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Positron emission tomography scan in cortical visual loss in patients with organophosphate intoxication
1Department of Ophthalmology, Taipei Veterans General Hospital, Taiwan, Republic of China.
Objective:
To determine the cerebral metabolism of patients with cortical visual loss.
Design:
Two observational case studies.
Testing:
Two patients who survived acute organophosphate poisoning with respiratory failure experienced severe visual loss despite relatively normal ophthalmic examination results. Magnetic resonance imaging of the brain revealed no abnormality of the visual system in either patient. Positron emission tomography (PET) was performed in these 2 patients and in 12 normal subjects with fluorine-18 fluorodeoxyglucose (FDG) as a tracer to measure cerebral glucose metabolism for the estimation of neurologic deficit in the visual cortex.
Main Outcome Measures:
The FDG uptake values were measured as nanoCurie per cubic centimeters of tissue (nCi/cc). The relative uptake index in visual cortex was computed as the ratio of uptake of FDG in each region of visual cortex to that of cerebellum (regional visual cortex/cerebellum).
Results:
Hypometabolism was observed in the visual cortex of both patients. The relative uptake index of FDG in visual cortex (visual cortex/cerebellum) was significantly decreased in those patients compared with normal subjects.
Conclusions:
In patients with cortical visual loss, conventional neuroimaging techniques can fail to visualize damage that can be detected by PET scanning, and PET analysis may be helpful in estimating the metabolic deficit of visual cortex and in establishing the organic nature of cortical visual loss in these patients.