Parkinson’s Disease (PD) is a progressive neurological disorder causing tremors, stiffness, and slowness. A wide range of non-motor symptoms significantly impacts the quality of life for those with PD, and gastrointestinal (GI) issues are among the most common. These GI problems, which affect most individuals with the disease, can include difficulty swallowing, delayed stomach emptying, and changes in bowel habits. This article explores whether diarrhea is a symptom of PD and examines the underlying causes for gut dysfunction.
The Connection Between Parkinson’s Disease and Diarrhea
Constipation is the most prevalent and widely recognized GI symptom in PD, often affecting up to 80% of patients and sometimes appearing years before the onset of motor symptoms. While less common as a direct, primary symptom of the disease itself, diarrhea can certainly occur in people with PD. A significant concern is “overflow diarrhea,” a secondary complication of severe, long-term constipation where liquid stool leaks around a blockage of hardened feces. Diarrhea is often considered a part of a broader dysregulation of the bowel rather than a standalone symptom of the neurodegeneration.
Neurological Reasons for Gut Dysfunction
The pathology of Parkinson’s disease involves the accumulation of a misfolded protein called alpha-synuclein, which forms clumps known as Lewy bodies. This pathology is not confined to the brain but also affects the nervous system of the gut, known as the Enteric Nervous System (ENS). The presence of Lewy bodies in the ENS disrupts its normal function, leading to impaired motility, or dysmotility. This disruption is the root cause of the irregular gut transit time seen in PD.
The alpha-synuclein pathology also affects the nerve cells in the lower brainstem that control gut function, specifically the dorsal motor nucleus of the vagus nerve. Damage to this nerve further contributes to the dysmotility. While this neurological damage most often results in the slowing of the digestive tract, leading to constipation and delayed gastric emptying, it represents a fundamental breakdown in the body’s ability to regulate intestinal movement. The resulting dysregulation of the GI tract can manifest as alternating periods of constipation and diarrhea, reflecting an unstable state of motility.
Medication Side Effects and Other Contributing Factors
Diarrhea is often a side effect of the medications used to manage Parkinson’s disease. Levodopa, frequently combined with carbidopa, is a standard treatment, and diarrhea is a recognized adverse reaction. For example, the combination drug carbidopa/levodopa/entacapone can cause diarrhea. Entacapone, which is added to prolong the effect of levodopa, is also known to cause diarrhea.
Small Intestinal Bacterial Overgrowth (SIBO)
Another frequent cause of chronic diarrhea is Small Intestinal Bacterial Overgrowth (SIBO), a condition where excessive bacteria colonize the small intestine. The slowed motility in the gut caused by PD creates an environment where bacteria can overgrow. SIBO can cause symptoms like bloating, abdominal pain, and chronic diarrhea, and it may also worsen the absorption of PD medications. The presence of SIBO is a concern because its symptoms can overlap with other GI issues, making a diagnosis difficult without specific testing.
Strategies for Managing Diarrhea
Management of diarrhea in PD begins with investigating the underlying cause, as treatment for medication side effects differs from that for SIBO or overflow diarrhea. If the cause is a medication side effect, a neurologist may adjust the dosage or explore alternative formulations. If SIBO is suspected, a gastroenterologist may perform a glucose hydrogen breath test and prescribe antibiotics to reduce bacterial overgrowth.
Dietary modifications can also help regulate bowel movements, though management is individualized. While fiber is often recommended for constipation, patients with diarrhea should be cautious, as too much fiber can worsen symptoms. Maintaining adequate hydration counters the fluid loss that occurs with persistent diarrhea. Patients should consult their healthcare provider before using over-the-counter anti-diarrheal medications, as these can be detrimental if the diarrhea is caused by severe constipation or SIBO.