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Biomedical subjects

G M Clark

Publications and source records attributed to G M Clark.

At least 271 records · Page 15Linked to original sources
PubMed1993

Comparison of half-band and full-band electrodes for intracochlear electrical stimulation.

It was hypothesized that intracochlear stimulating electrodes oriented toward the modiolus would require a lower stimulus current to elicit a threshold neural response than longitudinal band electrodes. Electrically evoked auditory brain stem responses (EABRs) and electrode impedances were recorded with full-band and oriented half-band scala tympani electrodes in anesthetized, deafened cats. To elicit a threshold EABR, the stimulus current required for stimulation through half-band electrodes oriented toward the modiolus was not significantly different from the current required for stimulation through full-band electrodes. The impedances of full-band electrodes were significantly lower than those of half-band electrodes. Considering the significantly higher impedance and current density of half-band electrodes in comparison to full-band electrodes, as well as the critical orientation of half-band electrodes during implantation, we believe that full-band electrodes have advantages over half-band electrodes for scala tympani implantation.

Acoustic Impedance Tests↗
PubMed1993

Effect of chronic electrical stimulation on cochlear nucleus neuron size in normal hearing kittens.

Very young cochlear-implant candidates may have undetected islands of residual hearing. Would the maturation of these functioning auditory neurons be affected by chronic cochlear stimulation? This was tested by examining neuron sizes in the cochlear nuclei of young, normal hearing kittens with and without chronic cochlear stimulation. Six animals received bilateral intra- or extracochlear implants and were electrically stimulated unilaterally for periods of 1,000-1,500 hours. After sacrifice, cross-sectional areas of approximately 11,000 neurons somata in the cochlear nuclei were measured with an image-analysis system. There were statistically significant differences between stimulated and unstimulated nuclei, especially the posteroventral cochlear nucleus (PVCN), in individual cats, but the directions of the differences were inconsistent. Overall, there was no significant effect of electrical stimulation on soma size. These results indicate that chronic electrical stimulation of the auditory nerve has no positive or negative trophic effects on otherwise innervated, maturing cochlear nucleus neurons.

Animals↗
PubMed1993

The effect of handedness in tactile speech perception.

This study examined differential performance of normally hearing subjects using a tactile device on the dominant versus non-dominant hand. The study evaluated whether tactual sensitivity for non-speech stimuli was greater for the dominant hand as compared with the non-dominant hand, and secondly, whether there was an advantage for speech presented tactually to the dominant hand, resulting from a preferential pathway to the language processing area in the left cerebral hemisphere. Evaluations of threshold pulse width, dynamic ranges, paired electrode identification, and a closed-set tactual pattern discrimination test battery showed no difference in tactual sensitivity measures between the two hands. Speech perception was assessed with closed sets of vowels and consonants and with open-set Harvey Gardner (HG) words and Arthur Boothroyd (AB) words. Group mean scores were higher in each of the tactually aided conditions as compared with the unaided conditions for speech tests, with the exception of AB words in the tactile plus lip-reading plus audition/lip-reading plus audition condition on the right hand. Overall mean scores on the closed-set vowel test and on open-set HG and AB words were significantly higher for the tactually aided condition as compared with the unaided condition. Comparison of performance between the dominant and non-dominant hand showed a significant advantage for the dominant hand on the closed-set vowel test only. No significant differences between hands in either tactually aided or unaided conditions were evident for any of the other speech perception tests. Factors influencing this result could have been variations in degree of difficulty of the tests, the amount of training subjects received, or the training strategy employed. Although an advantage to presenting speech through the dominant hand may exist, it is unlikely to be great enough to outweigh possible restrictions on everyday use.

Communication Devices for People with Disabilities↗
PubMed1993

Radiologic evaluation of multichannel intracochlear implant insertion depth.

Postoperative plain film x-rays are necessary in all multichannel cochlear implant patients to confirm intracochlear position, detect possible electrode kinking, and provide a reference if postoperative slippage occurs. In addition, precise documentation of multichannel intracochlear electrode insertion depths is necessary for comparison of speech recognition results among patients and may be of use for future speech processing strategies. In the present study, a method has been devised, using a modified Stenver's view, to more accurately document insertion depths of the electrode array and location of individual electrodes on 50 multichannel cochlear implant patients. Surgical estimates of insertion depth are shown to have great variability in regard to distance along the basilar membrane when compared with x-ray documentation. Additionally, there is preliminary evidence that insertion depth, as determined by x-ray studies, has a strong correlation with open-set speech discrimination.

Basilar Membrane↗
PubMed1993

The postnatal growth of the temporal bone and its implications for cochlear implantation in children.

The postnatal growth of the human temporal bone was examined by direct anatomical measurements on 60 cadaver specimens of all ages. The bones were dissected as one would perform cochlear implant surgery using a posterior tympanotomy approach. Nineteen anatomical/surgical landmarks with implications for cochlear implant surgery were identified on each bone and the distance between these points measured. The temporal bone was found to be a complex structure, phylogenetically, anatomically and functionally consisting of four different parts with independent postnatal development. The inner and middle ears were adult size at birth. The external auditory canal and most parts of the temporal bone were subject to significant lateral growth. The size of the pneumatised mastoid increased in all directions. In the facial recess, however, no postnatal growth was observed. Between birth and adulthood an average of 12 mm (SD 5 mm) of growth was seen directly between the sino-dural angle and the round window, the landmarks approximating the implantation site for the receiver-stimulator and the electrode entry point into the inner ear. However, if an electrode leadwire is fixed at a cortical fixation site such as the posterosuperior point of Macewen's triangle, the leadwire would be subject to approximately 20 mm of growth. These results indicate that a paediatric cochlear implant design incorporating an expandable leadwire to accommodate this growth should allow up to 25 mm of leadwire lengthening. The fossa incudis showed no growth relative to the round window and was found to be a convenient fixation site for the electrode array close to the cochlea. From an anatomical and surgical point of view, cochlear implantation in very young children is feasible, provided the electrode array is secured and the design accommodates for controlled leadwire lengthening.

Adolescent↗
PubMed1993

Comparison of engraftment and acute GVHD in patients undergoing cryopreserved or fresh allogeneic BMT.

Coordination of marrow donation for allogeneic BMT is a common logistical problem. The use of cryopreserved donor marrow would facilitate scheduling and avoid potential problems due to donor employment, injury, infection or death. We analysed results of 10 matched related BMTs performed with cryopreserved donor marrow and compared them with 33 matched related BMTs using fresh bone marrow over a 4 year period. No difference in time to engraftment of granulocytes and platelets or transfusion requirements were demonstrated for the two groups. However, there was less GVHD in patients who received cryopreserved donor marrow (chi 2, p = 0.03; Fisher's exact test (two-sided) p = 0.067) despite comparable risk factors. The reason for this difference is unclear. Our results indicate that the use of cryopreserved bone marrow for allogeneic BMT patients is at least equivalent to the use of fresh bone marrow. A prospective randomized trial is needed to determine if a true difference exists in the incidence or severity of acute GVHD and to determine if recurrence rate is different between the two groups.

Acute Disease↗
PubMed1993

Tactile-auditory speech perception by unimodally and bimodally trained normal-hearing subjects.

The following study compared the effectiveness of unimodal and bimodal training strategies at improving the perception of speech information under a variety of conditions. Normal-hearing subjects were trained in the perception of vowel and consonant stimuli. Speech information was provided either via a multiple channel electrotactile speech processing aid (the Tickle Talker), and/or by a 200-Hz low-pass filtered auditory signal. Two subjects were trained only in the combined tactile-plus-auditory (TA) condition; the remaining two were trained in both the tactile-alone (T) and auditory-alone (A) conditions; however, only one condition was used at any single time. All subjects were evaluated in the TA, T, and A conditions, both at the beginning of the study, prior to training, and at the completion of training, on closed-set vowel and consonant confusion tests, and on an open-set word test. Results indicated that whilst statistically significant improvements occurred from one evaluation period to the next, in both groups of subjects, the improvements per condition were not dependent on the type of training received. The results provide a preliminary indication that the provision of unimodal training does not impair the perception of speech information under bimodal perception conditions.

Acoustic Stimulation↗
PubMed1993

Evaluation of expandable leadwires for pediatric cochlear implants.

The development of cochlear implants for use in very young children (1-2 years old) will require techniques designed to accommodate temporal bone growth. Previous anatomic studies have shown that the leadwire of a cochlear implant must be capable of expanding up to 20 mm between the round window and the implanted receiver-stimulator in response to skull growth. In the present study morphologic and biomechanical evaluation of five expandable leadwire designs was conducted following their implantation in young cats. Two helical shaped leadwire designs frequently exhibited extensive fibrous tissue adhesions and broke during long-term implantation. In contrast, thin, flexible Silastic envelopes were effective in minimizing tissue adhesions. Residual V- and Z-shaped leadwires, placed in these envelopes, showed little evidence of fibrous tissue adhesions following implantation periods of up to 2 years. Moreover, these leadwires readily expanded both during the growth of the animal and when biomechanical expansion studies performed at the completion of the implant period. These expandable leadwire designs appear to be appropriate candidates for use in pediatric cochlear implants.

Animals↗
PubMed1992

Prognosis and treatment decisions in patients with breast cancer without axillary node involvement.

BACKGROUND: Every month, treatment decisions must be made for more than 6000 patients with breast cancer without axillary node involvement in the United States. Approximately 70% of these patients will survive more than 10 years after surgery and/or radiation treatment without additional systemic adjuvant therapy. If we had good methods to identify patients who are destined to have a recurrence of their disease, only those patients should receive adjuvant therapy. METHODS: The authors reviewed the literature supporting the use of currently available prognostic factors for patients with node-negative breast cancer, and formulated a framework on which prognostic factor information can be based to help make these treatment decisions. RESULTS: The steps involved in making treatment decisions are: use prognostic factors to determine the recurrence probability; calculate the expected treatment benefit; and weigh the expected benefits against the potential risks. CONCLUSIONS: Prognostic factors can be used to help make treatment decisions for patients with breast cancer without axillary node involvement. However, the final treatment decision must take into account all aspects of the patient and her disease, and the physician must help the patient evaluate her prognostic factors, arrive at an understanding of her particular risk of recurrence, and weigh the potential benefits and risks of adjuvant therapy.

Axilla↗
PubMed1992

Hispanic patients with head and neck cancer do not have a worse prognosis than Anglo-American patients.

This study was undertaken to determine whether the survival of Hispanic patients with squamous cell carcinoma of the head and neck was different from that of Anglo-American patients. The charts of 275 male patients with a diagnosis of squamous cell carcinoma of the head and neck at one Veterans Administration Hospital were reviewed in an attempt to identify prognostic indicators for both ethnic groups. No differences were observed between Anglo-American and Hispanic patients with respect to sites of the primary tumor, age at diagnosis, performance status, or the frequency of surgery, radiation therapy, or chemotherapy; however, there was a tendency for Hispanic patients to have received more treatment. There also was a trend (P = 0.12) for Hispanic patients to have a more advanced stage of cancer. Hispanic patients lost significantly more weight (P less than 0.001) and had significantly lower serum albumin levels (P less than 0.0001). According to the results of multivariate survival analyses, the variables that were predictive of a poor prognosis included advanced stage of disease, decreased serum albumin levels, increased weight loss, administration of chemotherapy, lack of radiation therapy or surgery, and advanced age. Ethnicity was not a significant predictor of survival either in univariate analyses, or within patients with the same stage of disease, or after adjustment for other prognostic factors. In conclusion, the natural history of squamous cell carcinoma of the head and neck is the same for Hispanic and Anglo-American patients.

Carcinoma, Squamous Cell↗
PubMed1992

A demonstration that breast cancer recurrence can be predicted by neural network analysis.

Neural Network Analysis, a form of artificial intelligence, was successfully used to predict the clinical outcome of node-positive breast cancer patients. A Neural Network was trained to predict clinical outcome using prognostic information from 1008 patients. During training, the network received as input information tumor hormone receptor status, DNA index and S-phase determination by flow cytometry, tumor size, number of axillary lymph nodes involved with tumor, and age of the patient, as well as length of clinical followup, relapse status, and time of relapse. The ability of the trained Network to determine relapse probability was then validated in a separate set of 960 patients. The Neural Network was as powerful as Cox Regression Modeling in identifying breast cancer patients at high and low risk for relapse.

Axilla↗
PubMed1992

Integrating prognostic factors.

This special issue of Breast Cancer Research and Treatment addresses the topic of how to evaluate new prognostic factors for breast cancer, and how the information provided by these new factors might be integrated with traditional factors to make better treatment decisions for these patients. Although the focus is on breast cancer, the techniques described are equally applicable to any situation involving survival or failure over time.

Breast Neoplasms↗