Improving patient outcomes with inguinal hernioplasty-local anaesthesia versus local anaesthesia and conscious sedation: a randomized controlled trial.

PURPOSE: Conscious sedation is regularly used in ambulatory surgery to improve patient outcomes, in particular patient satisfaction. We hypothesized that the addition of conscious sedation would provide greater patient satisfaction with inguinal hernioplasty compared to local anesthesia alone. METHODS: This trial was a single-centre, randomized, placebo-controlled, double-blinded trial where patients undergoing inguinal hernioplasty using local anaesthesia were randomized to receive local anaesthesia alone versus local anaesthesia and conscious sedation. The primary outcome of patient satisfaction was assessed using the Iowa Satisfaction with Anesthesia Scale (ISAS). The study was powered to detect a significant difference in ISAS scores between groups. Comparisons were made using T test and Chi square tests. A p value of less than 0.05 was considered significant. RESULTS: There were 149 patients randomized: 78 to the local anesthesia (LA) group and 71 to the local anaesthesia and conscious sedation (LACS) group. For the primary outcome measure of patient satisfaction, the mean ISAS score was significantly greater in the LACS group (p = 0.009). The experience of pain and pain severity was greater in the LA group (p = 0.016; p = 0.0162 respectively). No statistically significant difference was found between groups with respect to operative time, time to discharge or postoperative complications. CONCLUSION: The use of conscious sedation with local anesthesia for inguinal hernioplasty is safe, results in less pain experience and severity and is associated with better patient satisfaction. The use of conscious sedation does not delay patient discharge.

Should Delayed Cholecystectomy Following Acute Calculous Cholecystitis Be Discouraged in a Resource-restricted Setting?

BACKGROUND: Early cholecystectomy for acute calculous cholecystitis (ACC) reduces hospital stay and complications during the waiting period. The purpose of this study is to establish the patterns of management of ACC at the University Hospital of the West Indies (UHWI) and to evaluate the advantages of early versus delayed cholecystectomy. METHODS: This was a retrospective chart review of patients admitted with a diagnosis of ACC. Data collection included demographics, management strategy, timing to cholecystectomy, significant events while awaiting cholecystectomy and duration of hospital stay. Mann-Whitney U and Chi-squared tests were used for analysis. P-value of < 0.05 was considered significant. RESULTS: A total of 102 patient charts were extracted, 59 of which were managed conservatively and 43 managed with early cholecystectomy. The mean time to surgery after conservative management was 173 days. About 30% of persons managed conservatively had significant attacks while awaiting surgery, which included need for re-admission and earlier intervention. There was a trend toward longer mean total hospital stay in the conservative group (xsx = 5.03, xCons = 6.12; p = 0.054). CONCLUSION: Conservative management of ACC results in significant delays in definitive management and risks of complications during the waiting period. Early cholecystectomy should be encouraged even in a resource-restricted setting.

The Impact of Trans-thoracic Ultrasound on Cardiac Injuries.

OBJECTIVE: To determine the impact of trans-thoracic ultrasound (TTUS) in patients with chest trauma and potential cardiac injuries and to determine the outcome of patients with cardiac injury detected on TTUS. METHOD: Data were obtained from the Trauma Registry for all patients presenting alive to the University Hospital of the West Indies during the 10-year period commencing January 1, 2001 and who were subjected to a TTUS or emergency thoracotomy for cardiac injuries, or had cardiac injuries at postmortem. In addition to demographics, variables analysed included mechanism and site of injury and outcome. RESULTS: Of 405 patients being subjected to a TTUS during the period, 12 (3%) had cardiac injuries. During the same period, 63 patients in the Trauma Registry had proven cardiac injuries. Trans-thoracic ultrasound was thus conducted on 19% of all patients with cardiac injuries. Three patients had positive TTUS but no cardiac injuries. Of the patients with injuries, the mean age was 30.4 years, 92.1% were male and 65% were as a result of stab wounds, while 22% were as a result of gunshot wounds. The right ventricle was the most common site of injury, accounting for 41% of cases, while the left ventricle, both ventricles and other sites accounted for 27%, 17% and 14%, respectively. Ninety per cent of the group was subjected to emergency thoracotomy; mortality of the entire group was 48%, including one patient who had TTUS. CONCLUSIONS: This review demonstrates that cardiac injuries remain lethal, diagnosis is largely clinical and TTUS may be over-utilized, having little impact on clinical outcome of patients presenting with this injury.

Dengue fever mimicking acute appendicitis: A case report.

INTRODUCTION: Dengue fever is an acute viral disease, which usually presents as a mild febrile illness. Patients with severe disease present with dengue haemorrhagic fever or dengue toxic shock syndrome. Rarely, it presents with abdominal symptoms mimicking acute appendicitis. We present a case of a male patient presenting with right iliac fossa pain and suspected acute appendicitis that was later diagnosed with dengue fever following a negative appendicectomy. PRESENTATION OF CASE: A 13-year old male patient presented with fever, localized right-sided abdominal pain and vomiting. Abdominal ultrasound was not helpful and appendicectomy was performed due to worsening abdominal signs and an elevated temperature. A normal appendix with enlarged mesenteric nodes was found at surgery. Complete blood count showed thrombocytopenia with leucopenia. Dengue fever was now suspected and confirmed by IgM enzyme-linked immunosorbent assay against dengue virus. DISCUSSION: This unusual presentation of dengue fever mimicking acute appendicitis should be suspected during viral outbreaks and in patients with atypical symptoms and cytopenias on blood evaluation in order to prevent unnecessary surgery. CONCLUSION: This case highlights the occurrence of abdominal symptoms and complications that may accompany dengue fever. Early recognition of dengue fever mimicking acute appendicitis will avoid non-therapeutic operation and the diagnosis may be aided by blood investigations indicating a leucopenia, which is uncommon in patients with suppurative acute appendicitis.

Colorectal anastomotic leakage at the university hospital of the west indies: an analysis of risk factors.

BACKGROUND: Anastomotic leakage remains a concern in general surgical practice. The significance lies in the resultant abdominal sepsis, related morbidity and mortality, risk of anastomotic loss, permanent stoma creation and the effect on local recurrence and overall patient survival in colorectal cancer cases. OBJECTIVES: This study serves to determine the leak rates and the mortality thereof related to colonic and rectal anastomoses at the University Hospital of the West Indies (UHWI) in Kingston, Jamaica. Independent factors contributing to anastomotic leaks in these patients will also be assessed and correlations determined. METHODS: A review of the medical records of one hundred and thirty-three cases of colonic and rectal anastomoses identified retrospectively over a three-year period provided relevant information for analysis. RESULTS: Anastomotic leaks were identified in twelve patients, providing a leak rate of 9.0%. No 30-day mortality related to anastomotic leakage was noted. Based on a multivariate analysis, male gender was identified as the sole independent factor related to anastomotic leakage. CONCLUSION: Colorectal anastomotic leak rates at UHWI fell at the upper limit of leak rates typically quoted in the literature. No modifiable risk factor appeared to contribute to this leak rate. Early identification and intervention is critical in limiting mortality associated with colorectal anastomotic leakage.

Minimally invasive surgery training in the Caribbean: a survey of general surgical residents and their trainers / Entrenamiento de cirugía mínimamente invasiva en el Caribe: encuesta a los residentes de cirugía general y sus entrenadores

BACKGROUND: There has been debate on the feasibility of incorporating minimally invasive surgery (MIS) into surgical practice in developing countries due to resource and training limitations. Our study establishes the current and desired state of MIS training in surgical residency programmes in the Caribbean. METHODS: An adapted version of a previously administered questionnaire was issued to surgeons and residents involved in the general surgical residency programme of The University of the West Indies in Barbados, Jamaica and Trinidad and Tobago. Data were analysed using the Statistical Package for the Social Sciences, version 17.0. RESULTS: The questionnaire was sent to 41 surgeons and 41 residents with a 65% response rate. Most residents had performed less than 25 basic laparoscopic procedures. Up to 82% of residents felt that they would be unable to perform advanced laparoscopic procedures due to lack of training. The principal negative factors influencing MIS training included lack of operating room time, lack of equipment and lack of preceptor expertise. Both surgeons (83.4%) and residents (93.4%) strongly felt that a surgical skills laboratory would be helpful for the acquisition of MIS skills. Both surgeons (85.7%) and residents (100%) felt that there was a role for an MIS surgeon in fulfilling training obligations. CONCLUSION: The basic and advanced MIS experience of residents in the Caribbean is limited. Surgeon training and resource limitations are major contributing factors. There is a strong desire on the part of surgeons and residents alike for the incorporation of more effective MIS training into the residency programme in the Caribbean.

ANTECEDENTES: Ha habido debates en torno a la factibilidad de incorporar la cirugía mínimamente invasiva (CMI) a la práctica quirúrgica de los países en desarrollo, debido a las limitaciones de recursos y entrenamiento. Este estudio establece cual es el estado actual y el estado deseado en relación con la CMI en los programas de residencia quirúrgica en el Caribe. MÉTODOS: Una versión adaptada de un cuestionario previamente aplicado fue administrada a cirujanos y residentes participantes en el programa de residencia de cirugía general del Hospital Universitario de West Indies en Barbados, Jamaica, y Trinidad y Tobago. Los datos se analizaron usando el Programa estadístico SPSS para ciencias sociales, versión 17.0. RESULTADOS: El cuestionario fue enviado a 41 cirujanos y 41 residentes, y se obtuvo una tasa de respuesta del 65%. La mayor parte de los residentes habían realizado menos de 25 procedimientos laparoscópicos básicos. Hasta un 82% de los residentes, nsentían que no serían capaces de realizar procedimientos laparoscópicos avanzados, debido a la falta de entrenamiento. Los principales factores negativos que afectaban el entrenamiento de la CMI incluían la falta de un horario para usar el salón de operaciones, la falta de equipos, y la falta de preceptores expertos. Tanto los cirujanos (83.4%) como los residentes (93.4%) sentían fuertemente que un laboratorio de habilidades quirúrgicas sería útil para el desarrollo de las habilidades de la CMI. Tanto los cirujanos (85.7%) como los residentes (100%) sentían que el papel de un cirujano debía incluir la obligación de impartir entrenamientos. CONCLUSIÓN: La experiencia básica así como la experiencia avanzada de CMI de los residentes del Caribe es limitada. Las limitaciones en relación con los recursos y el entrenamiento a impartir por los cirujanos, es uno de los factores principales en tal sentido. Por parte de los cirujanos así como de los residentes, existe un fuerte deseo de incorporar entrenamientos más efectivos de CMI al programa de residencia del Caribe.

Minimally invasive surgery training in the Caribbean--a survey of general surgical residents and their trainers.

BACKGROUND: There has been debate on the feasibility of incorporating minimally invasive surgery (MIS) into surgical practice in developing countries due to resource and training limitations. Our study establishes the current and desired state of MIS training in surgical residency programmes in the Caribbean. METHODS: An adapted version of a previously administered questionnaire was issued to surgeons and residents involved in the general surgical residency programme of The University of the West Indies in Barbados, Jamaica and Trinidad and Tobago. Data were analysed using the Statistical Package for the Social Sciences, version 17.0. RESULTS: The questionnaire was sent to 41 surgeons and 41 residents with a 65% response rate. Most residents had performed less than 25 basic laparoscopic procedures. Up to 82% of residents felt that they would be unable to perform advanced laparoscopic procedures due to lack of training. The principal negative factors influencing MIS training included lack of operating room time, lack of equipment and lack of preceptor expertise. Both surgeons (83.4%) and residents (93.4%) strongly felt that a surgical skills laboratory would be helpful for the acquisition of MIS skills. Both surgeons (85.7%) and residents (100%) felt that there was a role for an MIS surgeon in fulfilling training obligations. CONCLUSION: The basic and advanced MIS experience of residents in the Caribbean is limited. Surgeon training and resource limitations are major contributing factors. There is a strong desire on the part of surgeons and residents alike for the incorporation of more effective MIS training into the residency programme in the Caribbean.

Open abdominal aortic aneurysm repair in the era of endovascular repair / Reparación abierta de la aorta abdominal en la era de la reparación endovascular

OBJECTIVES: The development of minimally invasive techniques for abdominal aortic aneurysm (AAA) repair and the establishment of specialized centres have resulted in improved patient outcomes. This study examines open AAA repair at a non-specialized centre where advanced techniques are not practised. METHODS: We conducted a retrospective analysis on a cohort of 83 patients presenting for AAA repair to a non-specialized hospital, the University Hospital of the West Indies (UHWI). The end points assessed included operative (30-day) mortality, postoperative complications, duration of operation, blood loss, intensive care unit (ICU) stay and overall hospital stay. RESULTS: The overall operative mortality was 9.4% (23% for ruptured aneurysms and 5% for unruptured aneurysms). Mean operating time, blood loss, ICU stay and hospital stay were 326 ± 98 minutes, 2420 ± 1397 mls, 3 ± 5 days and 9 ± 5 days, respectively with no significant differences noted between ruptured and unruptured aneurysms. Mean aneurysm diameter was 6.13 ± 1.59 cm. CONCLUSION: Mortality rates for open aneurysm repair at the UHWI are consistent with findings in the current literature. Open AAA repair remains a safe treatment option in this environment. Continued improvements need to be made with respect to minimizing blood loss and operation duration, particularly in repairs of unruptured aneurysms.

OBJETIVOS: El desarrollo de técnicas mínimamente invasivas para la reparación del aneurisma aórtico abdominal (AAA) y el establecimiento de centros especializados para esos fines, han traído consigo el mejoramiento de los resultados clínicos de los pacientes. Este estudio examina reparaciones del tipo AAA en un centro no especializado, en el que no se practican técnicas avanzadas. MÉTODOS: Se llevo a cabo un análisis retrospectivo en una cohorte de 83 pacientes que acudieron para reparación de AAA a un hospital no especializado - el Hospital Universitario de West Indies (UHWI). Los aspectos finales evaluados incluyeron la mortalidad operatoria (30 días), las complicaciones post-operatorias, la duración de la operación, la pérdida de sangre, la estadía en la unidad de cuidados intensivos, y la estadía general en el hospital. RESULTADOS: La mortalidad operatoria general fue 9.4% (23% para los aneurismas rotos y 5% para los aneurismas no rotos). El tiempo promedio de operación, la pérdida de sangre, la estadía en la UCI, y la estadía hospitalaria fueron 326 ± 98 minutos, 2420 ± 1397 mls, 3 ± 5 días y 9 ± 5 días respectivamente, sin que se observen diferencias significativas entere aneurismas rotos y no rotos. El diámetro promedio de los aneurismas fue 6.13 ± 1.59 cm. CONCLUSIÓN: Las tasas de mortalidad para la reparación abierta de aneurismas en el UHWI concuerdan con los hallazgos en la literatura corriente. Se necesita continuar los esfuerzos por lograr mejorías en cuanto a minimizar la pérdida de sangre, y reducir el tiempo de duración de la operación, especialmente en las reparaciones de aneurismas no rotos.

Laparoscopic colectomy for colonic neoplasms in a developing country.

AIM: To report the outcome of patients treated for colonic neoplasms using a laparoscopic assisted technique since its introduction at the University Hospital of the West Indies, Jamaica. SUBJECTS AND METHODS: All consecutive patients undergoing laparoscopic assisted colectomy were entered into a prospective database and this data analysed. Data collected included patient demographics, pre-operative diagnosis, operative events, post-operative morbidity and outcome. RESULTS: Over the thirty-six months period July 1, 2005-December 31, 2005 and July 1, 2006-December 31, 2008, thirty patients each underwent laparoscopic assisted colectomy for a colonic neoplasm. Their mean age was 63 years with M: F ratio of 1:2. Seventy-four per cent of the patients had carcinomas which was located on the right and sigmoid colon in 17 and 10 patients respectively. Mean operative time was 98 min for patients with right-sided lesions and blood loss for the entire group was minimal. Two patients were converted to open resections. Median duration of hospitalization was five days. There was no mortality but three patients had complications. After median follow-up of 30 months, there was no local or systemic recurrence. CONCLUSIONS: Appropriately selected patients with colonic neoplasms can be safely subjected to a laparoscopic assisted resection and expect to enjoy the advantages of this technique even in a developing country setting. The outcome of thirty consecutive laparoscopic assisted colectomies is reported demonstrating that this technique can be safely applied to selected patients with colonic carcinomas in developing countries.

Open abdominal aortic aneurysm repair in the era of endovascular repair.

OBJECTIVES: The development of minimally invasive techniques for abdominal aortic aneurysm (AAA) repair and the establishment of specialized centres have resulted in improved patient outcomes. This study examines open AAA repair at a non-specialized centre where advanced techniques are not practised. METHODS: We conducted a retrospective analysis on a cohort of 83 patients presenting for AAA repair to a non-specialized hospital, the University Hospital of the West Indies (UHWI). The end points assessed included operative (30-day) mortality, postoperative complications, duration of operation, blood loss, intensive care unit (ICU) stay and overall hospital stay. RESULTS: The overall operative mortality was 9.4% (23% for ruptured aneurysms and 5% for unruptured aneurysms). Mean operating time, blood loss, ICU stay and hospital stay were 326 +/- 98 minutes, 2420 +/- 1397 mls, 3 +/- 5 days and 9 +/- 5 days, respectively with no significant differences noted between ruptured and unruptured aneurysms. Mean aneurysm diameter was 6.13 +/- 1.59 cm. CONCLUSION: Mortality rates for open aneurysm repair at the UHWI are consistent with findings in the current literature. Open AAA repair remains a safe treatment option in this environment. Continued improvements need to be made with respect to minimizing blood loss and operation duration, particularly in repairs of unruptured aneurysms.

Duodenal fibrosarcoma mimicking Franz tumour complicated by post-resection chylous ascites.

This case report presents a young woman who underwent a Whipples resection for a large pan-creato-duodenal tumour. Pathology and immunohistochemical analysis of the tumour suggest duodenal fibrosarcoma. The patient's postoperative management was complicated by chylous ascites. A brief literature review is given to highlight this unusual case.

Hermaphroditism: cytogenetics, gonadal pathology and gender assignment: a case report / Hermafroditismo: citogenética, patología gonadal y asignación de género

True hermaphroditism is a rare intersex disorder in which individuals possess both testicular and ovarian gonadal tissue. A case of true unilateral hermaphroditism presenting with ambiguous external genitalia, right scrotal testis and left pelvic ovotestis is herein outlined Phallic, gonadal and genetic factors were considered before male gender was assigned. Gender assignment procedures have been questioned by intersex activists opposed to early genital surgery. Western societies have a binary perspective on gender and this leads to a stigma being placed on intersex cases. A multidisciplinary approach to this problem involving paediatric specialists in the field, of endocrinology, surgery and psychiatry is necessary, along with educational programmes that promote tolerance in society to variations in gender.

Hermaphroditism: cytogenetics, gonadal pathology and gender assignment: a case report.

True hermaphroditism is a rare intersex disorder in which individuals possess both testicular and ovarian gonadal tissue. A case of true unilateral hermaphroditism presenting with ambiguous external genitalia, right scrotal testis and left pelvic ovotestis is herein outlined Phallic, gonadal and genetic factors were considered before male gender was assigned. Gender assignment procedures have been questioned by intersex activists opposed to early genital surgery. Western societies have a binary perspective on gender and this leads to a stigma being placed on intersex cases. A multidisciplinary approach to this problem involving paediatric specialists in the field, of endocrinology, surgery and psychiatry is necessary, along with educational programmes that promote tolerance in society to variations in gender.

Does exogenous GM1 ganglioside enhance the effects of electrical stimulation in ameliorating degeneration after neonatal deafness?

This study examined the combined effects of administration of exogenous GM1 ganglioside and electrical stimulation on the cochlear nucleus (CN) of cats deafened neonatally by ototoxic drugs. Five normal hearing adult cats served as controls. Another 12 cats were deafened bilaterally by daily injections of neomycin sulfate (60 mg/kg) for 17-21 days after birth until auditory brainstem testing demonstrated profound hearing loss. Six of the deaf animals comprised the GM1 group, which received daily injections of GM1 ganglioside (30 mg/kg) for 28-38 days during the period after profound deafness was confirmed, and prior to receiving a cochlear implant. The non-GM1 group (n=6) received no treatment during this interim period. All the deafened animals underwent unilateral cochlear implantation at 6-9 weeks postnatal and received several months (mean duration, 32 weeks) of chronic electrical stimulation (4 h/day, 5 days/week). Stimulation was delivered by intracochlear bipolar electrodes, using electrical signals that were designed to be temporally challenging to the central auditory system. Results showed that in the neonatally deafened animals, both the GM1 and non-GM1 groups, the volume of the CN was markedly reduced (to 76% of normal), but there was no difference between the animals that received GM1 and those that did not. The cross sectional areas of spherical cell somata in both GM1 and non-GM1 groups also showed a highly significant reduction in size, to < or =75% of normal after neonatal deafening. Moreover, in both the GM1 and non-GM1 groups, the spherical cells in the CN ipsilateral to the implanted cochlea were significantly larger (6%) than cells in the control, unstimulated CN. Again, however, there was no significant difference between the GM1 group and the non-GM1 group in spherical cell size. These results contrast sharply with previous reports that exogenous GM1 prevents CN degeneration after neonatal conductive hearing loss and partially prevents spiral ganglion cell degeneration when administered immediately after ototoxic drug deafening in adult animals. Taken together, findings to date suggest that GM1 may be effective in preventing degeneration only if the GM1 is administered immediately at the time hearing loss occurs.

The effect of electrode configuration and duration of deafness on threshold and selectivity of responses to intracochlear electrical stimulation.

This report examines the effects of intracochlear electrode configuration and mode of stimulation (bipolar or monopolar) on neural threshold and spatial selectivity in the inferior colliculus (IC) of the cat. Single and multiunit IC recordings were made in three groups of animals; acutely deafened adults (controls), neonatally deafened animals studied at 6 to 18 months of age and neonatally deafened cats studied at 2.5 to 6.5 years. Response thresholds were plotted versus IC depth to measure the spatial distribution of responses. The response selectivity for each stimulating configuration was defined as the width of the resulting spatial tuning curve (STC) measured at 6 dB above threshold. Spiral ganglion cell (SG) survival was examined histologically in all neonatally deafened animals and correlated with physiological results. Animals studied at less than 1.5 years had SG densities of 23.5%-64.4% of normal (mean=42.7%) while animals studied at greater than 2.5 years had densities of 5.1%-18.3% of normal (mean=9.9%). Electrophysiological results include the following. (1) Monopolar thresholds were 7-8 dB lower than bipolar thresholds in the same animals. (2) Varying the configuration of bipolar contacts (measured as radial, offset radial and longitudinal pairs) did not systematically affect IC threshold in either controls or short-term neonatally deafened animals. In contrast, the long-term neonatally deafened animals showed a difference in threshold with each configuration. (3) The spatial distributions (Q(6 dB)) of responses to bipolar stimulation were approximately 40% more restricted than those for monopolar stimulation. (4) The spatial selectivity of neonatally deafened animals studied at ages up to 1.5 years was equal to that of control animals with normal auditory experience. However, selectivity was degraded in the older animals. (5) Selectivity was decreased in some animals with the longitudinal bipolar configuration and multiple response peaks were seen in several cases using this stimulus configuration.

Plasticity in central representations in the inferior colliculus induced by chronic single- vs. two-channel electrical stimulation by a cochlear implant after neonatal deafness.

The goal of this research is to examine the functional consequences of patterned electrical stimulation delivered by a cochlear implant in the deafened developing auditory system. In previous electrophysiological experiments conducted in the inferior colliculus (IC), we have demonstrated that the precise cochleotopic organization of the central nucleus (ICC) develops normally in neonatally deafened unstimulated cats and is unaltered despite the lack of normal auditory input during development. However, these studies also showed that chronic electrical stimulation delivered at a single intracochlear location by one bipolar channel of a cochlear implant induces significant expansion of the central representation of the stimulated cochlear sector and degrades the cochleotopic organization of the IC. This report presents additional data from a new experimental series of neonatally deafened cats that received chronic stimulation on two adjacent bipolar intracochlear channels of a cochlear implant. Results suggest that competing inputs elicited by electrical stimulation delivered by two adjacent channels can maintain the selective representations of each activated cochlear sector within the central auditory system and prevent the expansion seen after single-channel stimulation. Alternating stimulation of two channels and use of highly controlled electrical signals may be particularly effective in maintaining or even sharpening selectivity of central representations of stimulated cochlear sectors. In contrast, simultaneous stimulation using two channels of a model analog cochlear implant processor in one experimental animal failed to maintain channel selectivity and resulted in marked expansion and fusion of the central representations of the stimulated channels. This potentially important preliminary result suggests that under some conditions the central auditory system may be unable to discriminate simultaneous, overlapping inputs from adjacent cochlear implant channels as distinct.

Responses of inferior colliculus neurons to amplitude-modulated intracochlear electrical pulses in deaf cats.

Current cochlear prostheses use amplitude-modulated pulse trains to encode acoustic signals. In this study we examined the responses of inferior colliculus (IC) neurons to sinusoidal amplitude-modulated pulses and compared the maximum unmodulated pulse rate (Fmax) to which they responded with the maximum modulation frequency (maxFm) that they followed. Consistent with previous results, responses to unmodulated pulses were all low-pass functions of pulse rate. Mean Fmax to unmodulated pulses was 104 pulses per second (pps) and modal Fmax was 60 pps. Above Fmax IC neurons ceased responding except for an onset burst at the beginning of the stimulus. However, IC neurons responded to much higher pulse rates when these pulses were amplitude modulated; 74% were relatively insensitive to carrier rate and responded to all modulated carriers including those exceeding 600 pps. In contrast, the responses of these neurons (70%) were low-pass functions of modulation frequency, and the remaining (30%) had band-pass functions with a maxFm of 42 and 34 Hz, respectively. Thus temporal resolution of IC neurons for modulated frequencies is significantly lower than that for unmodulated pulses. These two measures of temporal resolution (Fmax and maxFm) were uncorrelated (r(2) = 0.101). Several parameters influenced the amplitude and temporal structure of modulation responses including modulation depth, overall intensity and modulation-to-carrier rate ratio. We observed distortions in unit responses to amplitude-modulated signals when this ratio was 1/4 to 1/6. Since most current cochlear implant speech processors permit ratios that are significantly greater than this, severe distortion and signal degradation may occur frequently in these devices.

Electrical cochlear stimulation in the deaf cat: comparisons between psychophysical and central auditory neuronal thresholds.

Cochlear prostheses for electrical stimulation of the auditory nerve ("electrical hearing") can provide auditory capacity for profoundly deaf adults and children, including in many cases a restored ability to perceive speech without visual cues. A fundamental challenge in auditory neuroscience is to understand the neural and perceptual mechanisms that make rehabilitation of hearing possible in these deaf humans. We have developed a feline behavioral model that allows us to study behavioral and physiological variables in the same deaf animals. Cats deafened by injection of ototoxic antibiotics were implanted with either a monopolar round window electrode or a multichannel scala tympani electrode array. To evaluate the effects of perceptually significant electrical stimulation of the auditory nerve on the central auditory system, an animal was trained to avoid a mild electrocutaneous shock when biphasic current pulses (0.2 ms/phase) were delivered to its implanted cochlea. Psychophysical detection thresholds and electrical auditory brain stem response (EABR) thresholds were estimated in each cat. At the conclusion of behavioral testing, acute physiological experiments were conducted, and threshold responses were recorded for single neurons and multineuronal clusters in the central nucleus of the inferior colliculus (ICC) and the primary auditory cortex (A1). Behavioral and neurophysiological thresholds were evaluated with reference to cochlear histopathology in the same deaf cats. The results of the present study include: 1) in the cats implanted with a scala tympani electrode array, the lowest ICC and A1 neural thresholds were virtually identical to the behavioral thresholds for intracochlear bipolar stimulation; 2) behavioral thresholds were lower than ICC and A1 neural thresholds in each of the cats implanted with a monopolar round window electrode; 3) EABR thresholds were higher than behavioral thresholds in all of the cats (mean difference = 6.5 dB); and 4) the cumulative number of action potentials for a sample of ICC neurons increased monotonically as a function of the amplitude and the number of stimulating biphasic pulses. This physiological result suggests that the output from the ICC may be integrated spatially across neurons and temporally integrated across pulses when the auditory nerve array is stimulated with a train of biphasic current pulses. Because behavioral thresholds were lower and reaction times were faster at a pulse rate of 30 pps compared with a pulse rate of 2 pps, spatial-temporal integration in the central auditory system was presumably reflected in psychophysical performance.

Behavioral and neurophysiological thresholds for electrical cochlear stimulation in the deaf cat.

Psychophysical detection thresholds for unmodulated electrical pulse trains or for sinusoidally amplitude-modulated (SAM) pulse trains were estimated in deaf juvenile cats using a conditioned avoidance paradigm. Biphasic current pulses (0.2 ms/phase) were delivered by scala tympani electrodes consisting of 4-8 electrode contacts driven as bipolar pairs. Electrical auditory brainstem response (EABR) thresholds were obtained periodically, and at the conclusion of behavioral training, response thresholds were obtained for neurons in the inferior colliculus (IC) and the primary auditory cortex (A1) in acute physiological experiments in the same animals. The results of the study include: (1) detection thresholds for unmodulated pulse trains and for SAM pulse trains were virtually identical; (2) EABR thresholds and behavioral thresholds were significantly correlated, although EABR thresholds consistently overestimated behavioral thresholds; (3) the lowest thresholds in the IC and the A1 were significantly correlated with behavioral thresholds, and (4) mean lowest thresholds in the IC and the A1 were essentially the same as the mean psychophysical detection threshold in the trained deaf cats.

Temporal properties of chronic cochlear electrical stimulation determine temporal resolution of neurons in cat inferior colliculus.

As cochlear implants have become increasingly successful in the rehabilitation of adults with profound hearing impairment, the number of pediatric implant subjects has increased. We have developed an animal model of congenital deafness and investigated the effect of electrical stimulus frequency on the temporal resolution of central neurons in the developing auditory system of deaf cats. Maximum following frequencies (Fmax) and response latencies of isolated single neurons to intracochlear electrical pulse trains (charge balanced, constant current biphasic pulses) were recorded in the contralateral inferior colliculus (IC) of two groups of neonatally deafened, barbiturate-anesthetized cats: animals chronically stimulated with low-frequency signals (< or = 80 Hz) and animals receiving chronic high-frequency stimulation (> or = 300 pps). The results were compared with data from unstimulated, acutely deafened and implanted adult cats with previously normal hearing (controls). Characteristic differences were seen between the temporal response properties of neurons in the external nucleus (ICX; approximately 16% of the recordings) and neurons in the central nucleus (ICC; approximately 81% of all recordings) of the IC: 1) in all three experimental groups, neurons in the ICX had significantly lower Fmax and longer response latencies than those in the ICC. 2) Chronic electrical stimulation in neonatally deafened cats altered the temporal resolution of neurons exclusively in the ICC but not in the ICX. The magnitude of this effect was dependent on the frequency of the chronic stimulation. Specifically, low-frequency signals (30 pps, 80 pps) maintained the temporal resolution of ICC neurons, whereas higher-frequency stimuli significantly improved temporal resolution of ICC neurons (i.e., higher Fmax and shorter response latencies) compared with neurons in control cats. Furthermore, Fmax and latencies to electrical stimuli were not correlated with the tonotopic gradient of the ICC, and changes in temporal resolution following chronic electrical stimulation occurred uniformly throughout the entire ICC. In all three experimental groups, increasing Fmax was correlated with shorter response latencies. The results indicate that the temporal features of the chronically applied electrical signals critically influence temporal processing of neurons in the cochleotopically organized ICC. We suggest that such plastic changes in temporal processing of central auditory neurons may contribute to the intersubject variability and gradual improvements in speech recognition performance observed in clinical studies of deaf children using cochlear implants.