Parkinson’s disease (PD) is a progressive neurological disorder primarily known for its motor symptoms, such as tremor, rigidity, and slowed movement. While these physical manifestations are the most recognized features, PD is a complex condition that also involves a wide range of non-motor symptoms. Gastrointestinal issues are among the most common non-motor complaints, often appearing years before the motor symptoms begin. Diarrhea is one such symptom, and understanding its connection to PD involves looking at both the disease’s underlying biology and the effects of its treatments.
The Gut-Brain Axis and Parkinson’s Disease
The body’s central nervous system (CNS) and the enteric nervous system (ENS), the intricate network of nerves lining the gut, communicate through the gut-brain axis. This bidirectional connection allows the brain to influence gut function and vice versa. The ENS is often referred to as the “second brain” because it independently manages digestion, motility, and secretion within the gastrointestinal tract.
In Parkinson’s disease, this communication axis is compromised, leading to gastrointestinal dysfunction. The pathology affecting the brain can also simultaneously affect the ENS, disrupting the coordinated signals needed for normal digestion. This systemic nerve damage means that gut muscles may contract too slowly (causing constipation) or too rapidly or erratically, which can result in diarrhea.
Diarrhea Caused by Disease Progression
The biological connection between PD progression and diarrhea lies in the accumulation of abnormal proteins within the gut’s nervous system. The protein alpha-synuclein, which misfolds and clumps into Lewy bodies in the brains of people with PD, is also found in the neurons of the enteric nervous system. This pathology in the gut can precede the onset of motor symptoms by a decade or more.
This accumulation of alpha-synuclein directly damages the nerves that control peristalsis, the wave-like muscle contractions that move contents through the intestines. Damage to these nerves leads to dysmotility, an impaired movement pattern that can manifest as either severely slow or rapid intestinal transit. While constipation is the most frequently reported bowel issue in PD, the nerve damage can occasionally cause the intestinal contents to move too quickly, resulting in diarrhea.
A less direct cause of diarrhea is “overflow diarrhea,” a consequence of severe constipation. When a large, hardened mass of stool blocks the colon, liquid stool can leak around the blockage, presenting as watery diarrhea. This symptom is a sign of underlying slow transit and severe obstruction, not rapid motility. Since the ENS pathology affects the entire length of the GI tract, it can also lead to small intestinal bacterial overgrowth (SIBO), a condition associated with diarrhea.
Medication Side Effects and Diarrhea
Beyond the direct effects of the disease on the gut, many medications used to manage the motor symptoms of PD can induce diarrhea. Levodopa, often prescribed with carbidopa, is a common culprit because it can irritate the gastrointestinal lining. This irritation or the medication’s effect on gut function can lead to increased motility or fluid secretion, resulting in loose stools.
Dopamine agonists, another common class of PD drugs that mimic dopamine’s effects, are also known to cause gastrointestinal side effects. These drugs act on dopamine receptors found throughout the body, including those in the gut, which can alter the normal rhythm of digestion. COMT inhibitors, sometimes used to prolong levodopa’s effect, have diarrhea listed as a recognized side effect, sometimes occurring months after starting the medication.
A change in dosage, timing, or formulation of any antiparkinsonian medication can trigger or alleviate these digestive side effects. For instance, taking medication with food may lessen gastrointestinal irritation. Medication-induced diarrhea is an important factor to consider, as it is often a manageable complication distinct from the disease’s progression.
Strategies for Managing Gastrointestinal Distress
Managing diarrhea in Parkinson’s disease requires a careful, multi-pronged approach starting with dietary adjustments. Maintaining adequate hydration is important to replace lost fluids and electrolytes, especially since people with PD may be prone to orthostatic hypotension. Adjusting fiber intake can also be beneficial, as balancing soluble and insoluble fiber helps normalize stool consistency, but this must be done gradually.
Before making any changes to PD medication, consult with a neurologist or movement disorder specialist. If the diarrhea is medication-related, they may adjust the dose, change the timing, or switch to an alternative formulation to mitigate the side effect. Crucially, if the diarrhea is suspected to be “overflow diarrhea” from underlying constipation, using over-the-counter anti-diarrheal medicines is strongly discouraged, as they can slow the gut further and worsen the impaction.
A gastroenterologist should be consulted to rule out other possible causes of diarrhea not directly related to PD, such as C. difficile infection, inflammatory bowel disease, or irritable bowel syndrome. A specialist can perform tests to check for issues like SIBO, which can be treated with specific antibiotics, or other concurrently occurring conditions. Comprehensive management involves addressing the gut symptoms while ensuring the neurological treatment remains effective.