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Biomedical subjects

S M Bowyer

Publications and source records attributed to S M Bowyer.

14 recordsLinked to original sources
PubMed2006

Retrospective review of MEG visual evoked hemifield responses prior to resection of temporo-parieto-occipital lesions.

Visual evoked cortical magnetic field (VEF) waveforms were recorded from both hemifields in 21 patients with temporo-parieto-occipital mass lesions to identify preserved visual pathways. Fifteen patients had visual symptoms pre-operatively. Magnetoencephalographic (MEG) VEF responses were detected, using single equivalent current dipole (ECD), in 17/21 patients studied. Displaced or abnormal responses were seen in 15 patients with disruption of pathway in one patient. Three of 21 patients had alterations in the surgical approach or the planned resection based on the MEG findings. The surgical outcome for these three patients suggests that the MEG study may have played a useful role in pre-surgical planning.

Adult↗
PubMed2005

Multi-Resolution FOCUSS: a source imaging technique applied to MEG data.

A variety of techniques are available for imaging magnetoencephalographic (MEG) data to the corresponding cortical structures. Each performs a functional optimization that includes mathematical and physical restrictions on source activity. Unlike other imaging techniques, MR-FOCUSS (Multi-Resolution FOCal Underdetermined System Solution) utilizes a wavelet statistical operator that allows spatial resolution to be chosen appropriately for focal or extended sources. Control of focal imaging properties is achieved by specifying P in an l(P) norm distribution template used to construct the wavelets. In addition, incorporation of a multi-resolution wavelet operator desensitizes the mathematical algorithm to noise, (regularization). Like the FOCUSS imaging technique, an initial estimate of cortical activity is recursively enhanced to obtain the final high resolution imaging results. Studies of model MEG data representing all regions of a realistic cortical model are performed to quantify MR-FOCUSS imaging properties. These modeled data studies included single and multiple dipole sources as well as an extended source model. Thus, MR-FOCUSS is found to be very effective for imaging language processing for pre-surgical planning and provides a high-resolution method to image sequential activation of multiple correlated sources involved in language processing.

Algorithms↗
PubMed2004

MEG localization of language-specific cortex utilizing MR-FOCUSS.

OBJECTIVE: To demonstrate noninvasive localization of cognitive cortical areas involved in language processing with magnetoencephalography (MEG) interpreted by multiresolution FOCUSS (MR-FOCUSS), a current density imaging technique. METHOD: MEG data were collected during verb-generation and picture-naming tasks from 18 right-handed control subjects and 24 right-handed patients with epilepsy. RESULTS: The averaged epic data from the verb-generation task, analyzed by MR-FOCUSS, showed initial activation in the left supramarginal gyrus, superior temporal gyrus, and angular gyrus at 239 +/- 31 ms in all subjects, consistent with other language mapping studies. Average amplitude of underlying cortical sources was approximately 452 pAm. The averaged epic data from the picture-naming task, analyzed by MR-FOCUSS, showed activation in the left inferior frontal gyrus (IFG) area starting at 436 +/- 40 ms in all subjects. Average amplitudes of underlying cortical sources were approximately 380 pAm. CONCLUSION: The time course of neuronal language processing can be imaged noninvasively with millisecond resolution by magnetoencephalography using the multiresolution FOCUSS technique.

Cerebral Cortex↗
PubMed2001

Magnetoencephalographic fields from patients with spontaneous and induced migraine aura.

We investigate and characterize the magnetoencephalographic waveforms from patients during spontaneous and visually induced migraine aura. Direct current neuromagnetic fields were measured during spontaneous onset of migraine auras in 4 migraine patients, and compared with recordings from 8 migraine-with-aura patients and 6 normal controls during visual stimulation of the occipital cortex. Complex direct current magnetoencephalographic shifts, similar in waveform, were observed in spontaneous and visually induced migraine patients, but not in controls. Two-dimensional inverse imaging showed multiple cortical areas activated in spontaneous and visually induced migraine aura patients. In normal subjects, activation was only observed in the primary visual cortex. Results support a spreading, depression-like neuroelectric event occurring during migraine aura that can arise spontaneously or be visually triggered in widespread regions of hyperexcitable occipital cortex.

Adult↗
PubMed2000

Relationships within and between genotypes of hepatitis B virus at points across the genome: footprints of recombination in certain isolates.

Hepatitis B virus (HBV) was partitioned into type, subtype and isolate categories and the average evolutionary distances within and between categories was plotted at each of 54 points along the genome. The graphs showed alternating variable and conserved domains within and between HBV subtypes and revealed that some specimens assigned to different groups are more similar across several contiguous intervals than specimens belonging to the same group. Isolates were screened individually to determine their conformation to type and mosaic structure was identified in 14/65 specimens. Two entire clades (six specimens) of genotype B had a B/C sequence switch in the core gene region, whereas six genotype D specimens showed D/A switching in one or more regions of the genome. Genotype E was not separate from genotype D in the X and C subgenomic regions. The nature and distribution of polymorphic sites in mosaic regions was mapped at both the nucleotide and protein levels and the position of the variant fragments was related to mutational hot spots and linear epitopes of HBV. Mosaic structure was demonstrated statistically in 11 isolates using bootstrap resampling and recombination, rather than random change, appeared to be the mechanism responsible. The sequence between and including the two DR regions was represented in all putative recombinants. The distribution of genetic distances over subgenomic regions showed that substitution rates are not constant among the lineages of HBV in the preS regions. Genotype F is the most diverse group. Only genotypes A, C and F partition consistently into subtypes.

Animals↗
PubMed1999

Analysis of MEG signals of spreading cortical depression with propagation constrained to a rectangular cortical strip. I. Lissencephalic rabbit model.

Magnetic fields arising from the rabbit cortex during spreading cortical depression (SCD) were measured in order to study the currents in the neocortex during SCD. SCD was constrained to propagate in a rectangular cortical strip perpendicular to the midline. This simplified in vivo cortical preparation enabled us to correlate magnetoencephalographic (MEG) signals to their underlying currents within the cortical strip. The propagation of SCD was monitored with an array of electrodes placed along the strip. The propagation speed for SCD in the lissencephalic rabbit brain was 3. 5+/-0.3 mm/min (mean+/-S.E.M., n=14). Slow, quasi-dc, MEG signals were observed as the SCD entered into the longitudinal fissure. The currents giving rise to the MEG signals were perpendicular to the cortical surface and directed from the surface to deeper layers of the cortex. A distributed dipolar source model was used to relate the data to the underlying cortical current. The moment of the single equivalent current dipole source was 38+/-9 nA-m (n=17). This study clarified the nature of the cortical currents during SCD in a lissencephalic in vivo preparation.

Animals↗
PubMed1999

Analysis of MEG signals of spreading cortical depression with propagation constrained to a rectangular cortical strip. II. Gyrencephalic swine model.

Currents produced during spreading cortical depression (SCD) in a gyrencephalic species (swine) were studied with magnetoencephalography (MEG) and electrocorticography (ECoG). SCD, initiated using electrical stimulation of the cortex, was constrained to propagate within a rectangular cortical strip in order to simplify the interpretation of the underlying currents. The ECoG signals monitored along the strip revealed that SCD propagated from an initiation site on the gyrus at a rate of 7.9+/-3.2 mm/min (n=23), entered the deep coronal sulcus and in most cases emerged from the other side of the sulcus, continuing to propagate across the next gyrus at a rate of 5.9+/-2.7 mm/min (n=22). The apparent propagation velocity within the sulcus was reduced to 1.7+/-0.8 mm/min (n=21). Strong MEG signals were observed as SCD entered the sulcus. The direction of magnetic field was opposite for SCD's on opposite banks of the sulcus. The currents were directed from a superficial layer to deeper layers of the cortex. The characteristics of SCD and associated MEG patterns from a gyrencephalic species may be similar to those in human patients during migraine aura.

Animals↗
PubMed1997

A unique segment of the hepatitis B virus group A genotype identified in isolates from South Africa.

The preS2/S genes of hepatitis B virus isolated from 29 acutely or chronically infected individuals in the Gauteng province of South Africa were sequenced. Phylogenetic analysis of these sequences in comparison with global isolates from the GenBank database showed that 24 sequences clustered with genotypic group A, three with genotypic group D and one each with genotypic groups B and C. Group A isolates had greater identity with groups D (variation of 6.6%) and E (6.8%) than with the Eastern groups B (7.4%) and C (8.1%) and were most different from group F (11.0%). Of the South African group A specimens, 59.1% clustered with two global sequences to form a discrete segment which we have called subgroup A. The amino acid differences that set these isolates apart from the rest of group A tended to cluster in the preS2 region (amino acids 7, 10, 32, 35, 47, 48, 53 and 54), with a few changes occurring in the major surface antigen (amino acid sites 207 and 209). Analysis of isolates showed that there was a 9-fold higher prevalence of the ay determinant in South Africa than previously reported.

Amino Acid Sequence↗
PubMed1987

Expression of the hepatitis B virus genome in chronic hepatitis B carriers and patients with hepatocellular carcinoma.

We examined the methylation status of CCGG sites in hepatitis B virus (HBV) DNA to determine whether methylation could be responsible for the selective expression of the HBV surface gene in chronic hepatitis B infection and hepatocellular carcinoma. Infected liver tissue from patients with low levels of viral replication was analyzed for HBV DNA copy number per haploid cell genome. Total cellular DNA, with sufficient HBV DNA, was digested with the restriction endonucleases Msp I and Hpa II, to determine whether the HBV DNA was methylated, or HindIII, to determine whether the HBV DNA was integrated or episomal. The cleavage fragments were analyzed by Southern blotting and hybridization to 32P-labeled HBV DNA. In replicative chronic hepatitis B, hypomethylation of the HBV genome correlated with HBV expression in both virions and infected tissue. In carriers with nonreplicative infection, it was difficult to ascertain the role of methylation as copy number was low. HBV DNA copy number was also low in 17 out of 29 of the tumor tissues tested and as many as 14 out of 16 of the adjacent non-neoplastic tissues tested. Integrated sequences were hypermethylated in the PLC/PRF/5 cell line and in six of the tumor tissues suggesting that methylation plays a role in HBV gene repression. However, since DNA from five other tumors was hypomethylated, the belief that methylation per se is an absolute determinant of HBV core gene repression does not hold for human hepatocellular carcinoma tissue. Additional factors, such as gene rearrangements, therefore, must influence HBV expression in hepatocellular carcinoma.

Base Sequence↗
PubMed1986

Small and asymptomatic hepatocellular carcinoma detected by alpha-fetoprotein screening in black hepatitis B carriers. A report of 2 cases.

Two patients in whom asymptomatic and small hepatocellular carcinomas (HCC) were detected by alpha-fetoprotein (AFP) screening are reported. Both patients were hepatitis B surface antigen-positive. In the first patient, the tumour grew slowly and was resected more than 2 years after a significant elevation in serum AFP levels had been first detected and 13 months after hepatic angiography confirmed the presence of a vascular tumour. In the second patient, a small encapsulated HCC was diagnosed by AFP screening and hepatic imaging. The clinical course in these 2 patients illustrates that small, asymptomatic and encapsulated HCCs do occur in southern African black hepatitis B carriers. Regular screening of patients at risk may be justified.

Adult↗
PubMed1985

Replication of hepatitis B virus in adult carriers in an endemic area.

We have examined serological markers of replicative and nonreplicative infection in 124 adult, black South African carriers of hepatitis B virus (HBV), in whom this infection is predominantly acquired in early childhood. The mean age of the group was 36 years. Antibody to hepatitis B e antigen (anti-HBe) was present in the serum of 93.5% of these carriers. Only 25.8% of the carriers were positive for HBV DNA in serum, and in the majority of these only trace amounts were detectable. IgM antibody to hepatitis B core antigen (IgM anti-HBc) was negative in 54% of the carriers, and only 26% had IgM anti-HBc in high titer. A significantly greater proportion of carriers who were positive for anti-HBe were positive for IgM anti-HBc (43.1%) than were positive for HBV DNA (24.5%). Serum aminotransferases were less than twofold elevated in 90.3% of the carriers. Only one carrier has thus far developed hepatocellular carcinoma. These results suggest that there is an inexorable progression to predominantly nonreplicative infection in the majority of southern African adult, black carriers, an occurrence that may take several decades. In areas endemic for HBV infection, antiviral agents effective against replicative HBV will have to be administered in childhood.

Adult↗
PubMed1985

Clinical and serological events accompanying changes in hepatitis B viral replication: case reports.

We measured serum markers of hepatitis B virus replication in two HBsAg-, HBeAg-positive hepatitis B carriers with chronic active hepatitis and cirrhosis. The first of these patients was HBsAg-, HBeAg-, HBV DNA- and HBV DNA polymerase-positive initially and spontaneously lost HBV DNA polymerase and HBV DNA. During the HBeAg-positive, DNA polymerase-negative "window phase", an increase in viral replication, characterized by the reappearance of HBV DNA and HBV DNA polymerase occurred, together with an aggravation of the underlying chronic hepatitis. In the second HBsAg-, HBeAg-positive carrier, spontaneous fluctuations in HBV replication were associated with clinical deterioration. Delta agent and hepatitis A virus superinfection were excluded. These observations suggest that spontaneous low-grade fluctuations of HBV replication accompanied by an increase in the biochemical activity of the underlying chronic hepatitis can be observed in certain HBV carriers.

Alanine Transaminase↗
PubMedDate not supplied

The occipital cortex is hyperexcitable in migraine: experimental evidence.

OBJECTIVES: Threshold for generation of magnetophosphenes has been reported to be lower in migraine. We compared the threshold for eliciting phosphenes by transcranial magnetic stimulation and the ability to visually trigger headache in a select group of individuals with migraine with and without aura to normal controls. METHODS: Transcranial magnetic stimulation was performed using the Cadwell MES-10 stimulator. A circular coil, 9.5 cm in diameter, was applied to the occipital scalp (7 cm above the inion). Stimulator intensity was increased in 10% increments until subjects reported visual phenomena or 100% intensity was reached. Stimulator intensity was then fine-tuned to determine the threshold at which phosphenes were seen. In the same subjects, visual stimulation was given in 3.0 T MRI and if a headache occurred the response was recorded. RESULTS: Fifteen subjects with migraine were compared to 8 controls. A significant proportion of the migraineurs (86.7%) developed phosphenes compared to the controls (25%) (P = .006). The probability of triggering a headache was also higher in the migraineurs (53%); no headache was triggered in the controls (P = .019). A significant correlation was found between the threshold for phosphenes on transcranial magnetic stimulation and visually triggered headache (P = .002). When only migraine was considered, there was again a significant trend (P = .084). CONCLUSIONS: There is a difference in threshold for excitability of occipital cortex in migraineurs and controls. The hyperexcitable visual cortex in migraine is predisposed to visually triggered headache.

Adult↗