A large amount of nitrites absorbed by the intestines causes methemoglobinemia, clinically presenting as generalized cyanosis of the skin and mucous membranes, known as enterogenous cyanosis, also referred to as nitrite poisoning.

bubble_chart Etiology

1. Many vegetables such as greens, bok choy, spinach, and beets contain nitrates and nitrites. When these vegetables spoil, are left too long after cooking, or are pickled for too short a time, the nitrates in them are reduced to nitrites by nitrate-reducing bacteria. Excessive intake can lead to poisoning. Bitter well water and repeatedly evaporated water from steamers also contain nitrates and nitrites, and excessive consumption can similarly cause poisoning.
2. When stomach acid is too low or digestive function is impaired, nitrate-reducing bacteria in the intestines proliferate. If a large amount of nitrate-rich vegetables is consumed, it can produce excessive nitrites, leading to poisoning.
3. Certain drugs and chemicals, such as aniline derivatives like vinegar aniline, antipyrine, phenacetin, nitro compounds like bismuth subnitrate, ammonium nitrate, sodium nitrite, and primaquine, can also cause poisoning.

bubble_chart Pathogenesis

When a large amount of nitrite enters the bloodstream, it intensifies the oxidation of hemoglobin. Once this exceeds the reducing capacity of coenzyme I, the concentration of methemoglobin increases (normally not exceeding 1%). Since methemoglobin is brownish-black and cannot bind with oxygen, it lacks oxygen-carrying capacity. When its concentration in the blood exceeds 10% (15 g/L), the skin and mucous membranes turn bluish-gray. At 20–30%, symptoms of hypoxia appear. At 55–60%, pronounced neurological symptoms emerge. Exceeding 70% can lead to death.

Symptoms typically appear within half an hour to three hours after ingestion. In mild cases, only the mucous membranes and the tips of the fingers and toes turn bluish-gray. In severe cases, symptoms include dizziness, headache, lack of strength, nausea, vomiting, abdominal pain, diarrhea, shortness of breath, weak pulse, and bluish-black discoloration of the skin and mucous membranes. The condition may progress to unconsciousness, convulsions, decreased blood pressure, and irregular heart rhythms, potentially leading to respiratory and circulatory failure and death.

bubble_chart Diagnosis

1. Medical history and signs Based on a history of consuming foods, medications, or well water containing nitrates and nitrites, along with sudden onset symptoms and bluish-gray skin and mucous membranes, the diagnosis can be considered.
2. Laboratory tests (1) Draw venous blood with heparin anticoagulation and shake. Normal venous blood appears dark red and turns bright red upon oxygenation. In methemoglobinemia, the blood remains brown after shaking for 15 minutes and only turns bright red after 5-6 hours. (2) Spectrophotometer or spectroscope analysis shows methemoglobin absorption bands at wavelengths of 520-630nm. Adding a few drops of 1% potassium cyanide causes the absorption bands to disappear immediately.

bubble_chart Treatment Measures

Mild cases can recover within 1-2 days by removing the disease cause and strengthening care. Severe cases should be treated promptly.

1. General first aid: Quickly rinse the stomach with potassium permanganate solution and administer magnesium sulfate for catharsis, while providing oxygen. Pay attention to the prevention and treatment of respiratory and circulatory failure. Severe cases may require transfusion of fresh blood or exchange transfusion.
2. Specific toxin-removing medication
   1. Methylene blue: For severe cases, 1% methylene blue at 0.1–0.2 ml/kg can be added to a glucose solution and administered intravenously over 10–15 minutes. Improvement can be observed within 15 minutes. If no effect is seen within 1–2 hours, the dose may be repeated once. For improved or mild cases, oral administration of 3–5 mg/kg, 3–4 times daily, is suitable. When methylene blue is used in small doses, it is reduced to leukomethylene blue under the action of reduced nicotinamide adenine dinucleotide (NADH) dehydrogenase. Leukomethylene blue can reduce methemoglobin to hemoglobin. However, large doses can oxidize hemoglobin to methemoglobin, so the dose should be carefully controlled.
   2. Vitamin C: 0.5–1.0 g per dose, added to 20–40 ml of glucose solution for intravenous injection. For mild cases, oral administration of 0.2–0.5 g per dose, 3 times daily, is appropriate.

bubble_chart Prevention

1. Eat vegetables fresh; pickled vegetables should be thoroughly fermented before consumption; avoid drinking bitter well water or water from steam pots.
2. Reduce vegetable intake when gastrointestinal function is abnormal.
3. Use aniline and nitro compounds with caution.

bubble_chart Differentiation

1. Respiratory and circulatory system diseases may present with cyanosis, accompanied by other corresponding signs; in grade I nitrite poisoning, there is obvious cyanosis but no significant respiratory distress, and the cyanosis should rapidly diminish and disappear after methylene blue injection.
2. In sulfhemoglobinemia, venous blood drawn and shaken for 15 minutes does not change color, and remains unchanged after 5-6 hours. Spectroscopic examination shows an absorption band peak at 620nm, and the absorption band does not disappear upon adding 1% potassium cyanide.