Magnetic Resonance Spectroscopy in the Neurologic Assessment of Adult Lead Poisoning

Robert Wright*^, Robert Mulkern^#, Roberta White^a , Antonio Aro* , Howard Hu*. The Channing Laboratory, Brigham and Women’s Hospital*, Department of Radiology, Boston Children’s Hospital^#, Harvard Medical School, Boston VA Hospital, Boston University Medical School^a, Boston, MA.and Department of Pediatrics, Brown Medical School^. Providence, RI.

Background: Magnetic Resonance Spectroscopy (MRS) is a noninvasive technique combining imaging with measurement of neurochemicals in discrete brain regions. The ratio of N-acetylcysteine (NAA) to creatine (CR) is a marker of neuronal density. We measured NAA/CR ratios in 2 identical twins with different body burdens of lead. Methods: Two 70 year old identical twin subjects were identified from a occupational medicine clinic roster with chronic lead poisoning. Both were retired painters, but one brother (Subject 1) primarily performed paint removal and was known to have higher lead exposure. After obtaining informed consent, patella and tibia bone lead concentrations were measured by X-ray fluourescence. Magnetic resonance studies of the hippocampus and frontal lobes were conducted with a 1.5 Tesla scanner (General Electric Medical Systems, Milwaukee, WI). Reported NAA/CR ratios are the average of the right and left sides. Neurocognitive testing was also performed with the results pending.

Results: Bone lead levels were high in each subject. Subject one had bone lead levels approximately 2.5 times higher than his brother. Subject one also had NAA/creatine ratios which were lower than his brother in both the hippocampus and frontal lobes. Subject one performed more poorly than his brother on cognitive testing. Conclusion: In genetically matched twins with identical education levels, the subject with higher bone lead concentration had lower neuronal density in the hippocampus and frontal lobes as measured by MRS. It should be noted that the bone lead levels for both subjects are 5-10 times that of the general population. Neurocognitive testing results will be presented While these results are promising, further study is necessary to determine whether MRS findings correlate both with markers of lead exposure and tests of cognitive function. .

Corresponding Author

Robert Wright MD; The Channing Laboratory; 181 Longwood Ave. Boston MA 02115

617-525-2731; 617- 525-0362 (fax)

Introduction

There has been a growing interest in the effects of exposure to lead on the nervous system as well as the mechanisms by which lead disrupts brain function. The effects of elevated blood lead levels have been examined primarily in the context of behavioral and neuropsychological evaluations. One of the most consistently reported impairments associated with lead exposure involves its negative impact on intellectual functioning. Little is known regarding the effects of lead on brain metabolism in vivo, and on structural and functional correlates of brain function. In the human brain, Magnetic Resonance Spectroscopy (MRS) provides a non-invasive method with which to monitor the biochemistry of acute and chronic stages of neurologic disease. The development of spatial localized spectroscopic methods which sample the relative levels of metabolites from a volume of tissue defined from an MRI image has provided a basis for integrating the biochemical information obtained by MRS with the anatomical and pathological information obtained from MRI. MRS has gained widespread acceptance as a method for assessing both neuronal viability as well as demyelination. MRS can measure both N-acetylaspartate (NAA) and choline in discrete tissue volumes. Choline is a constituent of myelin and changes in its concentration have been documented in demyelination disorders. In the cortex, NAA is located in neuronal cell bodies whereas in the white matter it is located largely in axons. A decrease in NAA has been proposed as an indicator of neuronal and axonal damage and loss (Arnold et al, 1997; van der Knapp et al 1992). In practice, the decrease in NAA is measured relative to the level of creatine, a stable metabolite whose level is constant following neuronal loss. Since there is evidence showing reduced NAA in disease processes involving intellectual deterioration, we hypothesize a decrease in NAA in the brains of adults with clinical evidence of lead exposure

Case History

JG and EG are 71 year-old monozygotic twins. Both are retired painters who worked in the Boston Metropolitan area. The brothers worked together but each predominantly performed separate well-defined tasks. JG performed paint removal by scraping, sanding, and heat-treatment with an electric iron. EG predominantly performed the painting, but would at times assist in paint removal. JG ate and smoked cigarettes at work without washing his hands, while EG reported that he was meticulous about washing his hands prior to eating at work. While at work both wore paper masks but did not use any sophisticated respiratory protective device. In 1984, JG developed chronic back pain for which he was referred to a neurosurgeon. Because of his occupational history of painting, a blood lead level was ordered and returned at 125 ug/dl. He was subsequently hospitalized and chelated with EDTA. Since this time he has been followed in the occupational medicine department of the Massachusetts Respiratory Hospital. Other chronic health problems for JG include hypertension. Otherwise both brothers are healthy. Both EG and JG completed the same high school, served together in the Navy and have worked together as painters for over 45 years. Because of this unique situation of differential lead exposures but complete genetic matching, as well as similar childhood and adult environments, we performed MRS studies and cognitive testing on these twins to determine differences which might be attributed to the differential lead exposures.

Methods

Bone Lead Measurement

K-x-ray fluorescence (KXRF) is technique using a 109Cd gamma‑ray source to provoke the emission of fluorescent photons from target tissue; these photons are then detected and counted and correlate with lead concentration(Chettle et al., 1991; Aro et al., 1994) A 30‑minute measurement was taken at the mid‑shaft of the left tibia and at the left patella after each region had been washed with a 50% solution of isopropyl alcohol.

Cognitive testing and MRS

The battery of cognitive tests included the Wechsler Adult Intelligence Scale-Revised and the mini mental status exam. A battery of specific tests for domains of attention, executive function, verbal language, visuo-spatial/motor testing, memory and behavior/personality were also performed.

Magnetic resonance studies were conducted with a 1.5 Tesla scanner(General Electric Medical Systems, Milwaukee, WI). In each subject, 6 separate acquisitions were performed, two imaging studies and four single voxel Point Resolved Echo Spectroscopy (PRESS) studies designed to sample spectra from the left and right frontal lobes and from the left and right hippocampal areas. The axial FSE acquisition provided the localizer image used for spectroscopic interrogation of the frontal lobes. The coronal acquisition provides the localizer image for the hippocampal areas. From the axial images acquired in the first series, a voxel of approximately 1.5 x 1.5 x 1.5 cm in the left frontal temporal lobe was graphically selected for spectroscopic interrogation. The PRESS sequence (90 ‑ t1 ‑ 180 ‑ t1 ‑ t2 ‑ 180 ‑ t2 ‑ acquire) was then applied to acquire water-suppressed 1H spectra from the selected voxel with a TR of 2000 ms, an echo time (TE = 2t1 + 2 t2) of 144 ms, and 128 scans. Water suppression was performed with three Chemical Shift Selective (CHESS) pulses applied with spoiling gradients prior to each water-suppressed acquisition.

Following the collection of PRESS data from the left frontal lobe, the process was repeated for the right frontal lobe with the same axial localizer image. The coronal FSE images were then acquired to provide a localizer image suitable for PRESS sampling of 1.5 x 1.5 x 1.5 mm voxels of the left and right hippocampal regions in two separate PRESS acquisitions performed in the manner described. The resulting spectrum was then phased to obtain an absorption mode spectrum by zooming the spectral region to the ‑4 ppm to 0 ppm range and performing first and second order phasing to achieve a flat baseline around the positively phased N‑acetylaspartate (NAA) resonance at ‑2 ppm. The choline (Cho), creatine (CR), and NAA resonances were then numerically integrated using the SAGE software to obtain peak areas. These areas were used to calculate the metabolite ratios NAA/Cho, NAA/CR, and Cho/CR.

Results

Neurocognitive Testing

Subject 1 (JG) was initially seen for testing in 1/90. At that time, he performed at average levels on most tasks. His naming abilities were well below average, however, and this was thought to reflect his childhood learning disability. He performed below expectation on tests of attention and sequencing, losing track of mental operations as he was carrying out tasks. He also had a remarkable tendency to perseverate and drawings were tremulous. At his second evaluation 9 years later, he performed at above average levels on visuospatial tasks and showed some slight improvement in this domain. His performance on attentional tasks was variable, but at times better than in 1990. He continued to evidence a naming deficit, which had not worsened. His performance when tested for his ability to retain newly learned information over delays was worse than it had been previously. Productions such as drawings were tremulous and much smaller than in 1990.

Subject 2 (EG) was tested only in 1999. He performed at average levels on most tasks but also showed below average performance on a test of naming (probably developmental in origin). He also performed at low average levels and below expectation for other abilities on tests tapping executive function (alternation of sequences, inhibiting distraction, development of effective strategies for task completion, inhibiting the tendency to perseverate).

Comparing the results from Subject 1 and Subject 2 in 1999, Subject 1 showed micrographia and tremor but Subject 2 did not. Subject 1 had deficits in verbal learning and on delayed recall of visuospatial information which were not seen in Subject 2.

Bone lead levels and MRS

The bone lead concentrations and NAA/creatine ratios from the MRS exams are summarized in Table 1. JG had much higher levels of trabecular (patella) lead and cortical (tibia) lead than EG. MRS exams demonstrated lower NAA/creatine ratios in JG in both the hippocampus and frontal lobe.

Table 1: Summary of Bone Lead Levels and NAA/creatine ratios

	Patella lead (ug/g bone)	Tibia lead (ug/g bone)	Hippocampal NAA/CR	Frontal Lobe NAA/CR
Subject 1 (JG)	343	189	1.26	1.38
Subject 2 (EG)	118	78	1.77	1.73

Discussion

Both subject’s bone lead levels are high for age , as both patella and tibia levels are typically less than 30 ug lead/gram bone. With respect to cognitive testing, patient 1 appears to have demonstrated deficits attributable to lead exposure. In addition, declines between 1990 and 1999 on tests assessing short-term memory and the changes in his drawings suggest a new progressive process, which may be related to the history of lead exposure. Patient 2 has some signs of frontal lobe dysfunction of unclear etiology. These may also be secondary to lead exposure as his bone lead levels are elevated for age. Overall, EG performed superiorly to JG in the neurocognitive tests and had NAA/creatine ratios which were higher, suggesting greater neuronal density in these regions. While we cannot conclusively attribute these differences to increases in lead exposure, the fact that these two subjects are genetically matched, as well as matched on education level would support the hypothesis that the differences in NAA/creatine ratios are secondary to higher lead exposure in JG. If so, this would suggest that chronic lead exposure caused a loss of neurons in the hippocampus and frontal. Possible mechanisms of cell loss would include lead induced oxidative toxicity (Adonaylo and Oteiza, 1999), cellular apoptosis without necrosis (Fox et al, 1999), and indirect oxidative toxicity via increases in the metabolite aminolevulinic acid, (Bechara, 1996).

In summary, we describe twin painters with differential lead exposures but otherwise similar life exposures. Bone lead levels in one subject were demonstrated to be markedly higher than his brother. This subject demonstrated greated deficits in performance during neurocognitive testing and had lower NAA/creatine ratios in the hippocampus and frontal lobes. These results are consistent with neuronal loss secondary to lead exposure although this case report cannot establish cause and effect. This report suggests that MRS may be a valuable research tool in determining the mechanisms of lead’s neurotoxicity.

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