Parthasarathy D. Thirumala, MD

1) Predictive value of somatosensory evoked potential monitoring during resection of intraparenchymal and intraventricular tumors using an endoscopic port.

Somatosensory evoked potentials (SSEP) have been used during skull base, spine and brain tumor operations to predict and reduce post operative neurologic deficits. Intraoperative monitoring of SSEPs allows for the dynamic assessment of the spinal cord dorsal columns, the brainstem medial lemnsical pathways to the thalamus, and thalamic connections to the primary sensory cortex. Persistent changes in the SSEPs during these procedures can predict post operative neurologic deficits. Intraoperative restoration of SSEP changes may prevent post operative neurologic complications. Regional and hemispheric changes of cerebral blood flow could potentially be identified with SSEP monitoring and thereby prevent neurovascular injury. Additionally, SSEP monitoring can evaluate potential peripheral injury related to neck position, brachial plexus injury or peripheral nerve injury during surgery. 

Endoscopic port surgery (EPS) presents an alternative approach for resection of deep tumors. The port itself is a transparent cylindrical retractor measuring 11.5 mm in diameter; the length of the port is tailored to the depth of the resection. The port is introduced into the brain over a bullet shaped dilator. In contrast to traditional microsurgical deep tumor resection, this technique attempts to limit retraction and dissection injury while improving the field of view at deep sites The endoscopic port has been used to resect tumors within deep white matter and the ventricles. Our study will be the first to evaluate the utility of intraoperative SSEP monitoring to prevent postoperative neurologic deficits after endoscopic resection of brain tumors.

2) Predicting hearing loss using brainstem auditory evoked potentials during microvascular decompression of cranial nerve VII for hemifacial spasm.

Microvascular decompression (MVD) of the cranial nerve (CN) VII is an effective treatment for hemifacial spasm (HFS). Hearing loss (HLS) is a well documented complication after MVD of CN VII for HFS. There are multiple mechanisms of damage to the cranial nerve VIII during the MVD including stretching of the CN VII when retracting cerebellum, manipulation of the labyrinthine artery and/or antero-inferior cerebellar artery, direct trauma due to instrumentation (iatrogenic injury), and venous injury. The incidence of HLS loss after MVD for HFS with the use of BAEP monitoring is 2.3 - 12.3. This variation is primarily due to different criteria used to evaluate HLS. The AAO-HNS criteria classifies HLS based on change in pure tone audiometry (PTA) and speech discrimination scores (SDS) and determines if further evaluation of “serviceable hearing” with hearing aids is warranted. BAEPs monitoring during MVD has demonstrated a decreased incidence of post-operative HLS when compared to series of patients who underwent MVD without intraoperative monitoring. Although multiple references to changes in BAEP waveforms and its correlation to HLS exist12, most have suggested that complete loss of waveform of BAEPs is the only reliable predictor of HLS. This latter information might be too late to help the surgeon manipulate or modify the procedure. Moreover, the neurophysiologist typically uses an arbitrary combination of increase in latency and /or a decrease in the amplitude of the wave V (AwV) of BAEPs, compared to baseline, as an alarm criteria to inform the surgeon about impending iatrogenic injury. Specific intraoperative BAEP waveform changes, however, can serve as more sensitive and earlier predictors of HLS and can eventually be implemented in the neurophysiologist’s alarm criteria for potential HLS. To date, no study has correlated specific intraoperative changes in latency, interpeak latency and amplitude of BAEP waveforms to pre and post operative audiogram data based on AAO-HNS criteria for HLS. Our current work intends to address the issue on the data collected and analyzed on patients who had their MVD for HFS at University of Pittsburgh medical Center.

3) The incidence of high frequency hearing loss after microvascular decompression for hemifacial spasm.

The auditory nerve is at risk during microvascular decompression (MVD) for hemifacial spasm (HFS). Although hearing loss (HL) has been reported following MVD surgery, there are no studies focused on high frequency hearing loss (HFHL) in patients who underwent MVD. Patients with HFHL often miss out on high-frequency components of speech, such as consonant sounds, and can have difficulties understanding speech in the presence of background noise. It has been proposed that higher frequencies may be more sensitive than lower frequencies to noise, acoustic trauma or ototoxic substances, thus early diagnosis of hearing loss in these frequencies may predict HL in lower frequencies. For the purposes of this article, we defined high frequencies as 4 kHz and 8 kHz, and an increase > 10dB in the pure tone audiometry to constitute HL. The primary aim of this study was to evaluate the incidence and discuss the pathogenesis of HFHL after microvascular decompression for hemifacial spasm.