Ep 160: A 32-Year-Old Pregnant Woman Seeking Advice on Exposures

This episode reviews the epidemiology of lead exposure in the United States and its health implications.

Read more on this topic in Harrison's.

Harrison's Principles of Internal Medicine, 22nd Edition

Transcript

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[Ms. Heidhausen] This is Katerina Heidhausen, executive editor of Harrison's Principles of Internal Medicine. Harrison's Podclass is brought to you by McGraw Hill's AccessMedicine, the online medical resource that delivers the latest content from the best minds in medicine. And now, on to the episode. 

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[Dr. Handy] Hi, everyone. Welcome back to Harrison's Podclass. We're your co-hosts. I'm Dr. Cathy Handy. 

[Dr. Wiener] And I'm Dr. Charlie Wiener, and we're joining you from the Johns Hopkins School of Medicine. Welcome to Harrison's Podclass. Today's episode is a 32-year-old pregnant woman seeking advice on exposures. Cathy, today's patient comes to you in clinic because she's pregnant and is worried about lead exposure in her environment. She has a number of health-related questions for you. 

[Dr. Handy] Heavy metals are an incredibly important topic to discuss because of the public health implications of exposure. The US Agency for Toxic Substances and Disease Registry maintains an updated list of all hazards present in toxic waste sites according to their prevalence and the severity of their toxicity. Can you guess the top three? 

[Dr. Wiener] Heavy metals? 

[Dr. Handy] That's right. The top three are arsenic, lead, and mercury. And then we have cadmium in seventh place, too. 

[Dr. Wiener] Lead is an important topic that received a lot of publicity recently due to public health studies in Flint, Michigan, and is an ongoing topic of investigation for children and adults. That's going to bring us to our question. So, today's question asks, which of the following statements regarding the heavy metal, lead is true? Option A. cumulative lead exposure may be measured radiographically. Option B. elevated maternal lead levels are associated with infant macrosomia. C. lead is a trace element that is essential for some biochemical reactions. Option D. lead levels in the US population have been increasing in the last 20 years. And option E. is, serum lead levels are associated with cognitive decline in adults. 

[Dr. Handy] First off, let's be clear that lead has no role in any normal human biochemical function, and it has potential toxicity at any level. That's in contrast to copper and selenium, for example, which are essential to normal metabolic function as trace elements, but they're toxic at high levels of exposure. 

[Dr. Wiener] Okay, so, option C. is false right out of the bat? 

[Dr. Handy] Yes. Next, let's talk about measuring lead levels. Lead is absorbed through ingestion or inhalation. Organic lead may be absorbed dermally. In blood, over 95% is sequestered in red blood cells. So, if you're measuring blood levels, it must be in whole blood, not serum. 

[Dr. Wiener] Okay, so, that sounds like option E. is false since you can't measure serum levels. 

[Dr. Handy] Yes, but the part about cognitive decline will come up later in the question. 

[Dr. Wiener] Okay, well, option A. also talks about measurement by using radiography to measure cumulative exposure. Is that correct? 

[Dr. Handy] Yeah, that's going to be the true answer. Lead is distributed widely in soft tissue with a half-life of about 30 days, and 15% of dose sequestered in bone with half-life of over 20 years. K X-ray fluorescence or KXRF instruments allow safe in-vivo measurements of lead levels in bone, and that, in turn, reflect cumulative exposure over many years, as opposed to blood lead levels, which mostly reflect recent exposure. This technique has greatly advanced our understanding of chronic lead exposure and toxicity. 

[Dr. Wiener] How so, and what have we learned? 

[Dr. Handy] Fortunately, blood levels of lead have been declining in the general US population over the past few decades following the removal of lead from gasoline, and plumbing, and food cans, and other consumer products. However, high levels of cumulative lead exposure are known to be a risk factor for chronic diseases. 

[Dr. Wiener] Okay, so that makes D. false also, blood levels in the US have been declining in the last decades. 

[Dr. Handy] Yes, but we've learned that higher bone lead levels measured by KXRF are linked to increased risk of hypertension and accelerated declines in cognition in both men and women living in urban communities. Higher bone lead levels, even with low blood levels, are a major risk factor for increased cardiovascular morbidity and mortality rates in both community-based and occupational-exposed populations. Chronic lead exposure at community levels has also been associated with increased risk of hearing loss, Parkinson's disease, and amyotrophic lateral sclerosis. 

[Dr. Wiener] So, our patient is pregnant and she's asking in particular to the risks around pregnancy. Why don't you talk about those a little bit? 

[Dr. Handy] That's another important area. High maternal bone lead levels were found to predict lower birth weight, head circumference, birth length, and neurodevelopment performance in children by age two. Children have also been shown to have higher blood pressures at age 7-14 years, and that's important because that's an age range at which higher blood pressures are known to predict an elevated risk of developing hypertension. 

[Dr. Wiener] Okay, so that's why option B. is false. Maternal lead exposure is associated with low birth weight, not elevated birth weight. 

[Dr. Handy] Correct. High birth weight, or macrosomia, is associated with maternal diabetes. 

[Dr. Wiener] Okay, so we finished the question. Any closing thoughts? 

[Dr. Handy] Yes, we've spoken mostly today about chronic lead exposure, but it's important for clinicians to know that subclinical exposures that can be measured with elevated whole blood levels are associated with a host of diseases. In children, elevated blood levels are associated with anemia, intellectual disabilities, and deficits across multiple domains. In adults, chronic subclinical exposures are associated with an increased risk of anemia, peripheral neuropathy, hearing impairment, accelerated declines in cognition, higher risk of cardiovascular disease, kidney disease, and reproductive issues with diminished sperm counts and an increased risk of spontaneous abortions. 

[Dr. Wiener] Wow, that's sobering. So, today's teaching points are that there is no safe level of lead, either acutely, sub-acutely, or chronically. Chronic exposure, as defined by bone deposition, is associated with cognitive decline, and subclinical exposures are associated with multi-system toxicity in children and in adults. 

[Dr. Handy] And you can find this question and other questions like it in the Harrison's Self-Review, and you can learn more about this topic in Harrison's chapter on heavy metal poisoning. Visit the show notes for links to helpful resources, including related chapters and review questions from Harrison's, available exclusively on AccessMedicine. If you enjoyed this episode, please leave us a review, so we can reach more listeners just like you. Thanks so much for listening. 

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