Aeromedical Report: Kidney Stones

Air Line Pilot, February 2001, p. 5
By Dr. Quay Snyder, ALPA Associate Aeromedical Advisor

Kidney stones afflict more than 500,000 Americans each year. The pain from these stones can be excruciating and incapacitating. Nearly 60 percent of people with kidney stones will have a recurrence within 10 years of the first stone. Approximately 80 percent of kidney stones are reported in men.

The presence of a kidney stone is generally disqualifying for FAA aeromedical certification. Once stone-free, a pilot may resume flying duties after clearance from the pilot’s AME or the FAA. Under certain circumstances, the FAA may also clear pilots with retained kidney stones to return to flying duties. In 2000, the physicians of the ALPA Aeromedical Office helped 196 pilots with kidney stones return to flight duty.

The anatomy of the urinary system has four main components: the kidneys, the ureters, the bladder, and the urethra. Urinary stone disease encompasses several types of stones found in several locations, though the common term for all of the locations is kidney stones.

• Renal stones are located in the kidney.

• Calyceal stones are found in the funnel-shaped area called the calyx between the kidney and the ureter.

• Ureteral stones are in the tube that drains urine from the kidney to the bladder. Stones are usually most painful when they migrate down, or are lodged in, the narrow part of the ureter. The narrowest area of the ureter is that portion that extends into the bladder, also known as the ureterovesicular junction or UVJ. Stones in the bladder do not usually cause symptoms unless they become lodged in the prostatic or penile urethra of men.

• "Saddle" stones, also known as "staghorn calculi," extend from the calyx into several portions of the kidney and rarely pass spontaneously.

Stones are made up of several different substances. Most stones are made of calcium oxalate, calcium urate, or calcium pyrophosphate. Occasionally, stones are made of primarily uric acid—cystine or struvite. Calcium stones are usually visible on x-rays, while uric acid stones are nearly invisible.

The composition of the stones is important in being able to locate the stones and deciding how to treat recurrent stones. The doctor can determine what the stone is composed of by sending a stone strained from the urine for chemical analysis. A 24-hour urine collection can also be analyzed and compared to blood chemistries to determine if the kidney is excreting too much or too little of a compound leading to increased risk for stone formation. Kidney function is also measured using 24-hour urine analysis called creatinine clearance.

Urinary stones usually manifest as severe, acute pain on one flank or the lower back. Pain may radiate into the scrotum or testicle on the same side for men and into the groin area of the same side for women. Urinalysis may show microscopic blood that is not visible to the naked eye. Occasionally, a stone without any pain is detected after a urinalysis shows blood or after an x-ray that is taken for another reason shows a stone.

Doctors often confirm a diagnosis and locate a stone by using x-rays. Some small stones are difficult to find even if the doctor is specifically looking for the stone. A more sophisticated diagnostic technique, called an intravenous pyelogram (IVP), involves injecting a dye that is visible on x-ray film and excreted by the kidneys. The IVP shows an outline of the kidneys, calyces, and ureters. If a stone is blocking the ureter, the dye swells the ureter above the stone and may not pass into the ureter below the stone. Later evaluation of the cause of the stone may involve a 24-hour collection of urine to determine the concentration of certain chemicals known to provoke stone formation.

At times, computerized tomography (CT) scans are used to locate stones, particularly smaller stones that may not be readily visible on x-ray film.

Many treatments exist for stones. The FAA does not dictate a particular treatment for stones and may grant a waiver for flying for any treatment that the pilot and the physician agree to and that eliminates the stone. The most conservative treatments involve the spontaneous passage of the stone after oral or intravenous hydration. A key element in preventing stones is adequate hydration and generous fluid intake.

• Medical treatment for stones involves increasing the urine output and pain relief. People with stones are instructed to drink large amounts of fluids (one or more gallons) a day in an attempt to flush the stone through the urinary system. Those who have enough pain to cause nausea and vomiting may need to use an IV to get large amounts of fluids rapidly. Pain relief may require narcotics, especially as the stone moves down the ureter.

• Extra-corporal shock wave lithotripsy (ESWL) may be used for some stones that are located high in the urinary system and that may not pass spontaneously. This procedure uses sound waves from several sources outside the body focused on the stone to fragment it into smaller pieces that can pass spontaneously. The procedure, which involves one of several techniques, requires anesthesia; and many patients report considerable soreness for several days after the ESWL.

Often, the surgeon will insert a stent, or hollow tube, into the ureter using a scope inserted through the bladder. The stent is to decrease the pain of the residual gravel from the shattered stone as it passes through the ureter. After all the fragments have passed, the stent is removed. Recovery and stone passage may take several days to weeks.

• Ureteroscopic stone removal (basket extraction)may be used for stones located lower in the collecting system (closer to the bladder). The surgeon inserts a probe through the urethra and bladder into the ureter. Using direct visualization of a fiberoptic scope, the surgeon grabs the stone and manipulates it through the probe, surrounding the stone with a clasp or expandable cage (basket). As the entire probe is removed, the stone is dragged out with the probe. Recovery is usually within several days. As with the ESWL, a stent is sometimes left in place in the ureter for several days to allow healing. Not all stones are located in areas where this technique is practical.

• Percutaneous nephrolithostomy is a more invasive procedure, which is usually reserved for stones in the kidney or calyx that are too large to pass and too large for ESWL. A surgeon makes a small incision through the patient’s back and into the ureter or calyx to directly visualize the stone through a small scope. The stone is then removed through the scope. Sometimes the stone must be broken up to be removed through the scope. The recovery time is somewhat longer for this procedure and may require staying in the hospital for several days. As in the above procedure, the advantage over ESWL is that the stone is removed rather than passing through the ureter.

If evaluation of the cause of stone formation reveals abnormal concentrations of certain substances in the urine or blood, medications may be prescribed to change the concentration and lower the risk of recurrent stone formation. Medication to prevent recurrences of kidney stones includes the diuretic hydrochlorothiazide (HCTZ), which decreases calcium excretion via the urine. Allopurinol (Zyloprim) is used to decrease the body’s uric acid production and hence lower the concentration in the urine.

The FAA may grant a waiver for using either medication after an observation period free of side effects. Because HCTZ lowers blood pressure and causes potassium loss, the FAA may require periodic reports that include blood tests for potassium (K+) on future medical applications. Other medications may be prescribed to change the acidity or alkalinity of the urine and decrease the risk of stones. The FAA may also grant a waiver for using these.

The formation of stones is best prevented by drinking enough water. The dry environment of the pressurized aircraft cabin leads to dehydration. A lack of a ready source of fluids and an unwillingness to leave the cockpit frequently to "attend to physiologic needs" during long flights also puts the pilot at increased risk for dehydration and stone formation. The commuter pilot who has little time between flights, often sits on a hot ramp, and has physical work to perform associated with the flight may also become dehydrated. The general aviation pilot on a long cross-country flight without enough water on board is also at risk.

Certain medications, such as Crixivan, are known to cause stones in some users. Certain foods that contain purines (meats, fish, and poultry), oxalate, or calcium may place some individuals at risk. Not all people will benefit from restricting these foods, however. A 24-hour urine collection and analysis will aid your physician in recommending whether to restrict certain foods.

The body is usually a liter dehydrated before an individual becomes thirsty. A good rule of thumb for adequate hydration is to drink enough to keep the urine clear. Pilots with previous kidney stones rarely forget this rule.

The FAA will certify a pilot to fly after all stones are cleared, the pilot is stable, and documentation is forwarded to the FAA for clearance. Many AMEs are willing to clear a pilot also, if they have the appropriate documentation.

For those pilots who have retained stones that do not pass, the FAA will consider granting a waiver if their physician can affirm that the stone appears stable and is unlikely to pass spontaneously. The hazard is that a retained stone may pass during flight and compromise flying safety. The FAA alone retains waiver authority for this condition.

For the first episode of a passed kidney stone, the FAA will not generally require follow-up reports from the pilot’s personal physician on subsequent FAA medical examinations. Pilots with a history of recurrent or retained stones should expect to be required to submit these reports annually at the time of their FAA medical examinations for several years.

Copyright © 2001 Virtual Flight Surgeons, Inc. Reprinted with permission.