved in the supine position . we have been concerned about the existence of posterior fossa crowding or subdural hematoma in patients complaining of orthostatic headache , which indicates the need for surgical intervention or a poor outcome.12,13 when presented with cases of orthostatic headache , clinicians should keep in mind not only posterior fossa crowding and subdural hematoma but also pneumocephalus .