Biomedical subjects

S N Merchant

Publications and source records attributed to S N Merchant.

At least 55 records · Page 3Linked to original sources

PubMed1996

Acoustic input impedance of the stapes and cochlea in human temporal bones.

The acoustic input impedance of the stapes and cochlea ZSC represents the mechanical load driven by the tympanic membrane, malleus and incus. ZSC was calculated from broad-band measurements (20 Hz to 11 kHz) of stapes displacement made with an optical motion sensor and of sound pressure at the stapes head in a human temporal-bone preparation. Measurements were made in 12 fresh temporal bones with the round window insulated from the sound stimulus. Below 1 kHz, the magnitude of ZSC was approximately inversely proportional to frequency, and ZSC angle was between 0.10 and -0.20 periods. This behavior is consistent with a mixed stiffness and resistance. Between 1 and 4 kHz, ZSC was resistance-dominated with a magnitude between 40 and 100 mks acoustic G omega that was roughly independent of frequency, and its angle was between -0.12 and 0 periods. Between 4 and 7 kHz, the magnitude of ZSC was either constant or increased with frequency while ZSC angle was near 0. Between 7 and 8 kHz, both ZSC magnitude and angle decreased sharply with frequency, and both increased somewhat at higher frequencies. The input impedance of the cochlea ZC was estimated in one ear from ZSC measurements made before and after draining the inner ear fluids. ZC was stiffness-dominated below 100 HZ, and resistance-dominated from 100 Hz to 5 kHz. The frequency-dependent magnitude of ZSC in our bones is similar to those reported by other investigators in cadaver temporal bones (Nakamura et al., 1992; Kurokawa and Goode, 1995). Our ZSC measurements are qualitatively similar to theoretical predictions (Zwislocki, 1962; Kringlebotn, 1988), but are a factor of 3 greater in magnitude, implying that ZSC may be more resistive and stiffer than previously thought. We found inter-ear variations of a factor of 4 (12 dB), which may explain some of the clinically observed variations in size of the air bone gap in individuals with middle ear lesions or after middle-ear reconstructive surgery.

Acoustic Impedance Tests↗

PubMed1995

Ménière's disease.

There now exists a vast literature on Ménière's disease. In this review, we examine closely the foundations of some of the current concepts regarding various aspects of the disorder, including definitions, clinical features, natural history, pathology, pathogenesis, pathophysiology, diagnosis and treatment. We will highlight areas where the current state of knowledge is incomplete and also suggest some avenues for further research.

Animals↗

PubMed1995

Mechanical and acoustic analysis of middle ear reconstruction.

Existing mechanical and acoustic models of the human middle ear are used to investigate the expected performance of various middle ear reconstructive procedures. Although the results of these analyses generally are consistent with observed surgical results, such an analytic approach enables the formation of some simple rules for maximizing middle ear function after stapedectomy and tympanomastoid surgery. Model analyses predict the following: (1) stapes prostheses of 0.6 to 0.8 mm in diameter produce air-bone gaps of less than 12 dB; smaller diameter prostheses produce larger air-bone gaps; (2) the mass of a stapedectomy or ossicular replacement prosthesis can be 16 times greater than that of the stapes with little effect on the hearing result; (3) the volume of the middle ear air spaces after tympanomastoid surgery should be at least 0.5 cc for optimal acoustic results; and (4) in a type IV tympanoplasty, the effectiveness of the round window graft (acoustic shield) is an important parameter determining the hearing result; the shield should be as impedant (stiff) as possible for optimal postoperative hearing. The model framework described in this report hopefully can be used to investigate the effect of variations in other surgical and help define new procedures.

Cochlear Implants↗

PubMed1995

Middle ear mechanics of type IV and type V tympanoplasty: I. Model analysis and predictions.

An analysis of type IV and type V tympanoplasty procedures was performed using a quantitative model of the acoustic and mechanical properties of the stapes, cochlea, round window shield, and cavum minor air space. Realistic values for the impedance of these structures were determined from anatomic and functional measurements in normal ears. These model values lead to predicted type IV hearing results that match well with the best surgical results over a broad frequency range (125-4000 Hz). A parametric study of alterations in the model impedances reveals that a good hearing result depends on a mobile stapes, proper aeration of the cavum minor air space, and a sufficiently stiff graft shield. Intersubject variations in the cochlear impedance also can have a significant effect on the postsurgical hearing response.

Acoustic Impedance Tests↗

PubMed1995

Middle ear mechanics of type IV and type V tympanoplasty: II. Clinical analysis and surgical implications.

Type IV and type V tympanoplasty operations are simple, robust, and well-established techniques to reconstruct middle ears that have been severely altered by chronic otitis media. In a previous paper, the authors developed a simple four-block physiologic model to describe hearing results after these procedures. This paper presents a comparison of model predictions to hearing results obtained from a detailed retrospective clinical review of 30 type IV and type V procedures. Audiograms predicted by the model and those observed clinically show good agreement over a wide frequency range (500-4000 Hz) and for many different clinical conditions. Thus, this model reliably predicts postsurgical hearing results. The application of quantitative analyses provided by this model permits the formation of a few simple surgical rules that may improve postoperative hearing results. (1) The footplate should be left as mobile as possible (e.g., by covering it with a very thin split-thickness skin graft, as opposed to a fascia graft, which will tend to stiffen it). If the footplate is ankylosed, it should be removed and replaced with a compliant tissue graft, such as fat. (2) The round window acoustic graft shield should be made as stiff as possible. If the shield material used is temporalis fascia, then one should consider using more than one layer, or reinforcing it with cartilage. (3) An attempt should be made to create an aerated cavum minor containing at least 0.03 cc of air.

Adolescent↗

PubMed1994

Histopathology of ossicular implants.

Ossicular and cortical bone grafts maintain their morphologic structure for long periods of time and show varying amounts of replacement of nonviable bone by new bone through a slow process of creeping substitution. Cartilage grafts develop chondromalacia, lose stiffness, and tend to be resorbed over time. Plastipore prostheses elicit foreign body giant cell responses with microscopic biodegradation of the implants. There is a great need for the study of well-documented human temporal bone cases with in situ ossicular implants.

Adolescent↗

PubMed1993

The National Temporal Bone, Hearing, and Balance Pathology Resource Registry.

The National Temporal Bone, Hearing, and Balance Pathology Resource Registry has been established with funding provided by the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health. The registry is meant to serve as a national resource for researchers and the public to stimulate and facilitate human otopathologic research. It will maintain a computerized database of currently active and inactive temporal bone and auditory brain-stem collections throughout the United States. In addition, it will encourage human temporal bone research by disseminating pertinent information, developing and fostering temporal bone professional educational activities, implementing a national temporal bone acquisition network, and encouraging investigative collaborations in the study of the human temporal bone and brain structures. It will also identify otopathologic collections at risk of being discarded or lost and will develop mechanisms and strategies to conserve them.

Brain↗

PubMed1990

Cadaver middle ears as models for living ears: comparisons of middle ear input immittance.

In vitro measurements of the middle ear input immittance in temporal bones extracted from human cadavers were directly compared with similar in vivo measurements from clinically normal subjects. The results of this comparison indicate that most otoscopically normal unfixed cadaver ears have middle ear input immittances that are indistinguishable from those of live subjects in the 0.1- to 2-kHz range--as long as they have been kept from drying and the static pressures on either side of the tympanic membrane are equal. The effects of the middle ear muscles on the measured input immittance are generally small and the cadaver ears can be maintained in the frozen state for several months with little change. Tympanometry appears to be a reliable indicator of normal middle ear immittance. Cadaver middle ears are useful models of human middle ear function.

Acoustic Impedance Tests↗

PubMed1989

Meniere's syndrome and endolymphatic hydrops. Double-blind temporal bone study.

A systematic double-blind assessment of case histories and histopathologic findings in temporal bones in the collection at the Massachusetts Eye and Ear Infirmary was performed to test the hypothesis that clinical Meniere's syndrome is associated with endolymphatic hydrops demonstrated histopathologically at death. Thirteen of 13 cases of clinical Meniere's syndrome were found to have endolymphatic hydrops not attributable to other causes. However, some patients with idiopathic endolymphatic hydrops did not exhibit clinical Meniere's syndrome as revealed in their medical records. These results challenge the dogma that endolymphatic hydrops per se generates the symptoms of Meniere's syndrome.

Aged↗

PubMed1988

Transcriptional and post-transcriptional regulation of the genes encoding cytochromes P-450c and P-450d in vivo and in primary hepatocyte cultures.

In both primary cell cultures of rat hepatocytes and in liver, polycyclic aromatic hydrocarbons (PAHs) were found to influence the accumulation of the cytochrome P-450c and P-450d mRNAs by both transcriptional and post-transcriptional mechanisms. Following treatment with PAHs, cytochrome P-450c mRNA levels increased approximately 100-fold in both hepatocyte cultures and in liver, while transcription rates, measured by run-on transcription of isolated nuclei, increased 3-fold in hepatocyte cultures and 10-fold in liver. The difference in the -fold increases of mRNA level and transcription rate suggests that post-transcriptional, as well as transcriptional, mechanisms contributed to the regulation of cytochrome P-450c mRNA levels. Following treatment with PAHs, cytochrome P-450d mRNA levels increased 200-fold in hepatocyte cultures and 70-fold in liver, while transcription rates remained unchanged in hepatocyte cultures and increased only 1.7-fold in liver. This suggests that post-transcriptional mechanisms were of primary importance in regulating cytochrome P-450d mRNA levels. The newly developed hepatocyte primary cell culture system used in these studies differs from previously reported systems in that the cytochrome P-450d gene, as well as the cytochrome P-450c gene, were expressed in response to PAHs. In this cell culture system the regulation of these two genes was quite similar, although not identical, to that found in liver. The mechanisms controlling the tissue-specific expression of the genes encoding cytochromes P-450c and P-450d were also examined. The cytochrome P-450c mRNA was found in kidney, heart, and lung, as well as in liver, of PAH-treated rats, while the mature cytochrome P-450d mRNA was detected only in liver. The substantial increase in cytochrome P-450c mRNA in kidney in response to beta-napthoflavone was not associated with a detectable change in the transcription rate of cytochrome P-450c gene, indicating that cytochrome P-450c mRNA levels must be regulated primarily post-transcriptionally in kidney. Even though mature cytochrome P-450d mRNA could not be detected in kidney, the cytochrome P-450d gene was transcribed at a substantial rate in this tissue; therefore, the lack of accumulation of mature cytochrome P-450d mRNA in kidney must have been due to post-transcriptional control.

Animals↗

PubMed1986

Experiences with the butterfly chart.

The bithermal caloric test remains, four decades after its first description, the single most valuable test of vestibular function. However, there are several variations described in the exact procedure of caloric stimulation and in the method of evaluating and representing the test results. For the past 10 years, we have followed a procedure using a constant standardized stimulus and a particular system for caloric nystagmus representation, viz., the Butterfly Chart, and have found it to be very satisfactory. This paper describes our experience with the use of the Butterfly Chart and its clinical application in over 4,500 cases.

Adult↗

PubMed1986

The caloric response in the contralateral ear in acoustic neuroma.

The caloric test response from the unaffected ear in a case of acoustic neuroma may be normal, hypoactive or hyperactive; each has a different pathophysiological connotation: a normal response is the anticipated finding; a hyperactive contralateral response is due to the presence of a large neuroma with brainstem compression; and a hypoactive response may result from several factors, such as lack of mental alertness during the test, drugs like barbiturates, raised intracranial tension, or the presence of bilateral neuromas.

Caloric Tests↗

PubMed1986

Vestibular effects of intravenous lidocaine used in the treatment of tinnitus.

Intravenous lidocaine is known to have an effect on the auditory system in that it is useful for suppressing tinnitus, albeit temporarily. We have used intravenously administered lidocaine as one of the treatment modalities for refractory, disturbing, tinnitus. Its effects on the vestibulo-ocular system were determined by electronystagmography performed before and immediately after injecting lidocaine: smooth pendular stimulus tracking was unaffected; spontaneous and positional nystagmus tended to be suppressed; directional preponderance was reduced or reversed; and the difference between the nystagmus responses in the two directions during the pendular rotation chair test was also reduced or reversed. These changes in the caloric and rotation tests were statistically significant. Lidocaine also appeared to have altered the balance between the two sides in the vestibulo-spinal system as indicated by the results of the stepping test during craniocorpography. The results lend support to the hypothesis that intravenous lidocaine acts at the level of the central nervous system rather than at the periphery.

Electronystagmography↗

PubMed1985