Middle cerebral artery strokes causing homonymous hemianopia: positron emission tomography

Spontaneous recovery and treatment effects in patients with homonymous visual field defects: a meta-analysis of existing literature in terms of the ICF framework

Homonymous visual field defects (HVFDs) are a common consequence of posterior brain injury. Most patients do not recover spontaneously and require rehabiliation. To determine whether a certain intervention may help an individual patient, it is necessary to predict the patient's level of functioning and the effect of specific training. We provide an overview of both the existing literature on HVFDs in terms of the International Classification of Functioning, Disability, and Health (ICF) components and the variables predicting the functioning of HVFD patients or the effect of treatment. We systematically analyzed 221 publications on HVFD. All variables included in these articles were classified according to the ICF, as developed by the World Health Organization, and checked for their predictive value. We found that ICF helps to clarify the scope of the existing literature and provides a framework for designing future studies, which should consider including more outcome measures related to Activities and Participation. Although several factors have been described that predict HVFD patients' level of functioning or the effects of training, additional research is necessary to identify more.

Nuclear medicine in cerebrovascular disease

Stroke and cerebrovascular diseases are major causes of mortality, morbidity, and disability. Nuclear Medicine, primarily via tomographic methods, has made significant contributions to the understanding of the hemodynamic and metabolic consequences of cerebrovascular diseases. In this review, the findings in acute, subacute, and chronic cerebrovascular diseases are described. Many of the pathophysiologic processes and consequences that follow stroke, including completed infarct core, adjacent penumbra, and diaschisis, have been investigated with Nuclear Medicine, and stroke outcome may be related to these phenomena. Additional topics included in this review are cerebrovascular reserve tests and multi-infarct dementia. Finally, Nuclear Medicine investigations of stroke recovery and cerebral plasticity appear to indicate that enhanced activity of preexisting networks, rather than substitution of function, represents the most important mechanism of improvement in chronic stroke rehabilitation.

Serial functional imaging poststroke reveals visual cortex reorganization

PURPOSE

Visual cortical reorganization following injury remains poorly understood. The authors performed serial functional magnetic resonance imaging (fMRI) on patients with visual cortex infarction to evaluate early and late striate, ventral, and dorsal extrastriate cortical activation.

METHODS

Patients were studied with fMRI within 10 days and at 6 months. The authors used a high-level visual activation task designed to activate the ventral extrastriate cortex. These data were compared to those of age-appropriate healthy control participants.

RESULTS

The results from 24 healthy control individuals (mean age 65.7 +/- SE 3.6 years, range 32-89) were compared to those from 5 stroke patients (mean age 73.8 +/- SE 7 years, range 49-86). Patients had infarcts involving the striate and ventral extrastriate cortex. Patient activation patterns were markedly different to controls. Bilateral striate and ventral extrastriate activation was reduced at both sessions, but dorsal extrastriate activated voxel counts remained comparable to controls. Conversely, mean percent magnetic resonance signal change increased in dorsal sites.

CONCLUSIONS

These data provide strong evidence of bilateral poststroke functional depression of striate and ventral extrastriate cortices. Possible utilization or surrogacy of the dorsal visual system was demonstrated following stroke. This activity could provide a target for novel visual rehabilitation therapies.

Interhemispheric effects of simulated lesions in a neural model of letter identification

Experimental studies have produced conflicting results about the extent to which the intact, nonlesioned cerebral hemisphere is responsible for recovery from cognitive deficits following focal brain damage such as a stroke. To obtain a better theoretical understanding of interhemispheric interactions during recovery, we examined the effects of simulated lesions to a bihemispheric neural model of letter identification under various assumptions about hemispheric asymmetries, corpus callosum influence, and lesion size. Among other results, the model demonstrates that the intact hemispheric region's participation in the recovery process is a function of preexisting lateralization and lesion size, indicating that interpretation of experimental work should take these factors into account.

Positron emission tomography in diagnosing brainstem vascular lesions that cause abnormal eye movements

PURPOSE

To evaluate the efficacy of positron emission tomography (PET) in aiding in the diagnosis of brainstem infarctions that cause abnormal eye movements.

METHOD

Cerebral glucose metabolism was examined by PET with 18F-fluorodeoxyglucose as a tracer in five normal control subjects and six patients with abnormal eye movements. The PET images were registered to and superimposed on magnetic resonance images (MRIs).

RESULTS

All control subjects showed little asymmetry of glucose metabolism in the brainstem, whereas all six patients demonstrated areas of low glucose metabolism in the brainstem. Areas of low metabolism seen by PET were wider than they appeared to be by MRI; MRIs even appeared normal in some patients. Asymmetry index measurements at the level of the ischemic lesion ranged between 19% and 45%.

CONCLUSIONS

Positron emission tomography detected metabolic abnormality in patients with brainstem lesions that caused abnormal eye movements. Superimposing PET images on MRIs accurately localized abnormally low metabolism in the brainstem. Combined imaging with PET and MRI can be used to diagnose ischemic lesions in the brainstem from functional (PET) and morphologic (MRI) viewpoints.

Do changes in oxygen metabolism in the unaffected cerebral hemisphere underlie early neurological recovery after stroke? A positron emission tomography study

BACKGROUND AND PURPOSE

Whether an initial depression of function in the unaffected hemisphere ("transcallosal diaschisis") plays a role in early neurological recovery after acute stroke remains controversial. Previous studies were confounded by lack of acute-stage assessment with follow-up and by the problem of defining a suitable control group, since preexisting stroke risk factors may influence prestroke cerebral metabolism. We evaluated with positron emission tomography (PET) the relationships between unaffected-hemisphere (ie, contralateral) oxygen consumption (cCMRO2) and quantitative neurological assessments (and their respective evolution over time) after ischemic stroke.

METHODS

Among 30 consecutive patients with first-ever middle cerebral artery ischemic stroke studied with the (15)O equilibrium method, we selected all survivors (n=19; mean age, 74.6 years) who were investigated both within the first 18 hours after stroke onset (PET1; mean, 11 +/- 4 hours) and 15 to 30 days later (PET2; mean, 24 +/- 10 days), with each patient serving as his/her own control. Neurological deficits were quantified using Orgogozo's middle cerebral artery scale (N score) at each PET session. Neurological changes were calculated as changes in the N score. A late CT scan coregistered with PET provided infarct topography and volume index.

RESULTS

At PET2, we observed the overall expected neurological recovery. There was a nearly significant trend for a decrease in cCMRO2 from PET1 to PET2, especially for the neocortex (P=.08, F test); in a subgroup of eight patients with large infarcts, this CMRO2 decline was significant (P<.05) in the mirror region to the infarct. There was no significant correlation (Spearman's tests) between acute-stage cCMRO2 and same-day N scores or between changes in cCMRO2 versus changes in N score from PET1 to PET2 (any region). There was a nearly significant trend for lower PET2 cCMRO2 in the subgroup of eight patients with large compared with small infarcts (P=.06).

CONCLUSIONS

We found no evidence for an influence of cCMRO2 on acute-stage neurological deficit or for a role of the unaffected hemisphere in early recovery after acute MCA ischemic stroke. The decline in unaffected-hemisphere metabolism from the acute to the subacute stage in the face of overall clinical recovery appears clinically irrelevant. The fact that the neocortical cCMRO2 at PET2 tended to be lower, and declined significantly from PET1 to PET2 in the mirror region in the subgroup of patients with large infarcts, suggests that this delayed effect represents transcallosal fiber degeneration.

Contribution of diaschisis to the clinical deficit in human cerebral infarction

BACKGROUND AND PURPOSE

Regions of decreased cerebral blood flow are often seen on single-photon emission computed tomography (SPECT) after stroke and have been widely reported to add to the clinical deficit. However, such reports have not distinguished between correlation and causation. We analyzed 124 serial SPECT scans performed in 50 patients to assess the role of diaschisis in the clinical deficit after stroke.

METHODS

SPECT with the use of 99mTc-hexamethylpropyleneamine oxime (99mTc-HMPAO) was performed in a prospective, unselected series of 50 patients with cerebral infarcts studied at a median of 1.1, 6.8, and 95 days after ictus. Patients were also assessed with the use of the Canadian Neurological Scale, the Barthel Index, a neuropsychological evaluation, and infarct volume measurement.

RESULTS

One hundred twenty-four serial SPECT scans were done in 50 patients. Diaschisis was identified at 168 sites. There was insufficient correlation between diaschisis and the clinical measurements to support the suggestion that diaschisis independently causes clinical deficits beyond those due to the infarct itself. Unlike the clinical status, diaschisis showed little tendency to resolve during the 3-month follow-up period of the study. Several of the instances of correlation were shown to be of a noncausal kind, with both the diaschisis and the clinical deficit being due to the lesion directly; there was no known mechanism for the diaschisis to cause the clinical deficit.

CONCLUSIONS

Diaschisis does not independently add to the clinical deficit after stroke. It is more likely that it simply represents part of the damage done by the stroke.

Transhemispheric diaschisis. A review and comment

We review here the literature in both animal models and humans concerning electrical activity, blood flow, and metabolism in the hemisphere contralateral to unilateral cerebral ischemia. We analyze the data by periods based on the time from initial injury to emphasize the time course of transhemispheric diaschisis. Contralateral electrical activity, such as evoked potential amplitude, is increased in the late stages after unilateral infarction, with the data from the more acute periods being inconclusive. Contralateral blood flow changes probably depend on the magnitude of the ischemic injury, with a larger insult resulting in a decrease not seen with smaller insults. Some studies have shown a decrease in contralateral blood flow over the first week followed by a gradual return toward baseline. Most measures of contralateral metabolism show a time course similar to blood flow, that is, a decrease followed by gradual recovery. The effects of corpus callosum section on transhemispheric diaschisis are not yet established. We provide examples to show that under certain conditions, diaschisis may represent a loss of remote inhibition rather than a loss of remote facilitation, as von Monakow originally suggested. By following the contralateral changes over time, particularly during the first minutes and hours of ischemia, insight will be gained into the brain's responses remote from the focus of ischemic injury. These responses should bear a relation to the brain's defense mechanisms ipsilaterally to the region of ischemia.