Parkinson’s Disease (PD) is a progressive neurological disorder primarily known for its effects on movement, such as tremor, rigidity, and slowed motion. However, the disease affects the entire body, and gastrointestinal (GI) problems are among the most common non-motor symptoms. These issues can often begin years, or even decades, before the physical symptoms that lead to a PD diagnosis become apparent. While constipation is the most recognized GI symptom in PD, the disease process and its treatments can also lead to episodes of diarrhea. This symptom is complex, often stemming from underlying nervous system dysfunction or as a direct result of necessary medications.
Why Diarrhea Occurs in Parkinson’s
The pathology of Parkinson’s disease extends beyond the brain and involves the enteric nervous system, the network of nerves embedded in the walls of the gastrointestinal tract. This involvement is characterized by the accumulation of misfolded alpha-synuclein protein within the nerve cells of the gut. This pathology disrupts communication, leading to dysautonomia (autonomic neuropathy), which impairs the involuntary control of bodily functions.
Dysautonomia results in irregular gut motility, making the movement of digestive tract contents unpredictable. This often manifests as slow transit time, leading to severe constipation, the predominant bowel issue in PD. In cases of severe blockage, however, the body can develop “overflow diarrhea,” where watery stool leaks around the impacted mass of hard stool, mimicking true diarrhea. Treating this with anti-diarrheal medication would worsen the underlying constipation.
The slowed movement of the small intestine, a consequence of PD-related dysautonomia, creates an environment where bacteria from the large intestine can migrate and flourish. This condition is known as Small Intestinal Bacterial Overgrowth (SIBO), and it is significantly more common in individuals with PD. SIBO symptoms often include bloating, abdominal pain, and diarrhea, making it a frequent direct cause of loose stools.
Medications That Can Cause Diarrhea
While the disease process can lead to diarrhea, the medications used to manage Parkinson’s symptoms are a major cause of gastrointestinal upset. Many antiparkinsonian drugs directly affect the digestive system, as they interact with neurotransmitters active in the gut. Diarrhea is a recognized adverse effect of Levodopa preparations, which are often combined with Carbidopa.
Catechol-O-methyltransferase (COMT) inhibitors, such as entacapone and tolcapone, are used to prolong the effect of Levodopa and are frequently associated with diarrhea. Entacapone, in particular, can cause significant loose stools that may necessitate a change in treatment regimen. Monoamine Oxidase B (MAO-B) inhibitors, like selegiline and rasagiline, which help prevent dopamine breakdown, can also contribute to GI distress, including diarrhea.
Patients often take other drugs that can exacerbate or directly cause diarrhea beyond the primary PD medications. Laxatives, prescribed to manage PD-related constipation, can easily overcorrect the problem and induce loose stools. Antibiotics used to treat unrelated infections, or those prescribed for SIBO, can disrupt the gut microbiome and lead to temporary diarrhea. The timing and dose of these medications are relevant, as taking certain drugs without food or at higher doses can increase gastrointestinal irritation.
Strategies for Diagnosis and Treatment
Addressing persistent diarrhea in Parkinson’s disease requires a systematic approach, often involving a gastroenterologist in addition to the neurologist. The first step involves detailed symptom logging, where the patient records the frequency, consistency, and timing of bowel movements relative to medication doses and meals. This information helps the care team determine if the diarrhea is medication-induced or linked to the disease process.
The medical team reviews all current prescriptions, including over-the-counter supplements and laxatives, to identify and potentially adjust or stop any medications known to cause loose stools. If SIBO is suspected, a breath test is typically ordered, which measures the gases produced by bacteria in the small intestine after ingesting a sugary solution. Stool analysis may also be performed to rule out infectious causes or malabsorption issues.
Treatment strategies are highly individualized and focus on correcting the underlying cause. For confirmed SIBO, a course of targeted antibiotics, such as rifaximin, is often effective. If the cause is chronic constipation with overflow, the focus shifts to aggressive treatment of the blockage, which may involve a temporary bowel cleanse followed by adjusting long-term laxative use.
Dietary and Medication Adjustments
Dietary adjustments are a common intervention, focusing on balancing fiber intake and ensuring adequate hydration, as chronic diarrhea risks dehydration. A low FODMAP diet, which reduces fermentable carbohydrates, may be recommended to alleviate symptoms of bloating and diarrhea associated with bacterial overgrowth. When medication-induced, optimizing the timing or formulation of PD drugs, such as switching to an extended-release option, can manage the side effects without compromising motor control.