Comparison of SPECT findings and neuropsychological sequelae in carbon monoxide and organophosphate poisoning

The Legacy of the TTASAAN Report-Premature Conclusions and Forgotten Promises: A Review of Policy and Practice Part I

Brain perfusion single photon emission computed tomography (SPECT) scans were initially developed in 1970's. A key radiopharmaceutical, hexamethylpropyleneamine oxime (HMPAO), was originally approved in 1988, but was unstable. As a result, the quality of SPECT images varied greatly based on technique until 1993, when a method of stabilizing HMPAO was developed. In addition, most SPECT perfusion studies pre-1996 were performed on single-head gamma cameras. In 1996, the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology (TTASAAN) issued a report regarding the use of SPECT in the evaluation of neurological disorders. Although the TTASAAN report was published in January 1996, it was approved for publication in October 1994. Consequently, the reported brain SPECT studies relied upon to derive the conclusions of the TTASAAN report largely pre-date the introduction of stabilized HMPAO. While only 12% of the studies on traumatic brain injury (TBI) in the TTASAAN report utilized stable tracers and multi-head cameras, 69 subsequent studies with more than 23,000 subjects describe the utility of perfusion SPECT scans in the evaluation of TBI. Similarly, dementia SPECT imaging has improved. Modern SPECT utilizing multi-headed gamma cameras and quantitative analysis has a sensitivity of 86% and a specificity of 89% for the diagnosis of mild to moderate Alzheimer's disease-comparable to fluorodeoxyglucose positron emission tomography. Advances also have occurred in seizure neuroimaging. Lastly, developments in SPECT imaging of neurotoxicity and neuropsychiatric disorders have been striking. At the 25-year anniversary of the publication of the TTASAAN report, it is time to re-examine the utility of perfusion SPECT brain imaging. Herein, we review studies cited by the TTASAAN report vs. current brain SPECT imaging research literature for the major indications addressed in the report, as well as for emerging indications. In Part II, we elaborate technical aspects of SPECT neuroimaging and discuss scan interpretation for the clinician.

Evaluation of MR perfusion abnormalities in organophosphorus poisoning and its correlation with SPECT

AIM

Acute organophosphate (OP) pesticide poisoning causes substantial morbidity and mortality worldwide. Many imaging modalities, such as computerized tomography (CT), magnetic resonance imaging (MRI), and single photon emission computed tomography (SPECT) of the brain, have been used for quantitative assessment of the acute brain insult caused by acute OP poisoning. Perfusion defects on SPECT in acutely poisoned patients with OPs have been described, however, MR perfusion abnormalities have not been described in the literature. MR perfusion Imaging has the advantage of having higher spatial resolution, no radiation, and better availability.

MATERIALS AND METHODS

In this prospective study, 20 patients who ingested OP compounds were included. All the patients underwent brain SPECT on a dual head SPECT gamma camera and MRI brain on a 1.5T MR system. Neurocognitive tests were performed for all patients.

RESULTS

SPECT showed perfusion defects in 7 patients and total number of perfusion defects were 29. On MR perfusion, based on the cut-off values of normalized cerebral blood volume (nCBV) ratios and normalized cerebral blood flow (nCBF) ratios, the total number of patients showing perfusion defects were 6 and 8; and the total number of perfusion defects were 29 and 45, respectively. There was significant difference of the nCBV ratios and nCBF ratios between the control group (n = 20) and positive patients group (n = 6 and n = 8, respectively) (P > 0.05). All the defects seen on SPECT were well appreciated on nCBF maps (MRI perfusion) suggestive of 100% correlation.

CONCLUSION

MR perfusion imaging can be used as an effective modality for evaluation in acute OP poisoning.

Correlation of defects in regional cerebral blood flow determined by 99mTc SPECT with residual neurocognitive testing abnormalities during and 3 months post exposure in acutely poisoned patients with organophosphates

INTRODUCTION

Following acute organophosphate exposure, morphological changes in certain regions of the brain have been reported to develop within a few hours and involve neuronal degeneration. Single photon emission computed tomography (SPECT) has been used to determine changes in the regional cerebral blood flow and attempts have been made to correlate these changes with long-term neurological sequelae.

PURPOSE OF STUDY

The aim of the study was to determine changes in the regional cerebral blood flow by 99mTc-ECD SPECT following acute organophosphate poisoning and to correlate these defects with abnormalities in neurocognitive testing carried out during admission and at 3 months post exposure, in order to determine whether any changes in the cerebral blood flow could help in predicting future development of neurocognitive deficit.

PATIENTS AND METHODS

Twenty-eight patients with acute organophosphorous poisoning were included in the study. The inclusion criteria were a history of ingestion or accidental exposure, clinical features of cholinergic crises, and low serum acetylcholinesterase (AChE). Twenty age- and sex-matched patients from a previous study were used as controls for the neurocognitive tests. There were no controls for SPECT.

RESULTS

Of the 28 patients studied, 27 had abnormalities in the regional cerebral blood flow on SPECT with men having significantly higher abnormalities than women (p < 0.05). The right side of the brain had more defects than the left, with the occipital lobes being the most commonly involved. Of seven neurocognitive function tests carried out on patients who had regional cerebral blood flow defects during admission, abnormalities were observed in six tests. In 18 of 26 patients who could be tested at 3 months post exposure, improvement was observed in Trail B and Visual retention tests. However, others tests remained significantly abnormal.

CONCLUSION

We conclude that a single episode of clinically significant organophosphate intoxication can lead to persistent residual neurocognitive deficits. Detection of regional cerebral blood flow defects on 99mTc-ECD SPECT can possibly help in predicting long-term deficits in neurocognitive functions in such patients.