Deep brain stimulation (DBS) is increasingly being used for treatment of movement disorders associated with Parkinson's disease (PD). The procedure involves functional lesioning of subthalamic nucleus (STN) and globus pallidus internus (GPI) after placement of stimulator leads via burr hole craniotomies. The procedure is usually performed under monitored anaesthesia care with minimal or no sedation. During these procedures we use bispectral index (BIS) monitoring regularly along with the routine monitors because it may reflect the hypnotic effect of anaesthetics and sedative drugs. Various complications have been reported associated with DBS surgery  although tension pneumocephalus has never been reported. We report a case where a patient undergoing DBS surgery developed tension pneumocephalus associated with lowering of BIS value intraoperatively.
A 63-yr-old male weighing 58 kg was scheduled for magnetic resonance imaging guided, stereotactic frame based bilateral placement of electrodes for DBS. His past medical history was significant for coronary artery disease 6 yr previously. A 2-D echocardiogram showed concentric left ventricular hypertrophy with mild aortic regurgitation along with a left ventricular ejection fraction of 45%. The patient was kept fasting overnight. The first part of the procedure, that is electrode placement, was done under monitored anaesthesia care. In the operating theatre intravenous access was secured using an 18-G cannula on the dorsum of the left hand. Monitoring included 5-lead ECG, non-invasive blood pressure, pulse oximetry, and A-2000 BIS monitor (Aspect Medical Systems, Newton). The BIS sensor was placed on the right side of the forehead. Supplemental O2 was administered via nasal prongs at a flow rate of 2L min−1. The patient was positioned supine on the operating table in a stereotactic frame with 10° head up tilt. At the initiation of surgery, the heart rate and blood pressure were 64 beats min−1 and 130/56 mmHg, respectively and BIS was 98. After local anaesthetic infiltration, burr holes were made bilaterally. Electrodes were placed on both the subthalamic nuclei. The patient's responses to the test stimulation as assessed by the neurologist were satisfactory. This part of procedure lasted approximately 5 h. It was then noticed that the patient had became drowsy but was still obeying to verbal commands. The BIS monitor showed a value of 75–85.
The second part of the DBS surgery was placement of a battery in the anterior chest wall for further regulation of the stimulations. This was performed under general anaesthesia. General anaesthesia was induced with fentanyl 2 μg kg−1 and thiopentone 200 mg intravenously. Tracheal intubation was facilitated with rocuronium 60 mg intravenously. Anaesthesia was maintained with isoflurane in a mixture of O2 and N2O (1:2). BIS was maintained at 40–60. The procedure was completed uneventfully in 30 min. At the end of the procedure the patient was reversed from residual neuromuscular blockade with neostigmine and glycopyrrolate. As the patient was maintaining adequate tidal volume and responding to verbal commands, though drowsy, the trachea was extubated. After 15 min, he was still drowsy with a BIS of 65–75. Oxygen was given by facemask and the patient was transferred to the intensive care unit (ICU). The drowsiness persisted even after 1 h in the ICU so a computed tomographic (CT) scan was performed which revealed a tension pneumocephalus (Fig. 1). As the patient was haemodynamically stable with no lateralizing signs, no active intervention was considered except close monitoring. BIS was continuously monitored in the postoperative period. For the initial 10 h after surgery, BIS remained in the range 65–80. Later, it started to rise gradually coinciding with improving conscious level. The patient was moved to the postoperative ward after 18 h fully orientated and with a BIS value of 98. A CT scan 18 h after surgery showed a complete resolution of the pneumocephalus.
Pneumocephalus has been known to occur after any craniotomy procedure with an incidence reported to be 100% following supratentorial craniotomies . Generally it is asymptomatic but occasionally high pressure may build up in the air cavity with development of tension pneumocephalus. Although tension pneumocephalus is more common after posterior fossa or cervical spine surgery in the sitting position, it is rare following the supine position. It may manifest as deterioration of consciousness with or without lateralizing signs, severe restlessness, generalized convulsions or focal neurological deficits. Several contributing factors have been implicated in the development of tension pneumocephalus. Prominent among them are surgical position of the patient, duration of surgery, gross hydrocephalus, excessive cerebrospinal fluid (CSF) loss, a functional VP shunt, nitrous oxide anaesthesia, and intraoperative administration of dehydrating agents like mannitol and furosemide .
Perioperative complications associated with DBS surgery include haemorrhage, seizure, confusion  and venous air embolism  but tension pneumocephalus has never been reported. During DBS surgery, there occurs a slow but continuous egress of CSF from the cranial burr holes. However, due to the prolonged duration of surgery, the loss of CSF may become significant over a period of time. The space created by the CSF loss may become filled with air causing pneumocephalus. This seems to be the likely cause of development of pneumocephalus in our case. It is also likely that during closure of the dura mater, subdural injection of saline to fill such space was insufficient.
We believe that the drowsiness along with resultant fall in BIS value could have been due to pneumocephalus. It is also suggested that the use of nitrous oxide during general anaesthesia for the placement of the battery could have further expanded the collected air resulting in development of tension pneumocephalus. This manifested as persistent drowsiness with a low BIS value in the postoperative period. It is probable that over a period of time, the accumulated air became reabsorbed, resulting in improved BIS values with simultaneous improvement in neurological status.
Diagnosis of pneumocephalus requires a high index of suspicion in DBS surgery. We believe the persistent low BIS after extubation indicated a neurological deterioration. The treatment of tension pneumocephalus requires burr-hole aspiration . No active intervention was required in our patient, as he remained haemodynamically stable with no further neurological deterioration. However the patient was monitored closely.
BIS monitoring in neurosurgical procedures has not gained popularity due to difficulty in placement of sensors in the recommended location. Nevertheless we believe that this monitoring can provide valuable information in diagnosing neurological events as literature has shown its utility in diagnosing global cerebral ischaemia during asystole  and other forms of localized cerebral ischaemia and brain injuries . Sen and colleagues highlighted the usefulness of BIS for detection of cerebral ischaemia secondary to vasospasm in neurointerventional procedures . In our case, the patient had frontal pneumocephalus, and the BIS monitor displayed readings that correlated with the neurological status of the patient.
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