Michael J. LaRouere

Hearing preservation following suboccipital removal of acoustic neuromas

Advances in the diagnosis and intraoperative management of acoustic neuromas have greatly reduced the incidence of neurologic deficits following their removal. Ninety‐three patients underwent acoustic tumor removal during a 4½‐year period, and hearing preservation was attempted in 20 cases. Hearing was preserved in 65% of the entire series, and excellent results were obtained in tumors less than 1.5 cm. No patient with a tumor greater than 1.5 cm had serviceable hearing preserved when total tumor removal was performed. Two patients, one with neurofibromatosis and one with an acoustic neuroma in an only‐hearing ear, had planned partial tumor removal with preservation of hearing. Preoperative auditory brainstem response results were not predictive of postoperative hearing preservation. Intraoperative auditory brainstem response monitoring demonstrated that loss of wave V consistently correlated with loss of hearing postoperatively, whereas persistence of wave V (with a latency prolongation not exceeding 3.00 ms) was predictive of successful hearing preservation regardless of latency increases.

Recent Advances in Cochlear Blood Flow Measurements

Changes in blood flow to the inner ear have been thought to influence or underlie a number of cochlear diseases, including some forms of noise-induced hearing loss, sudden hearing loss, and Menieres disease. Recently, important advances have been made in two technologies for the study of cochlear blood flow. The first is in the area of vital microscopic studies of cochlear microcirculation, and the second is based on the introduction of laser technology in the form of laser Doppler flowmetry. In this report, measurements are given of changes in cochlear circulation caused by carbon dioxide breathing, intravenous phenylephrine injection, systemic hemodilution, positive end expiratory pressure, and direct electrical stimulation of the cochlea. From these changes, we observe that cochlear blood circulation responds to systemic blood pressure alterations and is subject to local flow control mechanisms. Linearity and speed of response of the laser Doppler instrumentation also are shown. These advances show promise for contributing to our knowledge of control mechanisms of inner ear blood flow and for revealing the influence of various pharmacologic agents of potential clinical value.

A comparison of laser Doppler and intravital microscopic measures of cochlear blood flow

Many inner ear disorders may be caused by alterations in cochlear blood flow (CBF). However, each measurement technique used to monitor CBF has limitations in examining the relationship between otopathologic states and blood flow. This study Investigates laser Doppler flowmetry (LDF) and its fundamental drawback: The unknown relationship of LDF output to actual CBF. LDF readings are directly compared with concurrent intravital microscopy (IVM) measures of erythrocyte velocity in the lateral wall of the guinea pig cochlea. Positive end expiratory pressure, spontaneous respiration of 5% and 10% carbon dioxide, phenylephrine, and direct electrical stimulation of the cochlea were used to manipulate CBF. High, positive correlations were found between simultaneous LDF and IVM measurements of CBF. In addition, the study demonstrated that current microdissection techniques used to perform IVM do not cause changes in CBF. IVM measurements of CBF are a more sensitive indicator of CBF changes than are LDF measures. Despite the high correlation between measurement techniques within a single manipulation, simultaneous LDF and IVM measurements differed between manipulations. This may reflect regional changes in CBF affected by these manipulations and differences in the sampled vascular beds contributing to these two measures. It is unlikely that a single calibration factor can be defined that would allow the conversion of LDF output to actual units of blood flow across different manipulations used to alter CBF.

Tumors and Pseudotumors of the Endolymphatic Sac

This article reports on the presentation, diagnosis, management, and treatment outcomes of lesions of the endolymphatic sac in patients treated at a tertiary neurotology referral center. It summarizes survival results in the largest series groups and presents a new diagnostic entity of pseudotumor of the endolymphatic sac. The study includes retrospective review of all patients diagnosed with lesions of the endolymphatic sac within our practice between 1994 and 2005 as well as review of the literature. The primary outcome measure was survival, and the secondary outcome measure was disease-free survival following definitive resection. Postoperative complications were assessed. Survival characteristics of the largest reported case series groups were reviewed. Five cases of endolymphatic sac lesions were identified. Of these, three were true endolymphatic sac tumors and two were inflammatory pseudotumors of the endolymphatic sac. All three of the endolymphatic sac tumors patients survived (100%), and two of the three had disease-free survival (67%). Two of three patients maintained persistent facial paresis postoperatively. Both patients with benign pseudotumors survived (100%). Our study concluded that endolymphatic sac tumors are rare neoplasms of the temporal bone that, although locally aggressive and invasive, have excellent prognosis for survival with complete resection. We report a new entity of pseudotumor of the endolymphatic sac that mimics true sac tumors in every respect on presentation but which is non-neoplastic in origin.

Acute perilymphatic perfusion of the guinea pig cochlea

A method for the continuous perfusion of the perilymphatic space of the inner ear in the guinea pig is described. Artificial perilymph is supplied to the cochlea and drained away through a tubing system while flow rates from 10 microliters/min to 0.3 ml/min are established by gravity syphon pressure. Techniques are also presented which allow control over the temperature of the perfusate and over the level of dissolved oxygen in the perfusate. Alone with these variables, the pH of the artificial perilymph can be manipulated and various drugs can be added to the perfusate to test their effect on the inner ear. The function of the inner ear is monitored by continuous recording of the sound evoked bioelectric potentials, the cochlear microphonic and the compound action potential. The cochlear perfusion technique has many applications in the study of cochlear physiology and metabolism, and in testing the sensitivity of the inner ear to ototoxic drugs.

Three-Dimensional X-Ray Computed Tomography of the Temporal Bone as an Aid to Surgical Planning

Three-dimensional x-ray computed tomography (3DCT) has been useful in planning surgical procedures involving craniofacial reconstruction, the pelvis, hip, spine, knee and shoulder. The clinical use of 3DCT in temporal bone surgery has not been evaluated. We used 3DCT to assess temporal bone anatomy in 9 patients. These cases evaluated two encephaloceles, two postinfiammatory dehiscences, two temporal bone fractures, one glomus jugulare tumor, one acoustic neuroma, and one meningioma. We found 3DCT useful in these temporal bone applications as a supplement to standard two-dimensional CT scanning (2DCT). While 3DCT provided a better preoperative understanding of the underlying anatomy, no treatment was modified solely on the basis of the information derived from the 3-D presentation of data. We conclude that, although 3DCT images have some qualitative advantages over 2DCT techniques, their value in further defining temporal bone morphology must be evaluated against the cost entailed by their use.

Evaluation and management of a carcinoma arising in a thyroglossal duct cyst

Thyroglossal duct remnants are the most common congenital cystic lesions of the neck; however, a carcinoma arising in these structures is rare. Two new cases of a papillary adenocarcinoma arising in a thyroglossal duct cyst are presented. Preoperative evaluation, operative management, and postoperative care are discussed. Initial evaluation consisting of a thorough head and neck examination, palpation of the thyroid gland, thyroid function tests, and selective use of thyroid imaging is recommended. Removal of the cyst and tract in the manner described by Sistrunk is advocated. If an adenocarcinoma is found in the cyst and if a carcinoma is found in the thyroid gland or a thyroid scan reveals a nodule, a total thyroidectomy is recommended. A modified neck dissection and total thyroidectomy is advocated for cervical metastases. Postoperatively, thyroid suppression and long-term follow-up are encouraged. A squamous cell carcinoma arising in a thyroglossal duct remnant appears more aggressive and requires complete excision and, for confirmed cervical metastases, radical neck dissection and postoperative radiation therapy.

Endoscopic vascular decompression.

Objective: This article describes the technique and reports the results of endoscopic vascular decompression (EVD) in patients with trigeminal neuralgia (TGN), hemifacial spasm (HFS), and cochleovestibular nerve compressive syndrome. Study Design: Retrospective case review. Subjects and Methods: This study evaluates the outcome and length of stay (LOS) of 20 patients who underwent EVD for vascular compressive disorders from 2005 to 2007. It also evaluates LOS in 41 patients who underwent traditional microvascular decompression (MVD) by the same surgeons from 1999 to 2004. Results: Eighty-six percent (12 of 14) patients had resolution of their TGN, and 80% (4 of 5) had resolution of their HFS. There were no major complications. The EVD patients had an average LOS of 2.36 days as compared with 4.36 days for the MVD patient group (p < 0.001). Conclusion: Endoscopic vascular decompression for patients with vascular compressive syndromes such as TGN and HFS is a safe and equally effective procedure when compared with the traditional and widely successful MVD surgery, with the added benefit of less morbidity and shorter hospital stays.

The effects of carbogen, carbon dioxide, and oxygen on noise-induced hearing loss

An investigation into the effect of Carbogen (95% O2/5% CO2), 5% CO2/air, and 100% oxygen on cochlear threshold shifts caused by noise was undertaken. Five groups of eight pigmented guinea pigs were exposed to 105 dB broad band noise for 6 h per day for five consecutive days with each group receiving the various gaseous mixtures either during noise exposure or for 1 h immediately after noise exposure. A control group received the same noise exposure but respired air. Auditory threshold shifts, as measured by the auditory evoked brainstem response, were measured at 2,4,8,12,16, 20 and 24 kHz. Recordings were taken pre-exposure and at Day 1, 3, 5, and Weeks 2 and 3 after noise exposure. Carbogen, given during noise exposure, resulted in a trend toward less post noise exposure threshold shift (as compared to controls) which reached statistical significance by Week 3 at all frequencies except 2 and 20 kHz. Subjects given Carbogen after exposure also showed a general trend toward decreased noise induced threshold shifts, as compared to controls, but this was not statistically significant. The mixture of 5% CO2/air given during noise exposure yielded no difference in threshold shifts as compared to controls. When 100% oxygen was administered during noise exposure, a marked decrease in noise induced threshold shifts could be seen as compared to controls, with differences reaching statistical significance by day 5 at most frequencies. These results indicate that oxygen (i.e. cochlear-oxygenation) is a more important factor than CO2 (i.e., as a vasodilator) in protection of the cochlea from noise induced damage.

Electrically stimulated increases in cochlear blood flow: I. Frequency and intensity effects.

Charge-balanced, sinusoidal current was passed differentially between the apex and round window of the guinea pig cochlea. Cochlear blood flow was measured using a laser Doppler flow monitor. Systemic blood pressure was monitored from a cannula within the common carotid artery. Electrical stimulation increased cochlear blood flow, while systemic blood pressure was unaffected. A cochlear blood flow response parameter, normalized for transient changes in systemic blood pressure, was defined. The magnitude of the response parameter was found to be frequency selective and was also found to be an increasing function of current intensity, with maximum responses obtained with 500 Hz sinusoids. This cochlear blood flow response was not observed in dead animals; was present in preparations paralyzed with gallamine hydrochloride; and was correlated with an increase in cochlear red blood cell velocity, as directly observed by intravital microscopy. These observations imply that electrical stimulation induces a local vasodilation within the temporal bone. The fact that decreased cochlear blood flow was never observed with current injection implies that ischemia is not a likely mechanism of electrically induced tissue damage within the inner ear. The mechanism of this cochlear blood flow response is addressed in a companion report.