Near-infrared spectroscopy in stroke: from research to clinical practice.

welcomes Letters to the Editor and will publish them, if suitable, as space permits. They should not exceed 750 words (including references) and may be subject to editing or abridgment. Please submit letters in duplicate, typed double-spaced. Include a fax number for the corresponding author and a completed copyright transfer agreement form (published in every issue). To the Editor: We read with interest the editorial comment by Villringer and colleagues on the use of near-infrared spectroscopy (NIRS) as a routine diagnostic tool in stroke. 1 We agree that NIRS has great potential to become a valuable bedside tool; however, several limitations need to be overcome. Crucial to the successful use of NIRS is an appreciation of its ability to measure real-time changes in cerebral hemoglobin oxygen saturation. These equate to changes in cerebral blood volume only and not necessarily cerebral blood flow and may consequently lead to misinterpretation of clinical data. The relative contribution of different vascular compartments to the NIRS signal also needs to be established as it is not known whether venous changes in oxygenated hemoglobin makes a greater contribution than arterial changes. Extracranial tissues: melanin-containing skin, lipid-prevalent soft tissue, bone, and cerebrospinal fluid may also absorb a significant proportion of infrared light. This makes absolute intrapatient comparisons difficult. Even in individual patient analyses extracranial tissues may elicit varying degrees of absorption with changes in their extracranial blood supply. Attempts to quantify this contribution have proved difficult whether studying patients at carotid endar-terectomy 2 or using Monte Carlo simulation. 3 Smielewski and colleagues however suggest that cerebral oxygenation, as recorded by NIRS and cutaneous laser-Doppler flowmetry, is largely unaffected by extracranial tissue perfusion. 4 More work is needed to understand the behavior of near-infrared light in different biological tissues if NIRS is to be used as a reliable clinical monitor. NIRS use as a research tool in measuring cortical activation may help in the understanding of dynamic remodeling which may occur within brain architecture during neuroplasticity post-stroke. The work of Kato and colleagues using NIRS in conjunction with functional MRI has lent support to the role of activation of the ipsilateral primary sensorimotor and supplementary motor cortex during neuronal reorganization after ischemic stroke. 5 The use of NIRS in the acute clinical assessment of cerebral perfusion in stroke has also been supported in recent work by Treborg and colleagues wherein indocyanine green was used as a tracer. 6 …