Kidney stones afflict over 500,000 Americans each year. Approximately 10% of the US population will get kidney stones in their lifetime. The pain from these stones can be excruciating and incapacitating. Nearly 60% of people with kidney stones will have a recurrence within ten years of the first stone. The presence of a kidney stone is generally disqualifying for FAA aeromedical certification until cleared by an Aviation Medical Examiner or the FAA. Controllers also have to be cleared by the Regional Flight Surgeon. Pilots and controllers with retained kidney stones may also be cleared by the FAA to return to safety sensitive duties if no symptoms and the stones are stable.
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 usual cause symptoms unless they become lodged in the prostatic or penile urethra. “Saddle” stones extend from the calyx into several portions of the kidney and rarely pass spontaneously.
Composition of Stones
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, cysteine 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 locating the stones and deciding how to treat recurrent stones. Stone composition is determined 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.
Symptoms and Diagnosis of Kidney Stones
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 an x-ray that is taken for another reason shows a stone.
The diagnosis of a stone is often done with an x-ray. Some small stones are often difficult to find even if the physician is specifically looking for the stone. An Intravenous Pyelogram (IVP) involves injecting a dye that is visible by x-ray 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.
At times, Computerized Tomography (CT) scans are used to locate stones, particularly smaller stones that may not be visible on x-ray films.
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.
Treatments for Kidney Stones
Many treatments exist for stones. The FAA does not dictate a particular treatment for stones, so any treatment agreed to by the pilot/controller and the physician that results in elimination of the stone is okay for return to flying/controlling. As discussed above, the most conservative treatments involve the spontaneous passage of the stone after oral or intravenous hydration. Adequate hydration by generous fluid intake is also a key element in the prevention of kidney stones.
Medical treatment for stones involves the increasing of 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. For those who may have enough pain to cause nausea and vomiting, an IV may be used to provide large amounts of fluids. Pain relief may require narcotics, especially as the stone moves down the ureter.
Extracorporal Shock Wave Lithotripsy (ESWL)
Stones of a certain size range located high in the urinary system that may not pass spontaneously are candidates for extra-corporal shockwave lithotripsy (ESWL). 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. There are several techniques for lithotripsy. The procedure requires anesthesia and many patients report considerable soreness after the ESWL. Often a stent, or hollow tube, is inserted 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 passing 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)
Stones located lower in the collecting system (closer to the bladder) may be removed by a basket extraction technique. A probe is inserted through the urethra and bladder into the ureter. Using direct visualization of a fiberoptic scope, the stone is grabbed and surrounded by a clasp or expandable cage (basket) and manipulated through the probe. The entire probe is removed dragging the stone 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.
A more invasive procedure is the percutaneous nephrolithotomy. This is usually reserved for stones in the kidney or calyx that are too large to pass and too large for ESWL. An small incision through the back and into the ureter is made 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 remove through the scope. The recovery time is somewhat longer for this procedure and may require several days hospitalization. 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, medication 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 in to the urine. Allopurinol (Zyloprim) is used to decrease uric acid production by the body and hence lower the concentration in the urine. Both medications are waiverable by the FAA after an observation period free of side effects. Since HCTZ lowers blood pressure and causes potassium loss, the FAA may require periodic reports including 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. These are also waiverable.
Prevention of Kidney Stones
Prevention of stones is best accomplished by maintaining hydration. The dry environment of the pressurized aircraft cabin predisposes to dehydration. A lack of a ready source of fluids and an unwillingness to frequently leave the cockpit 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 tarmac and has physical work to perform associated with the flight may also get dehydrated. The general aviation pilot on a long cross country without sufficient water carried on board is also at risk. The body is usually a liter dehydrated before an individual becomes thirsty. A good rule of thumb is to drink enough to keep the urine clear. Pilots with previous kidney stones rarely forget this rule.
Certain medications, such as Crixivan, are known to cause stones in some users. Certain foods that contain purines, oxalate or calcium may place individuals at risk. Foods high in oxalates include rhubarb, asparagus, spinach, chocolate, tea and coffee. Foods high in purines include organ meats (liver, brain), sardines, beans, beer and red wine. Not all people will benefit from restricting these foods, however. A 24 hour urine collection and analysis will aid your physician in recommending whether or not to restrict certain foods. If stones are composed of calcium oxalate or calcium phosphate, the use of a medication, hydrochlorothiazide, may reduce the risk of recurrence. Risk of recurrence of stones of uric acid may be reduced by the use of allopurinol. Allopurinol is sometimes used with calcium oxalate stones as well.
FAA Policy on Flying/Controlling After Kidney Stones
Historically, the FAA would certify a pilot/controller who has had a single episode of kidney stones to fly after all stones are cleared, the individual is stable and documentation is forwarded to the FAA for clearance. As of Oct 2015, the FAA further authorized AME’s to clear airmen back to flying even with retained kidney or bladder stones as long as they are not having any pain or indications of impending passage. The documentation should include reports of the evaluation and treatment, as well as a report of x-ray confirmation that the pilot/controller is stone free or any retained stones are stable. Controllers do require specific clearance from the Regional Flight Surgeon prior to returning to controlling. The following criteria must usually be met:
- Radiographically stone free or stable retained stones not likely to pass or incapacitate
- Normal renal function,
- No evidence of metabolic stone disease.
Individuals with recurrent episodes of kidney stones are generally required to present information to the FAA that they are free of stones before returning to aviation duties. Recurrent episodes generally require a 24 hour urine analysis as well.
As noted, for those pilots/controllers who have retained stones that do not pass, the FAA or AME will consider clearance if their physician can affirm that the stone appears stable and is unlikely to pass spontaneously or if they are so small that they would likely pass without incapacitation. The hazard is that a retained stone may pass during flight and compromise flying safety.
For the first episode of a stone, the FAA will not generally require follow-up reports from the pilot or controller’s personal physician on subsequent FAA medical examinations. Those with a history of recurrent or retained stones should expect to be required to submit these reports for several years at the time of their FAA medical examinations.
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