MRI's predictive value may depend on patient selection

It's one thing for acute stroke imaging to predict final infarct volume or a patient's risk of 90-day mortality. It's quite another for imaging to predict whether a patient will recover upper extremity dexterity or language skills, or to what extent the patient will be able to function independently.

Researchers are only beginning to explore MRI's potential for making these types of functional outcome predictions, and although some studies have in fact found lesion volume on MRI to be predictive of function, these have been plagued by methodological flaws.1 In April 2006, an 82-patient study conducted by researchers from the University of Edinburgh concluded that imaging variables were not independent 3-month outcome predictors for a heterogeneous group of patients but suggested that MR diffusion-weighted imaging might have predictive value in patients with severe stroke.2

Because the National Institutes of Health Stroke Scale is such a strong predictor of outcome, any added value from imaging may be inconsequential in patients with less severe stroke, in whom small NIHSS changes have large effects on outcome. In those with more severe stroke, small NIHSS changes have less of an effect on outcome, opening the door for MRI's additional predictive information to carry more weight.

"It might be worth using MR lesion volume as a predictor in patients with severe strokes only, but not in all strokes," said Joanna M. Wardlaw, MD, a professor of neuroradiology at Edinburgh.

Increasingly, researchers are examining the predictive value of MRI in the shorter term, looking for associations between lesion volume and acute functional recovery. Investigators from Dusseldorf reported that MRI-based penumbral imaging was predictive of recovery of grip force and movement rate during repetitive finger-thumb pinching movements between day 1 and day 8 after acute hemiparetic stroke in 36 patients.3 Researchers from Johns Hopkins University found that a diffusion-clinical mismatch, which uses clinical language tests instead of imaging to approximate perfusion abnormality, was predictive of language improvement within 1 week in 81 patients with acute left hemispheric stroke.4

Still largely unknown is the extent to which individual impairments, such as lack of grip strength, translate to functional impairment in a patient population known for its ability to compensate both neurologically and functionally. Such compensation mechanisms may render imaging better able to predict impairment than function, but this too may vary depending on stroke severity or other factors.

"What matters to patients and healthcare providers in general is whether the patient can return to independent existence, and if not, then how much help do they need," Wardlaw said.

REFERENCES

• Schiemanck SK, Kwakkel G, Post MW, Prevo AJ. Predictive value of ischemic lesion volume assessed with magnetic resonance imaging for neurological deficits and functional outcome poststroke: A critical review of the literature. Neurorehabil Neural Repair 2006;20:492-502.

• Hand PJ, Wardlaw JM, Rivers CS, et al. MR diffusion-weighted imaging and outcome prediction after ischemic stroke. Neurology 2006;66:1159-1163.

• Weller P, Wittsack HJ, Siebler M, et al. Motor recovery as assessed with isometric finger movements and perfusion magnetic resonance imaging after acute ischemic stroke. Neurorehabil Neural Repair 2006;20:390-397.

• Reineck L, Agarwal S, Hillis AE. "Diffusion-clinical mismatch" is associated with potential for early recovery of aphasia. Neurology 2005;64:828-833.