Cerebrospinal fluid, or CSF, is the clear liquid that surrounds the human brain and protects it from injury. If the membrane surrounding this layer of liquid is ruptured, the liquid can leak out of the nasal passageways. (Photo: Fox Phoenix/Video Screenshot)

After years of suffering with what he thought was a chronic runny nose, Joe Nagy found out recently that his leaky snout was actually a leaky brain.

The Arizona resident told Fox 10 News that he carried tissues around in his pocket to combat his "embarrassing" runny nose condition. When he finally went to see a specialist for what he thought was a severe allergy situation, the doctor told Nagy that the clear liquid coming out of his nostrils was cerebrospinal fluid, or CSF -- a.k.a. brain fluid.

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It turned out that Nagy had a hole in the membrane that surrounds the human brain, and the transparent fluid had been leaking through his nasal passageway for over a year and a half.

"Our brains are really just above our noses all the time," Dr. Peter Nakaji, the doctor who fixed Nagy's brain fluid leak, told Fox 10. "This is one of the more common conditions to be missed for a long time, because so many people have runny noses."

He said that leaks in the brain can be extremely tiny. "Like a puncture on the bicycle tire," he explained.

And, as if things couldn't get any worse, Nagy also came down with a serious case of meningitis, an inflammation of cerebral membranes often caused by infections in the brain fluid.

After treating Nagy for meningitis, doctors were able to repair Nagy's brain using a bit of cartilage from his nose to plug up the leak in the membrane.

But why was Nagy's brain leaking in the first place?

The average human brain contains about 140 ml, or 4.7 ounces, of brain fluid at any given time. The body replenishes its brain fluid once every six hours, meaning we cycle through about 600 ml, or 20 ounces, of brain fluid every day.

Even ancient physicians knew of the presence of a fluid in the brain. According to a report in the Annals of Clinical and Laboratory Science, Hippocrates, an ancient Greek physician, described the fluid surrounding the brain as "water", Vesalius, a 16th-century Dutch anatomist, called it an "excremental liquid."

Because of the use of primitive techniques in autopsies (ancient physicians simply cut off the human head, which drained the CSF before it could be studied), the true nature of brain liquid wasn't known until much later. Emanuel Swedenborg, an 18th-century Swedish engineer who also studied human anatomy, wrote about the "highly gifted juice" he observed in the human skull.

"Swedenborg recognized the cerebral cortex as the seat of thought and the source of the sensory and motor functions of the extremities," the Annals of Clinical & Laboratory Science reports.  

A leaky brain affects about 1 in 100,000 to 200,000 patients, according to one surgeon in Arizona. A number of different things can cause a CSF leak. Trauma to the head is a major source of a leaky brain, but also certain head, brain or spinal surgeries can tear or puncture the membrane surrounding the brain.

When no cause can be determined, it's called a spontaneous CSF leak.

Nagy's case of leaky brain isn't the first in Arizona. Just last year, Aundrea Aragon, a 35-year-old mother from Tucson, had clear liquid pouring out of her nostrils from two cracks in the back of her sinuses. ABC News reports that had doctors not repaired the cracks, Aragon would have died from her leaky brain.