Kidney stones are hard, crystalized masses that form within the urinary tract, most commonly composed of calcium oxalate, uric acid, or struvite. While they originate in the kidneys, many people experiencing a stone episode report severe discomfort and issues related to the digestive system. This connection between a urinary problem and gastrointestinal (GI) distress is a well-understood physiological phenomenon. The body mistakenly perceives the pain from a kidney stone as originating from the stomach or intestines. Therefore, managing a kidney stone often involves addressing both the pain in the flank and the resulting digestive symptoms.
The Visceral Reflex Arc
The primary reason kidney stones cause digestive upset lies in the shared neural pathways of the body’s internal organs. Both the kidneys and the gastrointestinal tract receive sensory nerve fibers that converge on the same segments of the spinal cord. This phenomenon is known as the viscero-visceral reflex or referred pain.
When a stone becomes lodged in the ureter, it causes distension and intense pain called renal colic. The sensory nerves carrying this pain signal travel to the spinal cord, where they connect with other nerve cells. These same spinal cord segments also receive input from the nerves innervating the stomach and intestines.
The brain has a less precise map for internal organ pain compared to skin pain, causing it to misinterpret the intense signal from the blocked ureter. Since digestive organs are more frequently the source of abdominal discomfort, the brain projects the pain and associated symptoms to the stomach or bowel area. This neural convergence explains why a problem in the urinary system can manifest as nausea or generalized abdominal pain.
Direct Manifestation of Stone Obstruction
The intense, wave-like pain of renal colic directly triggers a cascade of gastrointestinal symptoms. Nausea and vomiting are the most common digestive complaints associated with a kidney stone episode. This is an involuntary, autonomic nervous system response to the overwhelming internal pain caused by the stone obstructing the flow of urine.
The severe pain signal activates the sympathetic nervous system, which slows down the normal movement of the digestive tract, a process called peristalsis. This reduction in gut motility can lead to abdominal bloating and general discomfort. This autonomic response can also lead to changes in bowel habits, resulting in either constipation or, less commonly, diarrhea.
When the stone causes a complete or near-complete blockage of the ureter, pressure builds up and leads to swelling of the kidney, known as hydronephrosis. This distension further exacerbates the visceral reflex, increasing the severity of the GI symptoms. The body’s reaction to the obstruction and subsequent inflammation makes the experience feel like an acute stomach illness, even though the issue is purely urological.
Gastrointestinal Effects of Treatment and Medication
Beyond the direct effect of the stone itself, the medications used to manage stone pain and prevent recurrence can also provoke significant gastrointestinal issues. Pain management is a primary concern, and strong narcotic analgesics are often prescribed for renal colic.
Narcotic Analgesics
These analgesics frequently cause severe constipation and nausea. They slow down the entire digestive system and can require additional anti-sickness drugs to counteract their side effects.
Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)
NSAIDs, such as ketorolac, are commonly used for stone pain because they reduce inflammation and ureteral spasm. While often causing less nausea and sedation than narcotics, prolonged use of NSAIDs carries a risk of stomach irritation and potential ulcer formation in the GI lining. This can lead to stomach pain or indigestion that complicates the initial symptoms.
Long-Term Prevention Medications
For stone prevention, certain long-term medications also introduce digestive side effects. Urinary alkalinizers, such as potassium citrate, are used to raise urine pH and dissolve specific stone types, and commonly cause stomach discomfort, gas, diarrhea, or nausea and vomiting. Similarly, medications like allopurinol, used to treat uric acid stones, can have gastrointestinal upset as their most frequent side effect.