A 69-year-old man with a medical history of hypertension, diabetes, and obesity with a body mass index of 33 was admitted with altered mental status. He and his wife were returning from a 14-day Alaskan cruise. On their return, the wife started noticing that her husband was behaving strangely and acting forgetful and had short-term memory loss. This gradually progressed to generalized confusion and altered mental status leading him to the emergency room. He denied any chest pain, shortness of breath, fever, or any gastrointestinal or urinary symptoms. He did not have any motor deficits on examination.
Given the neurological symptoms and recent travel, a stroke alert was called where he had extensive tests, including a head CT, brain MRI and chest CT scan.
All the tests ruled out transient ischemic attack (TIA), stroke, and pulmonary embolism. The patient had been to higher altitudes before and had no problems with altitude. Other laboratory workups, including complete blood count (CBC), comprehensive metabolic panel (CMP), including blood glucose, were normal. His blood cultures were normal too. Urine drug screen was negative, and blood alcohol level was undetectable.
The wife later mentioned, during his vacation in Alaska, he had consumed a lot of fish on the cruise ship. This led to suspicion of organic mercury toxicity. The whole-blood mercury level was sent for evaluation. The whole blood mercury level was noted to be 35 ng/mL (normal whole blood mercury is usually <10 ng/mL). He was diagnosed with organic mercury toxicity due to the consumption of excessive fish.
The patient later admitted to eating excessive amounts of fish for all three meals a day on the cruise and the various land excursions he had in Alaska, particularly eating a lot of halibut, lingcod and salmon shark during his land excursions and eating pink salmon and perch on the cruise ship, which are all high in mercury. For reference, the Alaskan government has developed guidelines for eating fish safely and gives points per meal size of different fishes.
Recommendations are not to exceed 12 points per week. One 6-oz serving each of halibut, lingcod and salmon shark will give anywhere from 18 to 30 points per day.
He admits to never eating these large volumes of fish before this cruise. His occupation involved a desk job, and he had never been exposed to occupational mercury.
His wife, who had also accompanied him, did not eat excessive amounts of fish and did not have symptoms suggestive of mercury poisoning.
Differential diagnosis of acute cerebrovascular event was considered. Acute pulmonary embolism, with cerebral infarct in the setting of intra-cardiac shunt, was also considered a possibility. Additionally, hypoglycemia, alcohol intoxication and use of other illicit drugs could also account for these symptoms, and were ruled out with normal blood sugar and negative alcohol and urine drug screen on presentation. With a history of excessive fish intake, as above, organic mercury poisoning was considered likely as well.
The patient was managed conservatively with gradual improvement in his symptoms. Chelation was not used since it has not been shown to be effective in organic mercury poisoning.
The patient’s outcome despite having severe organic mercury poisoning was good. His symptoms completely resolved after four days of admission with conservative management. His mercury levels a month after the hospitalization had normalized to 9.2 ng/mL.
Mercury exists in elemental, inorganic, and organic forms, all of which may be toxic. The toxic manifestation depends on the form of exposure.
Exposure to organic mercury is mostly via consumption of mercury-contaminated fish.
The concentration of mercury is very low in most food (<0.02 mg/kg). However, certain types of marine fish (such as tuna, swordfish, and shark) and certain fish taken from polluted freshwaters (such as walleye, bass, and pike) may contain high concentrations of mercury, almost completely in the form of methylmercury. According to the US Food and Drug Administration Monitoring Program 1990–2010, concentrations of methylmercury in these fish maybe 1 mg/kg (1 ppm) or even higher. A concentration of more than 0.5 mg/kg (0.5 ppm) is considered very high.
According to the Natural Resources Defence Council, fish with 0.3–0.49 mg/kg (0.3–0.49 ppm) may be safely consumed three times per month. Fish with >0.5 mg/kg should be avoided. The Environmental Protection Agency recommends a maximum daily exposure to mercury of 0.1 mcg/kg of body weight, which will typically result in a blood mercury concentration of less than 6 mcg/L.
The monitoring of mercury in blood or urine may be used to identify and quantify exposure to methylmercury. Heavy consumers of fish and, in particular, those who eat mercury-containing species such as swordfish or shark may have blood mercury levels in excess of 20 mcg/L (normal value <5 mcg/L). While adult patients with excessive organic mercury consumption may present with subtle neurocognitive effects such as mild deficits in fine motor, verbal memory, and attention, effects in children exposed as fetuses may be more severe.
The preferred test for determining toxicity from organic mercury is a whole-blood mercury level. Because organic mercury is eliminated via the fecal route, urine mercury testing is less useful in organic mercury poisoning.
Patients with potential organic mercury toxicity should cease all exposure to organic mercury. A 24-hour urine should be sent for measurement of mercury concentration. There is no effective treatment for patients with toxic exposure to organic mercury. Neurologic damage due to organic mercury exposure may be profound, permanent, and is generally resistant to treatment with chelating agents. Prevention of exposure to organic mercury is the most important aspect of management.
The use of chelating agents is controversial among such patients. Central nervous system toxicity due to exposure to organic mercury is usually resistant to treatment with chelation. Chelation with dimercaprol (British anti-Lewisite) should not be used in organic mercury exposure, because it increases the mobilization of mercury to the brain. Treatment with oral dimercaptosuccinic acid is unlikely to reverse neurologic damage.
Alaska Department of Environmental Conservation’s Fish Monitoring Program has shown that Alaska has some of the cleanest fish in the world. As methylmercury increases up the food chain, most exposure occurs through the consumption of larger, predatory fish and marine mammals. The Department of Health and Social Service of Alaska has established a fish consumption point system to help pregnant women and young children and the general public to select lower mercury-containing fish.
Deepti Bhandare is a cardiologist.
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