Biomedical subjects

Ahmed T Toosy

Publications and source records attributed to Ahmed T Toosy.

5 recordsLinked to original sources

PubMed2005

Adaptive cortical plasticity in higher visual areas after acute optic neuritis.

The ability to distinguish adaptive cortical reorganization may help to target future therapeutic strategies after neurological insult. We investigated cortical plasticity by prospectively applying visual functional magnetic resonance imaging (fMRI) and optic nerve MRI to 20 patients with acute optic neuritis at baseline, 1, 3, 6, and 12 months. We performed three types of correlation analyses to investigate the relationships between fMRI activity, clinical function, and optic nerve structure. The first analysis directly correlated the fMRI response to clinical function or optic nerve structure and found dynamic relations especially within the first 3 months. The second analysis used a novel technique that modeled the fMRI response and optic nerve structure together with clinical function, to determine the contribution fMRI made to clinical function after accounting for structural factors. Significant effects were found at baseline only, within the right peristriate cortex, and bilaterally in the lateral occipital complexes, which are normally involved in higher order visual processing. The third analysis investigated the relation between the modeled visual recovery rate and fMRI response but found no significant effects. The key findings of this study are from the second analysis and suggest a genuine adaptive role for cortical reorganization within extrastriate visual areas early after optic neuritis.

Adaptation, Physiological↗

PubMed2005

Optic radiation changes after optic neuritis detected by tractography-based group mapping.

Postmortem data suggest that trans-synaptic degeneration occurs in the lateral geniculate nucleus after optic nerve injury. This study investigated in vivo the optic radiations in patients affected by optic neuritis using fast marching tractography (FMT), a diffusion magnetic resonance imaging (MRI) fiber tracking method, and group mapping techniques, which allow statistical comparisons between subjects. Seven patients, 1 year after isolated unilateral optic neuritis, and ten age and gender-matched controls underwent whole-brain diffusion tensor MR imaging. The FMT algorithm was used to generate voxel-scale connectivity (VSC) maps in the optic radiations in each subject in native space. Group maps of the left and right optic radiations were created in the patient and control group in a standardized reference frame using statistical parametric mapping (SPM99). The reconstructed optic radiations in the patient group were localized more laterally in the posterior part of the tracts and more inferiorly than in the control group. Patients showed reduced VSC values in both tracts compared with controls. These findings suggest that the group mapping techniques might be used to assess changes in the optic radiations in patients after an episode of optic neuritis. The changes we have observed may be secondary to the optic nerve damage.

Adult↗

PubMed2004

A serial MRI study following optic nerve mean area in acute optic neuritis.

This study assessed optic nerve mean area on serial MRI in a cohort of patients with a first episode of acute unilateral optic neuritis to assess the effects of a single acute inflammatory demyelinating lesion. Twenty-nine patients with a median delay from onset of visual symptoms of 13 days (range 7-24 days) were recruited. After a clinical examination and visual evoked potential (VEP) measurement, each patient had their optic nerves imaged with a coronal fat-saturated short echo fast fluid-attenuated inversion recovery sequence. Twenty-one patients had serial examinations after 2, 4, 8, 12, 26 and 52 weeks. In addition, 32 control subjects had their optic nerves imaged up to three times. The mean cross-sectional area of the intra-orbital portion of each optic nerve was calculated by a blinded observer using a computer-assisted contouring technique. At baseline, the mean area of diseased optic nerves was 16.1 mm2 compared with 13.4 mm2 for healthy contralateral optic nerves (20.1% higher, P < 0.0001) and 13.6 mm2 for controls (18.4% higher, P = 0.0003). The diseased optic nerve mean area declined over time, from initial swelling to later atrophy. The mean decline at 52 weeks was -0.0018 mm2/day (95% confidence interval -0.0038 to -0.00051). At 52 weeks, the mean area of diseased optic nerves was 11.3 mm2 compared with 12.8 mm2 for healthy contralateral optic nerves (11.7% lower, P = 0.032) and 13.1 mm2 for controls (13.7% lower, P = 0.008). The 52 week diseased optic nerve mean area was not significantly affected by the baseline mean area. There was an association between baseline optic nerve mean area and logMAR visual acuity (rS = 0.46, P = 0.012) and visual field mean deviation (rS = -0.55, P = 0.002), but there was no evidence of an association between 1 year mean area and visual outcome. There was no evidence of association between baseline, rates of decline or 1 year diseased optic nerve mean areas and any of the baseline, 1 year or time-averaged VEP variables. The present study shows a consistent pattern of changes associated with individual inflammatory demyelinating lesions in the optic nerve. Acutely, there was swelling, consistent with the presence of acute inflammation, which was related to visual impairment. Over the longer term, there was loss of tissue. The lack of association between 1 year optic nerve mean area and visual outcome may reflect a mild loss of tissue, redundancy or remodelling of function.

Acute Disease↗

PubMed2004

Visual recovery following acute optic neuritis--a clinical, electrophysiological and magnetic resonance imaging study.

This study reports the prospective follow-up of a cohort of patients with acute optic neuritis examined with serial visual tests, visual evoked potentials (VEPs), conventional and triple-dose gadolinium (Gd)-enhanced magnetic resonance imaging (MRI) to examine which factors are important in visual recovery. Thirty-three patients were recruited with acute unilateral optic neuritis. A clinical and VEP assessment was performed on each. Optic nerve MRI was performed using fast spin echo (FSE) (on all) and triple-dose Gd-enhanced T1-weighted sequences (n = 28). Optic nerve lesion lengths were measured. Serial assessments were performed on 22 of the patients up to one-year. Serial Gd-enhanced optic nerve imaging was performed on 15 of the patients until enhancement ceased. The final 30-2 Humphrey visual field mean deviation (MD) was 2.55 dB higher in patients in the lowest quartile of initial Gd-enhanced lesion length compared with the other quartiles (p < 0.01) but recovery was not related to the duration of enhancement. The initial recovery of Humphrey MD was 4.60 dB units per day in patients with good eventual recoveries (MD > -6.0 dB) and 0.99 dB per day in poor-recovery patients (p = 0.02).Good-recovery patients had mean central field VEP amplitudes 2.29 microV higher during recovery than poor-recovery patients (p = 0.047). The results suggest that factors which are associated with a better prognosis are: having a short acute lesion on triple-dose gadolinium enhanced imaging, higher VEP amplitudes during recovery and a steep gradient of the initial improvement in vision.

Adult↗

PubMed2004

Characterizing function-structure relationships in the human visual system with functional MRI and diffusion tensor imaging.

A key objective in neuroscience is to improve our understanding of the relationship between brain function and structure. We investigated this in the posterior visual pathways of healthy volunteers by applying functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) with tractography. The optic radiations were segmented using the Probabilistic Index of Connectivity (PICo) tractography algorithm and extracted at several thresholds of connection confidence. The mean fractional anisotropy (FA) of the estimated tracts was found to correlate significantly with fMRI measures of visual cortex activity (induced by a photic stimulation paradigm). The results support the hypothesis that the visual cortical fMRI response is constrained by the external anatomical connections of the subserving optic radiations.

Adult↗