Physiological mechanisms of onset adaptation and contralateral suppression of DPOAEs in the rat.

An investigation was undertaken to measure medial olivocochlear (MOC) reflexes in anesthetized rats before and after sectioning of the middle-ear muscles. Distortion product otoacoustic emission (DPOAE) magnitude and phase temporal responses were measured ipsilaterally to study MOC-mediated "DPOAE onset adaptation" and in the presence of a contralateral noise to study MOC-mediated contralateral "suppression" (terms as used by previous researchers). Distortion product otoacoustic emission onset adaptation and contralateral suppression had predictable changes in direction of magnitude and phase that were dependent on the input-output function. After sectioning of the middle-ear muscles (MEMs), DPOAE onset adaptation and contralateral suppression were greatly reduced, and there were little, if any, changes in phase. These "residual" changes were interpreted as a result of the MOC reflex. The results suggest that what appears to be DPOAE onset adaptation and contralateral suppression can be mediated primarily by MEM reflexes. When studying MOC effects on otoacoustic emissions (OAEs) using acoustic stimulation, it is necessary to make recordings over a span of stimulus levels. In addition, looking at both magnitude and phase of the OAE may help separate what is due to the MOC reflex from MEM reflex.

Olivocochlear neurons in the chinchilla: a retrograde fluorescent labelling study.

Although the chinchilla is widely used as a model for auditory research, little is known about the distribution and morphology of its olivocochlear neurons. Here, we report on the olivocochlear neurons projecting to one cochlea, as determined by single and double retrograde fluorescent tracer techniques. 10 adult chinchillas were anesthetized and given either unilateral or bilateral injections of a fluorescent tracer (either Fluoro-Gold or Fast Blue) into scala tympani or as a control, a unilateral injection into the middle ear cavity. The results indicate that there are similarities as well as significant differences between the chinchilla and other species of rodents in the distributions of their olivocochlear neurons. Based on three well-labelled cases, there was a mean total of 1168 olivocochlear neurons in the chinchilla. Of these, the majority (mean 787) were small, lateral olivocochlear neurons found almost exclusively within the ipsilateral lateral superior olivary nucleus. The next largest group consisted of a mean of 280 medial olivocochlear neurons virtually all of which were located in the dorsomedial peri-olivary nucleus. Chinchilla medial olivocochlear neurons were more predominantly crossed in their projections (4:1) than in any known species. The smallest group of olivocochlear neurons (mean 101) consisted of larger lateral olivocochlear neurons (shell neurons) which were located on the margins of the superior olivary nucleus and which projected mainly (2.2:1) ipsilaterally. Double retrograde labelling was observed only in medial olivocochlear neurons and occurred in only 1-2% of these cells. The results confirm previous findings which indicated a relative paucity of fibers belonging to the uncrossed as compared to the crossed olivocochlear bundle. This, together with the strong apical bias of the uncrossed projection reported previously, offers possible explanations for the apparent absence of efferent-mediated suppressive effects of contralateral acoustic stimulation in this species. Regarding the lateral olivocochlear system, the chinchilla is shown to possess both intrinsic and shell neurons, as in the rat.

Noradrenergic and serotonergic projections to the superior olive: potential for modulation of olivocochlear neurons.

The distribution and density of noradrenergic (NA) and serotonergic (5-HT) varicosities in the superior olive (SO) and periolivary region (PO) and their relationship to olivocochlear neurons was studied. Antibodies against 5-HT and the NA precursor enzyme dopamine beta-hydroxylase were utilized to examine the density of innervation of SO and PO. To determine the relationship of these varicosities to efferent neurons projecting to the cochlea, olivocochlear neurons were retrogradely labeled with biotinylated dextranamine (BDA). NA and 5-HT varicosities were found adjacent to labeled olivocochlear neuron cell bodies and dendrites. More than 50% of labeled medial olivocochlear (MOC) neurons showed likely contact with NA varicosities and more than 90% of labeled MOC neurons with 5-HT varicosities. There was no apparent difference in the number of lateral olivocochlear (LOC) neurons in close proximity to NA and 5-HT varicosities versus MOCs in close proximity to NA and 5-HT varicosities. Our results suggest that the NA and 5-HT systems are in a position to modulate auditory brainstem processing. The specific relationship of NA and 5-HT varicosities to olivocochlear neurons suggests that one possible level of modulation is prior to signal transduction.

Inferior colliculus stimulation and changes in 2f1-f2 distortion product otoacoustic emissions in the rat.

The external, central and dorsal subnuclei of the inferior colliculus (ICX, ICC, ICD respectively) have different patterns of descending projections to the periolivary region. We found that electrical stimulation of these subnuclei in anesthetized rats causes suppression of 2f1-f2 distortion product otoacoustic emissions (DPOAE). The responses in DPOAEs to stimulation of ICX and ICC are characterized by a large (7-25 dB), initial suppression which lasted 25-30 s (early phase) followed by a sustained smaller (3-15 dB) suppression (late phase). ICX stimulation produces the largest suppressions of DPOAEs in both contralateral and ipsilateral ears equally. Stimulation of the ICC causes large suppressions in the contralateral ear DPOAEs but much smaller than the suppressions of DPOAEs caused by ICX stimulation. Stimulation of the ICC causes a much smaller suppression in the ipsilateral ear DPOAEs than in the contralateral ear DPOAEs. ICD stimulation, however, produces little or no suppression of DPOAEs in either ear when compared to other subnuclei. No frequency-specific changes in DPOAEs were seen with stimulation of any of the subnuclei of the inferior colliculus (IC). Sectioning of middle ear muscles did not negate the suppressive effects of IC stimulation on DPOAEs. These findings verify that the IC has a subnucleus-specific influence over cochlear micromechanics.

Microvascular decompression in glossopharyngeal neuralgia.

Glossopharyngeal neuralgia is a rare and often controversial cause for odynophagia and otalgia. The otolaryngologist, head and neck surgeon may be the primary physician called upon to diagnose and treat this entity. In this study, vascular decompression, or more specifically, elimination of contact between the ninth cranial nerve and the posterior inferior cerebellar artery, was employed as treatment in three patients. All achieved relief of their symptoms with this intervention. A review of the neurosurgical literature and the experience with vascular decompression in trigeminal neuralgia, hemifacial spasm, and glossopharyngeal neuralgia is presented. The authors conclude that vascular decompression is effective in carefully selected patients, and the role of the skull base surgeon in managing this problem is expanding.

Unilateral sensorineural hearing loss rehabilitation.

The Audiant Bone Conductor has been heralded as an aid for use in conductive hearing loss; however, its possible use in unilateral sensorineural hearing loss (SNHL) has also been proposed. Between July 1987 and July 1989, profound unilateral sensorineural hearing loss has been rehabilitated in 43 patients using the Xomed Audiant Bone Conductor. Patients who were willing to participate in this clinical trial and who were felt to be good contralateral routing of signals (CROS) aid candidates were selected preoperatively. Audiometric followup, selection criteria, patient satisfaction, and complications are discussed.

Acoustic neuroma: a cost-effective approach.

A cost-effective approach to the diagnosis and treatment of acoustic neuromas continues to evolve as diagnostic methods improve. In the past 7 months, since gadolinium-enhanced magnetic resonance imaging (MRI) has become available in our practice, our screening and presurgical workup has changed. The purpose of this article is to outline the current philosophy of the senior authors in relation to acoustic neuroma management on the basis of 72 patients diagnosed from July 1988 to February 1989. With more sensitive diagnostic means, older less sensitive studies may be eliminated from the routine workup, thus maintaining cost-effectiveness while preserving the highest standard of patient care. The body of this article will review our current use of the many available diagnostic options and emphasize a cost-effective approach.

Petrous apex cholesteatoma.

In this chapter we have reviewed the etiology, diagnosis, and surgical treatment of petrous apex cholesteatoma. The etiology is usually congenital for those tumors arising within the apex, but acquired for lesions secondarily invading the apex. Diagnosis is primarily radiographic. CT remains an excellent modality for determining the extent of disease, whereas recent advances with MRI help in differential diagnosis. In our experience the middle fossa approach and the translabyrinthine-transcochlear approach allow the surgeon the best opportunity to effectively treat this rare and challenging tumor.

Glomus tympanicum tumors: contemporary concepts in conservation surgery.

The management of glomus tympanicum tumors is not a new subject. A review of the contemporary management concept is, however, needed for two reasons: 1. the recent literature has suggested radiation therapy as a treatment option in this surgical disease, and 2. patients who are referred to us for management of these tumors continue to exhibit errors in diagnosis and surgical approaches that are unnecessarily radical. The purpose of this paper is to review the diagnosis of these lesions and to prospectively differentiate them from the more extensive jugulare tumors. When type and extent of the tumor are properly determined, a surgical procedure can be planned that conserves as much normal ear anatomy and function as possible. Between May 1970 and July 1988, 60 patients with glomus tympanicum tumors were treated at the Otology Group, P.C., in Nashville, Tennessee. In 47 patients (78%), the tumors were removed using an extended facial recess approach. The external auditory canal wall was removed in seven patients (12%), and six patients underwent a transcanal approach. Total tumor removal was obtained in 90% of patients. Surgical management remains the mainstay of treatment for glomus tympanicum tumors.

Management of bilateral acoustic tumors.

Bilateral acoustic tumors (Neurofibromatosis 2) have now been shown to arise in patients with gene abnormalities separate from those with von Recklinghausen's disease (Neurofibromatosis 1). The management of patients with this disease has long posed a particularly challenging problem for the neurotologist. This retrospective study reviews our series of 38 patients and updates a previous report. Thirty-four patients underwent surgical excision of tumors on one or both sides. Hearing conservation was attempted in 20 with hearing preservation possible in eight. Recent advances in understanding the hereditary etiology of this disease, changing methods for diagnosis, preferred surgical techniques, and guidelines for appropriate patient management are discussed.

Use of steroids and a long-acting vasoconstrictor in the treatment of postintubation croup. A ferret model.

There is little in the literature to support recommendations for the use of steroids and vasoconstrictors in the treatment of postintubation croup. We developed a ferret model to study this condition. Ninety-eight ferrets were used in these experiments, which were undertaken to evaluate the efficacy of steroids and/or a long-acting vasoconstrictor, to develop dose-response curves for dexamethasone sodium phosphate if it proved efficacious, and to evaluate the possible synergistic effects of combination therapy. With this animal model, we were able to document a statistically significant early effect of dexamethasone sodium phosphate, but only when administered in large doses (greater than 0.5 mg/kg). Our data also support the tapering of these large doses within a 20-hour period. The long-acting vasoconstrictor oxymetazoline hydrochloride has a clear and statistically significant beneficial effect that is apparent by 20 hours and continues through 40 hours. We did not find any additive or synergistic effect of the combination of dexamethasone and oxymetazoline. Further studies are underway in our laboratory to elucidate other aspects of this interesting syndrome.

Readily available full-thickness mucous membrane graft.

A uniformly thin, nonfatty, full-thickness mucous membrane graft measuring a minimum of 4 X 5 cm is described for an unusual clinical problem, total symblepharon. The donor graft's ready availability from a normal maxillary sinus, its ideal grafting characteristics, and minimal donor site morbidity make it an ideal choice in many other clinical situations. This thin yet full-thickness graft minimizes graft contracture and maximizes rapidity of graft "take," making it an excellent choice for tracheal or laryngeal relining procedures, release of oral cavity scar contractures, or pharyngeal reconstructions.

Noradrenalin, serotonin, and the dorsal horn.

There is anatomical, pharmacological, and physiological evidence that descending systems from the brain stem using noradrenalin and serotonin (5-hydroxytryptamine, 5-HT) participate in the control of segmental sensory processing. Study of these systems is described in this paper. L-dopa was given intravenously to cats to cause the release of noradrenalin and 5-HT. The resultant effects on the responses of dorsal horn cells to innocuous and noxious thermal stimulation of skin were determined. Using the catecholamine cell neurotoxin, 6-hydroxydopamine, the dopamine beta-hydroxylase inhibitor, fusaric acid, and the tryptophan hydroxylase inhibitor, parachlorophenylalanine, the available central stores of noradrenalin or 5-HT were altered, thus allowing separation of the effects of noradrenalin and 5-HT release on dorsal horn cells. The results indicate that noradrenalin facilitates the responses of dorsal horn cells to innocuous cutaneous stimuli, and has an inhibitory effect on the responses to noxious stimuli. Serotonin inhibits the responses of dorsal horn cells that respond to noxious stimuli, to both innocuous and noxious stimuli. The results are discussed in light of current developments concerning aminergic control of segmental sensory processing.

The effects of L-dopa on dorsal horn cell responses to innocuous skin stimulation.

The effects of L-DOPA on responses of dorsal horn cells to innocuous mechanical skin stimulation were studied. Following intravenous administration of L-DOPA (10--40 mg/kg) to cats with intact spinal cords, dorsal horn cells, which could be activated by only innocuous or innocuous and noxious stimuli, demonstrated increased reponses manifested by an increase in the average number of spikes per stimulus, increased receptive field size and occasional changes in adequate stimuli. When cats with acute cord transections were studied, L-DOPA increased the responses of cells located in lamina 4 and those cells which responded only to innocuous stimuli; cells which responded to noxious stimuli and those located in other laminae had depressed responses following L-DOPA administration. The inhibitory effects of L-DOPA were in part abolished, in spinal cats, by pretreatment with parachlorophenylalanine, a serotonin depletor. Studies of dorsal cells in chronic spinal cats indicated that the effects of L-DOPA were largely dependent on functioning terminals of descending suprasegmental pathways. The conclusion was reached that descending noradrenergic pathways, indirectly, cause an increase in transmission from cutaneous afferents to dorsal horn cells and that some of the inhibitory effects of L-DOPA described by others are a manifestation of L-DOPA interactions with inhibitory serotonergic systems.