Intracranial air on computerized tomography ANNE G. OSBORN, M.D., JONATHAN H. DAINES, M.D., S. DOUGLAS WING, M.D., AND ROBERT E. ANDERSON, M.D.

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1 J Neurosurg 48: , 1978 Intracranial air on computerized tomography ANNE G. OSBORN, M.D., JONATHAN H. DAINES, M.D., S. DOUGLAS WING, M.D., AND ROBERT E. ANDERSON, M.D. Department of Radiology, University of Utah College of Medicine, Salt Lake City, Utah Intracranial gas may be epidural, subdural, subarachnoid, parenchymal, or intraventricular. Intracranial air can be easily diagnosed and its location correctly assessed by computerized tomography. Potentially serious complications of intracranial air, such as tension pneumocephalus, can be rapidly and accurately identified, facilitating appropriate clinical therapy. KEY WORDS 9 computerized tomography 9 pneumocephalus 9 trauma C RANIAL computerized tomography (CT) has made possible the rapid, accurate identification of a wide variety of intracranial lesions. In many instances, a definitive diagnosis is possible on the basis of the CT scan alone. In others, the CT scan may provide additional information facilitating the management of complex neurological or neurosurgical problems. The detection of intracranial gas is such an instance. In the absence of prior diagnostic or surgical procedures such as lumbar puncture, pneumoencephalography, or craniotomy, the presence of intracranial gas is usually of serious clinical significance. In addition, intracranial gas may also be an unsuspected cause of intracranial mass effect and its clinical sequelae. We have performed over 6000 CT scans in our institution. Eighteen patients (0.3%) had intracranial gas (Table 1). We have selected scans from several of these cases to illustrate both the clinical significance and varied appearance of intracranial gas on CT scans. Summary of Cases The appearance of intracranial gas on CT scans is quite characteristic. Air appears as a region of very low attenuation (-1000 H) with a white rim surrounding the gas pocket (Figs. 1 and 6). This "halo" effect is a reconstruction artifact caused by the marked, abrupt change in attenuation between the air and surrounding cerebral parenchyma, a and should not be mistaken for an associated hematoma. EpMural Air Epidural air is uncommon. As with epidural hematomas, extradural intracranial air strips the tightly adherent dura from the inner space. Therefore, epidural gas usually appears as a biconvex air pocket that does not change with position (Fig. 2). Subdural Air Because the subdural compartment is a thin, slit-like potential space between the dura J. Neurosurg. / Volume 48 / March,

2 A. G. Osborn, J. H. Daines, S. D. Wing and R. E. Anderson TABLE 1 Clinical summary in 18 patients with intracranial air seen on computerized tomography scanning Case Age No. (years) Clinical History Radiographic and CT Findings mos mos 22 facial and basilar skull fractures 44 chronically draining right ear 38 bilateral burr holes & ventriculostomy 60 resection Pancoast tumor with postoperative pneumothorax & bronchopleural fistula 13 chronic massive hydrocephalus secondary to aqueduct stenosis; post-ventriculojugular shunt 69 facial trauma; postop removal of bilateral subdural hematomas 49 subdural empyema following removal of chronic subdural hematoma 48 previous pneumoencephalogram 24 It temporal & mastoid skull fracture 76 evacuation of bilateral subdural hematomas basilar skull fracture basilar skull fracture & evacuation of acute bilateral subdural hematomas postop removal of subarachnoid cyst carcinoma of breast; recent pneumoencephalogram postop removal It frontal parenchymal hematoma ethmoid fracture postop rt posterior communicating & anterior choroidal artery aneurysms evacuation of massive bilateral chronic subdural hematomas subarachnoid, subdural, & intraventricular air rt temporal air-containing abscess colloid cyst; large postop rt tension pneumatoma with rt-to-lt subfalcine shift of lateral ventricles massive bifrontal subdural pneumatomas with posterior shift of ventricles bifrontal subdural pneumatomas bifrontal subdural pneumatomas, rt-to-tt subfalcine shift of lateral ventricles air-containing subdural empyema intraventricular & subarachnoid air depressed skull fracture & subdural air bifrontal subdural pneumatomas with subdural hematoma; residual rt chronic rt-to- It shift of ventricles small bifrontal subdural pneumatomas bifrontal subdural pneumatomas subarachnoid air residual subarachnoid & intraventricular air small bifrontal subdural pneumatomas suprasellar subdural air small right tension subdurat pneumatoma bifrontal & supraseuar subdural pneurrmtomas FIG. 1. Case 1. Computerized tomography scans without contrast enhancement. Left,: Intraventricular air is identified as an area of low attenuation within the left temporal horn (arrow). Center: Subarachnoid air is seen as focal, non-confluent bubbles in the quadrigeminal plate and superior cerebellar cisterns (arrows). Right: Small bifrontal subdural air collections are present (arrows). Air is also seen within the frontal horn of the left lateral ventricle. Note the artifactual white "halo" surrounding the air collections. 356 J. Neurosurg. / Volume 48 / March, 1978

3 Intraeranial air on computerized tomography Fro. 2. Epidural mastoid pneumatocele. Left: Computerized tomography scan reveals a communication between the gas-filled mass and the mastoid air cells (arrow). Right: Higher scan shows a well localized, biconvex epidural air collection. Note the thinning of the adjacent calvaria. (Reproduced from Madeira JT, Summers GW: Epidural mastoid pneumatocele. Radiology 122: , 1977, with permission.) and arachnoid without natural adhesions, acute subdural air characteristically shifts with changing head position (Fig. 3). Subdural air may dissect along, and hence outline, the falx or tentorium. Subdural air commonly forms a well defined gravitational level with subdural fluid collections. If it is of sufficient size and is unilateral or asymmetrical, subdural "tension pneumocephalus" may develop, producing significant anteroposterior or subfalcine displacement of the cerebral hemispheres (Fig. 3). Subarachnoid Air Subarachnoid air is easily identified as small, nonconfluent bubbles of low attenuation conforming to the sulci and cerebrospinal fluid cisterns (Fig. 4). While subarachnoid gas may change position, the air pockets are nonconfluent and can thus usually be easily distinguished from subdural collections. One exception is a gas-forming subdural empyema, where the gas may become loculated within the thick empyema (Fig. 5). FIG. 3. Case 4. Bronchopleural fistula with pleural-subdural communication. Huge bifrontal subdural pneumatomas are present (black arrows). Note the marked posterior displacement of the cerebral cortex. Subdural air is also present over the tentorium (outlined arrows). J. Neurosurg. / Volume 48 / March,

4 A. G. Obsorn, J. H. Daines, S. D. Wing and R. E. Anderson FIG. 4. Computerized tomography scan following diagnostic pneumoencephalography. Left." Intraventricular air (arrows) forms a distinct gravitational level with the cerebrospinal fluid. Right." Higher level scan shows subarachnoid air within the cerebral sulci (arrows). Parenchymal Air Parenchymal gas is seen as an area of low attenuation lying within the cerebral substance. An air-fluid level may be present (Fig. 6). Intraventricular Air Intraventricular air is easily distinguished since it conforms to the ventricular spaces (Figs. 1, 4 left). Air-CSF levels can usually be identified. Since most CT scans are per- FIG. 5. Case 7. Subdural empyema. A massive right frontoparietal subdural empyema has displaced the lateral ventricles completely under the falx. Small loculated bubbles of subdural air (arrows) are trapped within the thick empyema. 358 J. Neurosurg. / Volume 48 / March, 1978

5 Intracranial air on computerized tomography air collection should alert the clinician to the likelihood of a basilar skull fracture and its potentially serious sequelae. Intracranial gas may be epidural, subdural, subarachnoid, parenchymal, or intraventricular. Intracranial air can be easily diagnosed and its location correctly assessed by CT scanning. Potentially serious complications of intracranial air, such as tension pneumocephalus, can be rapidly and accurately identified, facilitating appropriate clinical therapy. FIG. 6. Case 2. Right temporal lobe abscess. Computerized tomography scan without contrast enhancement shows a right temporal lobe mass with an intraparenchymal air pocket (arrows). formed in the brow-up position, intraventricular gas is most commonly present in the frontal and temporal horns. Discussion The introduction of air for diagnostic procedures such as pneumoencephalography, ventriculography, or lumbar puncture is the most common cause of pneumocephalus. 2 Intracranial gas has also been reported in association with fracture or infection of the paranasal sinuses or the petrous temporal bone, 5'6'8,9 cerebral abscess, 2a tumors of the sinuses or skull base, 4,1~ and following craniotomy or intraventricular drainage? 1 Intracranial gas has also been identified in the dead fetus, 2 and as a complication of penetrating skull wounds? Although moderate amounts of air can be easily identified on routine skull films, we have found CT scans of unique value in detecting very small amounts of intracranial air. Because of its extremely low attenuation coefficient, as little as 0.5 cc air can be readily identified by CT scans. In patients undergoing routine scanning for acute head trauma, the identification of even a small intracranial References 1. Alker GJ Jr, Oh YS, Leslie EV, et al: Postmortem radiology of head and neck injuries in fatal traffic accidents. Radiology 114: , Azar-Kia B, Sarwar M, Batnitzky S, et al: Radiology of intracranial gas. Am J Roentgenol Radium Ther Noel Med 124: , Davis KR, Taveras JM, Roberson GH, et al: Computed tomography in head trauma. Semin Roentgenol 12:5-62, Farooki WQ, Brodovsky DM, Verver D: Mucocele of the sphenoid sinus presenting as spontaneous pneumocephalus. J Otolaryngol 5: , Franklin G: Cranial pneumatocele. A clue to the diagnosis of occult epidural abscess. Arch Neurol 33: , 1976 (Letter) 6. Genieser NB, Becker MH: Head trauma in children. Radiol Clin North Am 12 (2): , Handel SF, Klein WC, Kim YW: Intracranial epidural abscess. Radiology 111: , Horowitz M, Ramsden RT, Block J: Traumatic pneumocephalus. J Laryngol Otol 90: , Madeira JT, Summers GW: Epidural mastoid pneumatocele. Radiology 122: , Sage MR, McAllister VL: Case report: Spontaneous intracranial "aerocoele" with chromophobe adenoma. Br J Radiol 47: , Witcombe JB, Torrens M J, Gye RS: Intracerebral pneumatocele: an unusual complication following intraventricular drainage in a case of benign intracranial hypertension. Nenroradiology 12: , 1976 Dr. Osborn is a James Picker Advanced Academic Fellow in Radiology. Address reprint requests to: Anne G. Osborn, M.D., Department of Radiology, University of Utah Medical Center, Salt Lake City, Utah J. Neurosurg. / Volume 48 / March,