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Biomedical subjects

D M Levi

Publications and source records attributed to D M Levi.

At least 19 recordsLinked to original sources
PubMed2006

Refractory ascites after liver transplantation: an analysis of 1058 liver transplant patients at a single center.

A retrospective study of 1058 liver transplant recipients was performed to determine: (i) the incidence, etiology, timing, clinical features and treatment of refractory ascites (RA), (ii) risk factors for RA development, (iii) predictors of RA disappearance, (iv) predictors of survival following RA and (v) the impact of RA on patient survival. Sixty-two patients (5.9%) developed RA and its disappearance occurred in 27/62 cases. Patients having hepatitis C virus (HCV) had a significantly higher hazard rate of developing RA (p < 0.00001). No other baseline characteristic was associated with RA. Cox stepwise regression analysis of the hazard rate of RA disappearance found two significant factors: HCV recurrence as the reason for developing RA implied a poorer outcome (p = 0.006), whereas an unknown reason implied a favorable outcome (p = 0.02). In addition, survival following RA was significantly poorer among patients having bacterial peritonitis or HCV recurrence. Finally, the mortality rate was significantly (nearly 8.6 times) higher in patients following RA development while it was ongoing (p < 0.00001); however, if the RA disappeared, then the additional risk of death also disappeared. This study illustrates the importance of developing an optimal treatment strategy to (i) effectively treat RA if it develops and (ii) prevent hepatitis C recurrence.

Adolescent↗
PubMed2005

The impact of Campath 1H induction in adult liver allotransplantation.

BACKGROUND: We report our experience with Campath 1H in adult liver allotransplantation. METHODS: Between December 2001 and February 2004, 77 patients underwent liver transplantation using Campath 1H induction and low-dose maintenance tacrolimus immunosuppression. The control group consisted of 50 patients with similar baseline characteristics and the same eligibility criteria, transplanted under our standard Tacrolimus/steroids regimen. Hepatitis C patients were excluded from the study. RESULTS: Patient and graft survival were similar for both groups. The incidence of rejection was significantly lower in the Campath vs the control group (51% vs 65% at 12 months, P = .009). Tacrolimus trough levels and conversion from Tacrolimus or the addition of other immunosuppressive drugs due to nephrotoxicity were also significantly lower in the Campath 1H group. CONCLUSION: Campath 1H induction with low-dose Tacrolimus maintenance immunosuppression is an effective regimen in reducing acute rejection in adult liver transplantation, while maintaining lower tacrolimus levels and less nephrotoxicity than our conventional immunosuppressive regimen.

Adult↗
PubMed2005

Transplantation for the treatment of intra-abdominal fibromatosis.

MATERIALS AND METHODS: During the last 9 years we treated 14 patients with a diagnosis of intra-abdominal fibromatosis. The 11 patients who received an intestinal allograft included isolated intestine (n = 6), liver-intestine (n = 1), intestine-kidney (n = 1), multivisceral (n = 1), multivisceral-kidney (n = 1), multivisceral-no liver (n = 1). Three patients received an intestinal autograft after partial abdominal evisceration and ex vivo tumor resection. Three patients additionally underwent an abdominal wall allograft. RESULTS: At follow-up until August 2004, all autotransplant patients are alive. Four intestinal transplant patients died within the first postoperative month. There were three graft losses. A patient who lost his graft early postoperatively was retransplanted but died of sepsis shortly there after. Two more patients lost their graft due to severe rejection and were retransplanted successfully. Two patients developed desmoid tumor recurrence in their abdominal or thoracic wall. Ten patients are alive 1 to 9 years posttransplantation. Nine have fully functioning grafts and one patient requires TPN supplementation at night due to dysmotility of her autograft. CONCLUSION: Intestinal allo-, or autotransplantation combined with transplantation of the abdominal wall can be lifesaving for patients suffering from extensive intra-abdominal fibromatosis.

Fibromatosis, Abdominal↗
PubMed2004

Trends in serum citrulline and acute rejection among recipients of small bowel transplants.

A test for detecting acute cellular rejection (ACR) of small intestinal transplants (ITx) would be a major advance. Small preliminary studies suggest that serum citrulline levels correlate with ACR. The results for a group of 26 isolated intestinal and multivisceral transplant recipients are summarized here. Serum citrulline concentrations were determined by ion exchange chromatography and compared to biopsy-based grade of ACR. Other factors considered included patient and donor age and sex, ischemia time, and serum creatinine. Straight-line fits were employed to describe how each patient's citrulline levels changed over time. Estimated times to achieve normal citrulline (>or=30 micromol/L) ranged from 1 to 730 days posttransplant for 21 patients demonstrating increasing citrulline levels over time. Using stepwise linear regression, patients' ranks for time required to achieve normal citrulline levels were the only independent predictors of both maximum ACR (P <.0001) and average ACR (P =.0059) after 14 days posttransplant. The rate and direction of change in citrulline over time may be an indicator of the risk of acute rejection. We plan to further examine the use of citrulline as a marker for rejection in larger prospective studies.

Adult↗
PubMed2004

Expanded use of transplantation techniques: abdominal wall transplantation and intestinal autotransplantation.

Surgical principles and techniques derived from organ transplantation surgery can provide novel applications in general surgery. We present an update on our 5-year experience with intestinal autotransplantation and abdominal wall transplantation. Nine patients underwent intestinal or multivisceral transplantation with the addition of 10 abdominal wall grafts to cover the large open areas from previous surgeries. Seven patients underwent near-total abdominal evisceration, ex vivo resection of masses at the base of the mesentery, followed by intestinal autotransplantation; 44% of the abdominal wall graft recipients are alive, but none of the fatalities were related to the graft itself. In two cases the graft had to be removed due to venous thrombosis. Of patients with intestinal autotransplants, 71% are alive with two mortalities due to recurrent metastatic malignancy. In only one case, the intestinal autograft had to be removed because of venous thrombosis. All surviving patients but one are on a regular diet; two are on supplemental enteral feeds. These results show that anastomotic and resection techniques derived from the experience in solid organ transplant can be utilized in complex wound closure, as is the case of abdominal wall transplantation, or resection of large retroperitoneal tumors with intestinal autotransplantation.

Abdominal Wall↗
PubMed2003

Intestinal transplantation: advances in immunosuppression and surgical techniques.

Campath-1H is being used as induction immunosuppression for intestinal/multivisceral transplantation. Patient and graft survival in this preliminary experience is similar to previous studies but there has been a significant decrease in the incidence and severity of acute rejections without increase of opportunistic infections. Collage of the abdominal wall (transplantation of a composite graft of the abdominal wall) can provide biologic coverage of the newly transplanted abdominal organs if necessary. Partial abdominal exenteration, ex vivo resection, and intestinal autotransplantation may be useful in removing otherwise unresectable lesions of the root of the mesentery.

Graft Rejection↗
PubMed2001

Serum citrulline and rejection in small bowel transplantation: a preliminary report.

BACKGROUND: There is no known serum marker for intestinal rejection. Serum concentrations of the amino acid citrulline arise almost exclusively from the intestinal mucosa. We examined the impact of acute cellular rejection (ACR) of intestinal allografts on serum citrulline levels. METHODS: Citrulline concentrations were assayed in serum samples of healthy volunteers (n=6) and seven patients who underwent small bowel transplants (SBTx). Trends in mean citrulline concentrations versus degree of ACR were assessed by matching posttransplantation citrulline concentrations with patients' grade of ACR at time of serum collection. Rejection was confirmed by biopsy and graded by following standardized criteria. An additional patient had citrulline concentrations determined for 31 sequential specimens 3-60 days posttransplant. RESULTS: Mean citrulline concentrations in controls were significantly higher than posttransplantation samples at any rejection grade. Mean concentrations declined significantly as rejection severity increased. The overall downward trend was statistically significant (P<0.05). In sequential measurements, citrulline levels increased significantly over time with declining severity of rejection. The increase in mean citrulline concentration between posttransplant days 3-16 and 52-60 was significant (P<0.01). CONCLUSIONS: Serum citrulline levels decline with increasing grade of ACR and may be a useful serum marker for intestinal rejection.

Adult↗
PubMed2001

Integration of local features into a global shape.

It is a well-established fact that the cortical filters selective for spatial frequency and orientation have a limited spatial extent. The present study investigated how information from local filters is integrated into global shapes. Specifically, we were interested in whether identification of a global pattern consisting of small oriented, spatially-bandpass features (Gabor patches) depends on the orientations of those features. The observer was presented with a C-like stimulus shape comprised of oriented Gabor elements on a blank background, and the performance measure was the threshold contrast for identifying the global orientation of the C-shape (four possible rotated orientations). Performance was significantly enhanced (i.e. contrast thresholds were lower) when the local features were aligned with the C-contour than when the features were orthogonal to it or they had mixed local orientations (randomly both aligned and orthogonal). Control experiments showed that the alignment of local features improves global pattern perception only when the features are close to each other. In contrast, a close spacing between the features interfered with global pattern perception when the local orientations were mixed.

Algorithms↗
PubMed2001

Spatial-frequency and contrast properties of crowding.

Crowding, the difficulty in recognizing a letter flanked by other letters, has been explained as a lateral masking effect. The purpose of this study was to examine the spatial-frequency and contrast dependencies of crowding, and to compare them with the properties of pattern masking. In experiment 1, we measured contrast thresholds for identifying the middle letters in strings of three randomly chosen lower-case letters (trigrams), for a range of letter spacings. Letters were digitally filtered using a set of bandpass filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter. Bandwidth of the filters was 1 octave. Frequencies of the target and flanking letters were the same, or differed by up to 2 octaves. Contrast of the flanking letters was fixed at the maximum value. Testing was conducted at the fovea and 5 degrees eccentricity. We found that crowding exhibits spatial-tuning functions like masking, but with generally broader bandwidths than those for masking. The spatial extent of crowding was found to be about 0.5 deg at the fovea and 2 deg at 5 degrees eccentricity, independent of target letter frequency. In experiment 2, we measured the contrast thresholds for identifying the middle target letters in trigrams for a range of flanking letter contrasts at 5 degrees eccentricity. At low flanker contrast, crowding does not show a facilitatory region, unlike pattern masking. At high flanker contrast, threshold rises with contrast with an exponent of 0.13-0.3, lower than corresponding exponents for pattern masking. In experiment 3, we varied the contrast ratio between the flanking letters and the target letters, and found that the magnitude of crowding increases monotonically with contrast ratio. This finding contradicts a prediction based on a grouping explanation for crowding. Our results are consistent with the postulation that crowding and masking may share the same first stage linear filtering process, and perhaps a similar second-stage process, with the additional property that the second-stage process in crowding pools information over a spatial extent that varies with eccentricity.

Analysis of Variance↗
PubMed2001

Is second-order spatial loss in amblyopia explained by the loss of first-order spatial input?

The purpose of the study was to determine whether amblyopes show detection loss for second-order spatial information, and if present, whether the loss is explained by the loss of first-order spatial input. We psychophysically determined detection thresholds for the amblyopic and non-amblyopic eyes of five adult amblyopes and the dominant eyes of three control observers. We found that four amblyopic eyes and two non-amblyopic eyes showed second-order loss relative to the control eyes. The second-order loss was greater than the first-order loss at the carrier spatial frequency (first-order input). The extra second-order loss indicates an early amplification of cortical neural loss that we speculate is due to deficient binocular input to second-order neurons.

Adult↗
PubMed2001

Surround modulation of perceived contrast and the role of brightness induction.

We studied iso- and cross-orientation surround modulation of perceived contrast (contrast-contrast phenomenon) with a contrast-matching method. Our results indicate (1) iso-oriented surrounds at all contrasts suppress perceived contrast of the test pattern. Cross-orientation surrounds, however, tend to enhance the perceived contrast of the test, particularly for high-contrast test patterns. Iso-orientation modulation acts over larger distances than does cross-orientation modulation. Surround modulation of perceived contrast is not accompanied by a simultaneous change of discrimination threshold. (2) Iso-orientation surround suppression is phase insensitive when brightness induction due to local luminance contrast is eliminated by a small center-surround gap. (3) Perceived contrast is similarly affected when the surround spatial frequency is equal to or higher than the center spatial frequency, but lower spatial frequency surrounds markedly enhance perceived contrast as a result of brightness induction. These data indicate that the contrast-contrast phenomenon is often mixed with brightness induction when it is measured with sinusoidal grating stimuli, and we suggest that this may account for some of the individual differences. After excluding the role of brightness induction, surround modulation of perceived contrast appears to be a second-order process that is phase independent and not tuned or very broadly tuned to spatial frequency.

Contrast Sensitivity↗
PubMed2000

Amblyopes see true alignment where normal observers see illusory tilt.

Amblyopia ("lazy eye") is an impairment in visual acuity resulting from abnormal neural development in the visual cortex. We tested the responses of ten amblyopic and six normal observers to illusions of perceived orientation in textures of Gabor patches: the "Fraser illusion," the "phase illusion," and a "tilted chain" illusion. The illusory tilt of the stimulus rows was matched by actual tilt in the opposite direction by using the method of constant stimuli. Amblyopes showed a significant increase in the Fraser illusion, a decrease in the phase illusion, and a reversal of the tilted chain illusion. Amblyopic performance could be simulated in normal observers by reducing the length of the rows. These results can be modeled by a theory which places the neural abnormality in amblyopia at the level of second stage grouping processes. Additionally, the illusions might be useful in the early diagnosis of amblyopia without the need for prior refractive correction.

Amblyopia↗
PubMed2000

Perception of mirror symmetry reveals long-range interactions between orientation-selective cortical filters.

We investigated perceptual interactions between orientation selective cortical filters using a task in which the observer was to detect mirror symmetry in briefly flashed visual patterns composed of oriented Gabor elements. On each trial, the Gabor patches were randomly placed in one half of the stimulus region, and symmetry was generated by reflecting the positions of the patch centers across the vertical axis. A prespecified proportion of patches were in mirror symmetrical positions with the remaining patches placed at random positions. The perception of mirror symmetry was measured for three stimulus conditions: (1) same orientation (i.e. all the local Gabor elements were either vertical or horizontal), (2) mixed matching (i.e. the orientations could be randomly both vertical and horizontal with the constraint that the orientations in a mirror symmetrical pair were matching), and (3) mixed opposing (i.e. the orientations were both vertical and horizontal, but in a mirror symmetrical pair they were always orthogonal). We found that the perception of global symmetry was poorer (thresholds were elevated) when the local orientations of feature pairs were orthogonal than when they were matched. This result is consistent with the properties of the neurons in the corpus callosum, which selectively interconnect cortical filters with identical orientation specificity.

Brain Mapping↗
PubMed2000

Liver transplantation with renoportal anastomosis after distal splenorenal shunt.

BACKGROUND: The distal splenorenal shunt (DSRS) is designed to maintain hepatopetal portal vein flow while decompressing gastroesophageal varices. However, over time, as the underlying liver disease progresses, the DSRS loses its selectivity. The most common method of addressing this issue during orthotopic liver transplantation is shunt ligation with or without splenectomy. Dismantling the shunt increases the complexity of the transplantation, and splenectomy may increase the risk of infection. HYPOTHESIS: Anastomosis of the donor portal vein to the left renal vein without dismantling the shunt is an effective method of portal vein reconstruction for patients with a patent DSRS. DESIGN: Retrospective analysis. SETTING: University-based teaching hospital, Miami, Fla. PATIENTS: Five liver transplant recipients with patent DSRS who received an orthotopic liver transplant between September 1996 and August 1999. INTERVENTIONS: The donor portal vein was anastomosed end-to-end to the left renal vein during liver transplantation. MAIN OUTCOME MEASURES: Perioperatve morbidity, portal vein flow by Doppler study, patient survival, and graft survival. RESULTS: In all patients, the graft liver reperfused promptly via flow through the left renal vein with adequate decompression of the bowel. Normal portal venous flow was demonstrated by intraoperative and postoperative Doppler ultrasound studies. At the mean follow-up of 16 months, 4 patients were alive with well-functioning grafts. CONCLUSIONS: This novel technique has the advantage of decreasing the complexity of the procedure, without requiring splenectomy, while securing adequate portal perfusion. Additionally, it can be applied without modifications in patients with portal vein thrombosis.

Aged↗
PubMed2000

Detecting disorder in spatial vision.

In normal foveal vision, visual space is accurately mapped from retina to cortex. However, the normal periphery, and the central field of strabismic amblyopes have elevated position discrimination thresholds, which have often been ascribed to increased 'intrinsic' spatial disorder. In the present study we evaluated the sensitivity of the human visual system (both normal and amblyopic) to spatial disorder, and asked whether there is increased 'intrinsic' topographical disorder in the amblyopic visual system. Specifically, we measured thresholds for detecting disorder (two-dimensional Gaussian position perturbations) either in a horizontal string of N equally spaced samples (Gabor patches), or in a ring of equally spaced samples over a wide range of feature separations. We also estimated both the 'equivalent intrinsic spatial disorder' and sampling efficiency using an equivalent noise approach. Our results suggest that both thresholds for detecting disorder, and equivalent intrinsic disorder depend strongly on separation, and are modestly increased in strabismic amblyopes. Strabismic amblyopes also show markedly reduced sampling efficiency. However, neither amblyopic nor peripheral vision performs like ideal or human observers with added separation-independent positional noise. Rather, the strong separation dependence suggests that the 'equivalent intrinsic disorder' may not reflect topographic disorder at all, but rather may reflect an abnormality in the amblyopes' Weber relationship.

Adolescent↗
PubMed2000

Seeing circles: what limits shape perception?

Humans are remarkably adept at judging shapes and discriminating forms. Forms and shapes are initially sampled by discrete localized visual filters (or receptive fields) in 'early' stages of visual processing. However, more complex higher level filters which integrate or pool information from many local filters may be needed to discern shapes. In order to understand the mechanisms that limit shape perception we asked observers to detect distortions in the shape of briefly presented circles constructed out of samples (Gabor patches, which are well matched to the early visual filters), and varied the radius of the circle, and the number and orientation of the samples. Our results show that shape perception is determined by two factors: the primary determinant is the separation between the samples; however, the orientation of the samples can modulate performance. At small separations, performance is best when the samples are aligned with the global shape, poorer when they are orthogonal, and intermediate when they are all horizontal or vertical. At larger separations these contextual differences disappear; however at all separations, performance is reduced when the orientations of the samples are mixed (i.e. each sample is randomly either aligned or orthogonal, or randomly either horizontal or vertical.). These results suggest an important role for sample separation in shape perception for sampled shapes and suggest that the mechanisms involved in feature binding may modulate the responses of the mechanisms underlying shape perception.

Form Perception↗