Diagnostic dilemma: Junk-food diet caused a teen's permanent blindness

The patient: A 14-year-old boy in the United Kingdom

The symptoms: The teen was brought to his family's general practitioner because he felt unusually tired. His doctor asked about the boy's eating habits and described him as a "fussy eater" in notes from the appointment. The doctor also indicated that the teen had no symptoms other than tiredness, according to a report of the case.

Tests revealed that the teen had mild anemia and low levels of vitamin B12, which comes from a person's diet because the body does not produce it. The body needs vitamin B12 for maintaining the health of red blood cells and the nervous system. The teen's doctor provided dietary advice and prescribed supplementary B12 injections.

What happened next: Later, at age 15, the teen began developing problems with his sight. An eye exam detected no abnormalities, and the ophthalmologist was unable to determine the cause of the vision loss, which worsened over time. When the patient was 17 years old, his general practitioner referred him to a neuro-ophthalmologist for further testing. (Neuro-ophthalmologists can assess vision problems related to the nervous system.)

Additional vision tests showed that the patient's visual acuity — a measurement of a person's ability to accurately see details from certain distances — was 20/200 in both eyes. In other words, objects that would be visible to a person with normal (20/20) vision at a distance of 200 feet were not clearly visible to him beyond a distance of 20 feet. Vision of 20/200 or lower falls within the parameters of legal blindness, according to the American Foundation for the Blind.

When asked, the patient denied using drugs, alcohol or tobacco, and his eyes showed no signs of disease, nor were they scarred or injured. The results of a neurological exam were normal.

However, blood tests showed that his red blood cells were enlarged, which can suggest vitamin deficiencies. His blood levels of copper and vitamin D were lower than normal. Furthermore, elevated concentrations of the amino acid homocysteine and methylmalonic acid — compounds that B12 helps to break down — further hinted that he was deficient in the vitamin, as at earlier examinations.

The diagnosis: The patient admitted that he was no longer taking the recommended B12 injections. (The case report did not indicate exactly when he had stopped.) His body mass index (BMI) was normal, and he was of average weight for his height. But because his B12 levels were low, the doctors questioned him about his diet.

He replied that he could not tolerate certain food textures; as a result, since elementary school, he had avoided most foods. For years, his daily diet consisted of fast-food french fries, white bread, potato chips, sausages and processed ham.

This diet lacked vital nutrients, leading to a condition called nutritional optic neuropathy, a rare type of vision loss that is often caused by severe deficits in B12 and other B vitamins. It causes atrophy of the optic nerve, which relays information from the eye to the parts of the brain that process vision. Copper deficiencies have also been linked to this type of optic neuropathy and to enlarged red blood cells, according to the report's authors.

The treatment: The doctors prescribed the teen supplements to correct his nutritional deficiencies and referred him to mental health services to treat his food aversions, which the doctors identified as a type of eating disorder.

Following treatment, the patient's vision did not deteriorate further. However, there was no improvement in his sight. Nutritional optic neuropathy may be reversed when caught early, but if the optic nerve becomes atrophied, the vision loss is permanent, the physicians explained in the report.

What makes the case unique: Nutritional optic neuropathy is rarely caused by dietary deficiencies alone. Historically, cases in the medical literature have generally been associated with malnutrition caused by war and famine. More recent cases have usually involved both nutritional deficits and smoking or abuse of alcohol or drugs.

In some instances, this type of neuropathy results from inflammatory bowel disease or bariatric surgery, which limit the body's absorption of certain nutrients. It can also stem from restrictive diets that are not adequately supplemented with missing vitamins and minerals.

The teen's case is also unusual because his normal BMI masked the vitamin deficiencies that caused his blindness. In other words, doctors may have overlooked his dietary deficits because his weight was within a normal range.

Moving forward, "nutritional optic neuropathy should be considered in any patient with unexplained vision symptoms and poor diet, regardless of BMI," the authors wrote in the report.

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