:Acta Nfurochirurgica

Acta Neurochir (Wien) (1986) 83:112 115

9 by Springer-Verlag1986

Pneumocephalus and Tension Pneumocephalus After Posterior Fossa Surgery in the Sitting Position: a Prospective Study N. Di Lorenzo 1, R. Caruso 1, R. Floris 2, V. Guerrisi 2, L. Bozzao 2, and A. Fortuna 1 Sections of 1Neurosurgery and 2Neuroradiology, Department of Neurological Sciences, University of Rome, Rome, Italy

Summary Thirty patients subjected to posterior fossa surgery in the sitting position were evaluated postoperatively by CT scans in order to assess the true incidenceofpneumocephalus and subsequent development, if any, of tension pneumocephalus. This prospective survey shows that the postoperative incidence of intracranial air was t 00%, while no progression towards tension pneumocephalus producing severe neurological deterioration was observed. It can thus be claimed that, although the sitting position may favour the development of tension pneumocephalus also other factors contribute to its pathogenesis. Finally, neither nitrous oxide nor temperature play any significant role in the development of this complication.

Keywords: Posterior fossa surgery; sitting position; tension pneumocephalus.

Introduction There is n o d o u b t that the sitting p o s i t i o n in n e u r o s u r g i c a l o p e r a t i o n s involving the posterior fossa offers the considerable a d v a n t a g e s of good surgical exposure, i m p r o v e d cerebrospinal fluid (CSF) a n d v e n o u s d r a i n a g e a n d better p a t i e n t access. O n rare occasions these a d v a n t a g e s can be offset by several adverse effects, some of which are serious a n d potentially fatal, such as air embolism, p o s t u r a l hypot e n s i o n a n d i n t r a c e r e b r a l h a e m o r r h a g e remote from the operative site s, 6, 29 A further c o m p l i c a t i o n of the sitting position has been identified in recent years, n a m e l y the large intracranial a c c u m u l a t i o n of air u n d e r pressure, i.e., t e n s i o n p n e u m o c e p h a l u s , which results in progressive obt u n d a t i o n , with or w i t h o u t lateralizing signs 3, 7, 9, 10, 12--14, 19, 21 I n practice, however, the incidence of p n e u m o c e p h alus after c r a n i o t o m y with the p a t i e n t in the sitting position, a n d the s u b s e q u e n t d e v e l o p m e n t , if any, of t e n s i o n p n e u m o c e p h a l u s a n d its pathogenesis has yet to

be assessed. The present study therefore has a twofold aim: on the one h a n d , to evaluate the true incidence of p n e u m o c e p h a l u s after c r a n i o t o m y with the p a t i e n t in the sitting p o s i t i o n and, o n the other, to discuss the u n d e r l y i n g m e c h a n i s m a n d the predisposing factors involved in its progression towards tension pneumocephalus.

Materials and Method Thirty patients, aged from 2 to 72years, with neoplastic pathology (9 medulloblastomas, 8 astrocytomas, 3 acoustic neurinomas, 2 ependymomas, 2 angioreticulomas, 2 carcinomatous metastases), and dysraphic (2 Arnold-Chiari malformations) or vascular pathology (2 spontaneous haematomas) of the posterior fossa and operated on in the sitting position, underwent a postoperative computed tomography (CT) scan in order to ascertain the presence of any intracranial air. Preoperative hydrocephalus was present in 21 cases, while the ventricular size was normal in the remaining 9, including 3 patients previously having had a CSF shunt. In hydrocephalic cases a ventricular drainage was inserted immediately prior to opening the posterior fossa and kept clamped during the operation. The ventricular catheter was then maintained open at 15 cm above head level for the first 48 hours after the operation. Anaesthesia consisted of a balanced technique of barbiturate, narcotic, muscle relaxant and controlled ventilation with 60% nitrous oxide in oxygen in 20 cases and 60% air and oxygen in 10 cases. Dehydrating agents (mannitol and/or furesemide) were never used peroperatively in these cases. All the patients underwent a CT scan within 12 hours postoperatively, depending on available radiological facilities. Whenever a certain amount of air was detected, a further CT scan was scheduled 24 hours after the first and, in some cases, repeated at longer intervals. A total of 62 CT scans were performed. The CT scans were reviewed to detect the presence and location (i.e., subdural, cisternal and intraventricular) of the air. Furthermore, the amount of air was defined as "non significant" (Fig. 1) or "significant" (Fig. 2) according to a subjectiveevaluation of the compression of the frontal horns

N. Di Lorenzo et al.: Pneumocephalus and Tension Pneumocephalus After Posterior Fossa Surgery in the Sitting Position

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Table 1. Amount and Location o f Intracranial Air in 30 Patients Subjected to Posterior Fossa Surgery in the Sitting Position 1

Significant air collection: 7 cases

Non significant air collection: 23 cases

Location of air

No. of cases

Location of air

No. of cases

Subdural Cisternal Intraventricular

72 2 3

subdural cisternal intraventricular

20 5 16

1 Most of the cases presented air in more than one CSF compartment. 2 Two cases presented mild neurological depression without lateralizing signs. Fig. 1. Representative case of a pneumocephalus defined as "non significant". Air collection in the subdural and intraventricular compartment

In the seven cases with significant p n e u m o c e p h a l u s it was found that nitrous oxide had been used in only two. Clinically speaking, only 2 cases (1 hydrocephalic patient and 1 with preexisting functional shunt) with significant air collection experienced confusion and somnolence without focal deficit during the first 4 postoperative days. The subsequent i m p r o v e m e n t in their clinical conditions coincided with the spontaneous reabsorption of the air. The postoperative course was completely a s y m p t o m a t i c in the remaining 28 patients.

Reabsorption of the Air

Fig. 2. Representative case of a pneumocephalus defined as "significant". Compression of the frontal lobes by subdural air

and/or frontal lobes. Likewise, from the clinical standpoint, any further symptoms such as generalized neurological depression and/or new focal deficit were noted.

Results The CT scan revealed the presence of air in all 30 patients, although the actual quantity varied considerably. The air was subdural in 27 cases, intraventricular in 19 and cisternal in 7. In m o s t cases the air was present in m o r e than one C S F c o m p a r t m e n t although the larger collections o f air were usually found in the subdural space. P n e u m o c e p h a l u s was deemed significant in 7 cases and nonsignificant in 23 cases (Table 1).

In the cases evaluated with serial C T scans, the time required for the air to be reabsorbed was directly p r o p o r t i o n a l to its quantity. Air collection defined as n o n significant was generally reabsorbed within one week and significant air within two weeks. In two cases, however, a very large collection of subdural and ventricular air lasted up to three weeks. Cisternal air a p p e a r e d to reabsorb m o r e quickly than subdural and intraventricular air.

Discussion An increasing n u m b e r of reports have recently appeared in the literature 3, 7, 9, 10, 12-14, 19, 21 concerning patients subjected to posterior fossa surgery in the sitting position and subsequently suffering from severe postoperative neurological deterioration with shift of the midline structures or antero-posterior cerebral distortion related to tension pneumocephalus. Although increasing our awareness of this potential complication of the sitting position and its treatment, these papers m a y also have led to an overestimation of its true incidence.

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N. Di Lorenzo et al.: Pneumocephalusand Tension PneumocephalusAfter Posterior Fossa Surgeryin the Sitting Position

The present study shows that there was 100% incidence of intracranial air after craniotomies performed with the patient in the sitting position, while no cases of severe or alarming neurological symptoms due to the tension effects of the air were observed. These data confirm the findings of Harders e t a l . 6. On the other hand, a certain degree of drowsiness and confusion without focal deficit was observed in two patients with significant pneumocephalus. This depressed level of consciousness could also have been unrelated to the intracranial air, although the improvement in the neurological status coincided with its reabsorption. Moreover, another 5 patients with significant pneumocephalus developed no symptoms that could be ascribed to it. The presence of air inside the intracranial cavity, i.e., pneumocephalus, after a neurosurgical procedure with the patient either in the sitting or the recumbent position is not an uncommon finding in postoperative skull roentgenograms or CT scans. In the sitting position, the decisive factor in the physiopathology of pneumocephalus 3, 12, 13, 19, 21 consists of gravity allowing the cerebral hemispheres to settle into the inferior cranial vault and the CSF to leak out via the wound, especially in hydrocephalic patients. Also, other brain volume reducing factors such as hyperventilation, intracranial dehydration, removal of a space-occupying mass, loss of brain substance, hydrocephalus, brain atrophy and the presence of active CSF drainage, can create or further increase an aircontaining space 1, 2, 4, 6, 8, 11, 15--17,2o, 21. Indeed, these factors can act either in the sitting position or the recumbent position. With closure of the surgical wound the air bubbles become entrapped intracranially. In the vast majority of cases however, the pneumocephalus thus formed is quite small, without clinical significance and tends gradually to reabsorb spontaneously. On the other hand, when several of the abovementioned factors act together they can create a very large air-filled space of poor compliance which reduces the total volume available for the constituents of the calvarium, i.e., CSF, blood and brain mass itself. During the immediate postoperative period, the reexpansion of the brain owing to rehydration and a certain amount of cerebral oedema due to surgery and the reaccumulation of the depleted CSF may compete with the intracranially entrapped air 3, 13, 21. This conflict will cause an increase in endocranial pressure. The diffusion of nitrous oxide into the entrapped air

owing to its greater solubility in blood and the fact that nitrous oxide has a higher partial pressure than nitrogen has also been implicated in further increasing the size of the pneumocephalus l, 8,11,13,16,22 and thus in its tendency to progress towards tension pneumoeephalus. However, nitrous oxide can diffuse out of the air-filled space as fast as it diffuses into it. Therefore, the pressure returns to its original level in less than 10 minutes 3,13, Is, 21 Indeed, Friedman et al. 3 speculate that high inspired nitrous oxide concentrations may in fact be beneficial prior to dural closure. Leunda e t a l . 10 claim that the ventricular system could act as an intracranial pressure buffering system by reducing its volume when subdural air comes under tension, thus acting as a protective factor for the brain parenchyma. As mentioned above, other factors when in summation can generate tension pneumocephalus even when hydrocephalus is absent and in patients operated on in the recumbent position. We personallyhave observeda similar case during the course of the present study. It consisted of a 35-year-oldmate with a huge olfactorygroovemeningiomaoperated on through a bifrontal flap in the recumbent position. In order to obtain a slack brain, mannitol, furesemideand lumbar drainagewereused. After surgerythe patient awakenedpromptlyfrom anaesthesia.Four hours postoperativelyhe becamelethargicand unresponsiveto verbalcommands, althoughno lateralizingneurologicaldeficitwas observed.ACT scanrevealedthe presence of a huge amount of bifrontaI subdural air (Fig. 3). The subdural collection was tapped percutaneously under sterile conditions and more than 45 ml of air under pressure was withdrawn. In the space of only a few minutes the patient became alert and was

Fig. 3. Severecompression of frontal parenchyma by subdural and intraventricular air

N. D i Lorenzo et al.: Pneumocephalus and Tension Pneumocephalus After Posterior Fossa Surgery in the Sitting Position responding appropriately to commands. A later CT scan disclosed only a few subdural air bubbles. The subsequent postoperative course was uneventful. 9. I n conclusion, this experience shows that the incidence of t e n s i o n p n e u m o c e p h a l u s after posterior fossa surgery in the sitting position p r o d u c i n g severe

10.

a n d a l a r m i n g neurological d e t e r i o r a t i o n is rare, at least when no peroperative d e h y d r a t i n g agents are a d m i n istered a n d when postoperative ventricular drainage is

11.

used. It is also i m p o r t a n t to realize that t e n s i o n p n e u m o c e p h a l u s is by n o m e a n s exclusively related to the sitting position a n d that the other c o n t r i b u t o r y

12.

factors discussed above can create c o n d i t i o n s facilitating its occurrence. Finally, neither n i t r o u s oxide n o r the w a r m i n g of the air from r o o m t e m p e r a t u r e to b o d y t e m p e r a t u r e play

13.

any

14.

relevant

role

in

the

development

of

this

complication. 15.

Acknowledgements We are grateful to Guido Giuliani, Saverio Giuliani and Vittorio Spano, technicians, for their kind and expert assistance.

16.

17.

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Authors' address: Dr. N. Di Lorenzo, Section of Neurosurgery, Department of Neurological Sciences, Viale dell'Universitfi 30, 1-00185 Roma, Italy.