vestibular schwannoma interface

Otolaryngology Head and Neck Surgery Volume 115 Number 2

lin-like positive immunostaining could not be found in either the maculae or the cristae. 179

Ultrastructural Evaluation of Eighth Cranial Nerve/ Vestibular Schwannoma Interface MARCO CARNER, VITTORIO COLLEI-FI,ANDREA SBARBATI, and LORENZO PAClNI, Verona, Italy

Objective: The topic of the eighth cranial nerve/vestibular schwannoma interface has been investigated to assess the feasibility of surgery aimed at the preservation of hearing function in acoustic neurinoma. Methods: Twelve acoustic neurinomas removed en bloc, ranging from 8 to 21 mm in size, were examined under light and electron microscopes. These included the vestibular (12 o f 12) and the cochlear (3 of 12). Results: Electron microscopy revealed a capsule consisting of a collagen layer at the neurinoma/nerve interface. In 10 specimens, this structure was hypertrophic because of a nonspecific reaction process of the tumor, and in two cases a double membrane of Schwann cell derivation was observed. The fibers of the cochlear and vestibular nerves closer to the tumor presented features of nonspecific degeneration. In the sections of cochlear and vestibular nerve obtained medially in relation to the tumor, the fibers presented substantially normal morphology. The sections including Scarpa's ganglion exhibited a preserved morphology both in the ganglion cells and in the nerve fibers. Conclusions: In small to medium acoustic neurinoma, because ultrastructural examination makes it possible to identify a cleavage plane between the eighth cranial nerve and the neurinoma, conservative surgery should be tried for removal of the tumor. 180

The Role of Amplitude Modulated Signals for Acoustic Reflex Decay RAYMOND D, COOK, MSIV, MICHAEL O. FERGUSON, MSIV, JOSEPH W. HALL Ill, PhD, HAROLD C. PILLSBURYIII, MD, and JOHN H. GORSE, PhD, Chapel Hill, N.C.

The acoustic reflex is an aural feedback control system that is activated by relatively intense sounds. Temporal decay of this reflex provides the basis of an objective audiologic test that differentiates cochlear from retrocochlear pathologies. The classic sign of a retrocochlear lesion is a rapid decay of the reflex under conditions of sustained pure-tone stimulation at frequencies below 1000 Hz. This restriction to lower frequencies is due to the fact that even normal ears show decay at higher frequency signals. It is unclear whether the acoustic reflex decay (ARD) seen in normal ears is related to frequency-specific channels or whether the critical variable is the timing information coded within the channels. This study seeks to clarify the issue by measuring ARD with the amplitude-modulated (AM) signals. The rationale is that, by superimposing on high-frequency stimuli tempo-

Scientific Posters

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ral structures that have low-frequency characteristics, it will be possible to ascertain whether resistance to ARD is determined solely by the frequency region of stimulation. Methods: This study examined ARD in 10 subjects with normal hearing and middle ear function, as determined by audiometry and tympanometry. The degree of ARD was measured for both modulated and unmodulated carrier frequencies of 500, 1000, 2000, and 4000 Hz. The AM stimuli had modulation rates of 50 to 400 Hz. A high-pass-filtered pulse-train stimulus was also used. Both ipsilateral and contralateral measures of ARD were obtained. The dependent variable was the half-life of the decaying reflex (D50%) over a 20-second stimulation interval. Results: In all 10 subjects, D50% was present for the 2000 and 4000 Hz unmodulated stimuli. AM of these stimuli diminished ARD for all subjects, and in 90% of the subjects ARD was not measurable for the modulated stimuli. D50% was not measurable for any of the seven subjects tested on the pulse-train stimulus. Conclusions: Amplitude modulation diminishes ARD. This suggests that resistance to ARD is mediated by the temporal information coded within the frequency channels of the cochlea rather than by the channels themselves. AM stimuli may allow the upper frequency limit of 1000 Hz to be extended, thus enhancing the sensitivity of the test. Further work is directed toward similar measurements in patients with confirmed lesions of the eighth cranial nerve. (Supported by the American Hearing Research Foundation.)

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Compllcatlons of the Slstrunk Procedure PANKAJ GUPTA, MD, and JOHN MADDALOZZO, MD, Chicago, IlL

Thyroglossal duct cysts are one of the most common congenital lesions located in the neck. Surgical excision performed in the manner described by Sistrunk is recommended because these lesions are prone to infection, ulceration, and infrequently, malignant degeneration. Although much has been written on the Sistrunk procedure with reference to a low rate of recurrence, discussion of its complications has been limited to case reports. A retrospective review of 45 pediatric patients who underwent the Sistrunk procedure for a preoperative diagnosis of thyroglossal duct cyst was performed. Patients had either a primary thyroglossal duct cyst or were operated on previously using technique other than that described by Sistrunk. Preoperative evaluation was obtained using ultrasound, CT, or radionuclide scanning. Complications were separated into major and minor complications. Major complications were defined as death or situations requiring hospital admission and/or return to the operating room. Minor complications included local wound infection, stitch abscess, and seroma. There were no major complications or recurrence. The minor complication rate was approximately 20%. We conclude that the Sistrunk procedure, as a definitive surgical treatment for thyroglossal duct cysts, is associated with minimal morbidity.