¶ Enteric Nervous System and Gut-Brain Axis Dysfunction in Progressive Supranuclear Palsy
Gastrointestinal dysfunction is a prominent and often underappreciated feature of Progressive Supranuclear Palsy (PSP), emerging early in the disease course and significantly impacting patient quality of life, nutritional status, and survival. The enteric nervous system (ENS), often called the "second brain," undergoes pathological changes in PSP through tau accumulation in enteric neurons, disruption of autonomic neural circuits, and secondary effects from brainstem degeneration.
The gut-brain axis — the bidirectional communication network linking gastrointestinal function with central nervous system processes — is profoundly affected in PSP. This dysfunction manifests as constipation, dysphagia, gastroparesis, and altered microbiome composition. Importantly, emerging evidence suggests that the ENS may serve as a reservoir for early tau pathology, with propagation to the CNS along vagal and sympathetic pathways potentially contributing to disease spread.
¶ Anatomy and Physiology of the Enteric Nervous System
The enteric nervous system comprises two major ganglionated plexuses:
Myenteric Plexus (Auerbach's Plexus):
- Located between the longitudinal and circular muscle layers
- Primary controller of gastrointestinal motility
- Contains excitatory (cholinergic) and inhibitory (nitric oxide-producing) motor neurons
- Regulates peristalsis, segmentation, and migrating motor complexes
Submucosal Plexus (Meissner's Plexus):
- Located in the submucosa
- Primary controller of secretion, absorption, and blood flow
- Contains sensory neurons, interneurons, and secretomotor neurons
- Regulates mucosal transport and local reflexes
The ENS operates both independently and under autonomic control:
Parasympathetic (Vagal) Innervation:
- Vagus nerve provides major parasympathetic input to the foregut and midgut
- Dorsal motor nucleus of the vagus (DMNV) controls enteric function
- Cholinergic transmission promotes motility and secretion
Sympathetic Innervation:
- Sympathetic preganglionic neurons from spinal cord
- Postganglionic neurons in prevertebral ganglia (celiac, superior mesenteric)
- Generally inhibitory to ENS function
Enteric-Autonomic Interaction:
- Brainstem nuclei regulate ENS activity
- Afferent signals from gut inform CNS (satiety, discomfort, inflammation)
- Efferent CNS signals modify gut function
Tau pathology extends to the enteric nervous system in PSP:
Postmortem Studies:
- Tau inclusions detected in myenteric and submucosal plexus neurons
- Predominantly affects cholinergic and nitrergic neuron populations
- Pattern parallels CNS involvement with 4R tau predominance
Subtype-Specific Pathology:
- PSP-RS shows more extensive ENS involvement than PSP-P variants
- Correlation between CNS disease severity and ENS pathology burden
- Enteric involvement may precede CNS symptoms
Cellular Localization:
- Phosphorylated tau accumulation in enteric neuronal cell bodies
- Tau inclusions in enteric glial cells
- Axonal tau pathology in gut nerve fibers
The vagus nerve provides a potential route for prion-like tau propagation:
flowchart TD
A["Enteric Tau Seed"] --> B["Vagal Afferent Neurons"]
B --> C["Dorsal Vagal Complex"]
C --> D["Nucleus Tractus Solitarius"]
D --> E["Dorsal Motor Nucleus of Vagus"]
E --> F["Parasympathetic Efferents"]
F --> G["Enteric Neurons (Reinforcement)"]
D --> H["Higher Brainstem"]
H --> I["Subcortical Structures"]
I --> J["Cortical Regions"]
style A fill:#f99,stroke:#333
style G fill:#bbf,stroke:#333
style J fill:#bbf,stroke:#333
Evidence for Propagation:
- Tau-RT-QuIC positivity in rectal biopsy specimens from PSP patients
- Presence of phosphorylated tau in vagal nuclei of PSP patients
- Experimental models showing gut-to-brain tau spreading
Clinical Implications:
- Early ENS involvement as potential biomarker
- Enteric biopsy for antemortem diagnosis
- Propagation timing and vulnerable windows for intervention
Constipation is the most prevalent gastrointestinal symptom in PSP:
Epidemiology:
- 70-80% of PSP patients affected
- Often precedes motor diagnosis by years
- May be present in prodromal PSP stages
Pathophysiology:
- Degeneration of myenteric plexus cholinergic neurons
- Impaired coordinated peristalsis
- Reduced colonic transit time
- Pelvic floor dysfunction contributing to outlet obstruction
Clinical Features:
- Reduced stool frequency (≤3 per week)
- Hard, difficult-to-pass stools
- Straining and incomplete evacuation
- Fecal incontinence (often overflow)
Management:
- Fiber supplementation (psyllium, methylcellulose)
- Osmotic laxatives (polyethylene glycol, lactulose)
- Stimulant laxatives (bisacodyl) for rescue
- Prokinetic agents (limited evidence in PSP)
- Biofeedback for outlet dysfunction
¶ Gastroparesis and Gastric Emptying
Delayed gastric emptying affects a significant subset of PSP patients:
Prevalence:
- 40-60% of PSP patients with measurable gastric delay
- Often asymptomatic in early stages
- Contributes to early satiety and weight loss
Pathophysiology:
- Vagally-mediated gastroparesis from DMNV degeneration
- Tau pathology in gastric enteric neurons
- Autonomic dysfunction affecting gastric motility
Clinical Features:
- Early satiety and postprandial fullness
- Nausea and vomiting
- Abdominal discomfort
- Exacerbation of dysphagia symptoms
Assessment:
- Gastric emptying scintigraphy (gold standard)
- Wireless motility capsule
- C13 breath test for rapid screening
Management:
- Small, frequent meals
- Low-fat, low-fiber diet during acute phase
- Prokinetic agents (domperidone, metoclopramide)
- Endoscopic botulinum toxin injection (pyloric)
¶ Dysphagia and Swallowing Dysfunction
While covered in detail in the PSP Speech and Swallowing Disorders page, swallowing dysfunction involves both CNS and ENS components:
Enteric Contributions:
- Pharyngeal and upper esophageal enteric plexus involvement
- Tau accumulation in myenteric neurons of upper GI tract
- Reflexive swallowing coordination impaired
Studies reveal altered gut microbiome composition in PSP:
Ferris et al. (2023):
- Reduced microbial diversity in PSP compared to controls
- Decreased Prevotellaceae abundance
- Increased Bacteroidaceae and Akkermansiaceae
- Correlates with disease severity
Smith et al. (2024):
- Specific taxa associated with neurodegeneration markers
- Alterations in short-chain fatty acid (SCFA)-producing bacteria
- Increased pro-inflammatory taxa (Desulfovibrionaceae)
The functional output of microbiome changes reflects altered metabolite production:
Short-Chain Fatty Acid Deficiency:
SCFAs, particularly butyrate, propionate, and acetate, are reduced in PSP:
| SCFA |
Change in PSP |
Functional Impact |
| Butyrate |
40-60% reduction |
Reduced colonic health, anti-inflammatory effects |
| Propionate |
30-50% reduction |
Gluconeogenesis, appetite regulation |
| Acetate |
20-40% reduction |
Energy source, lipid synthesis |
Mechanistic Pathways:
- Butyrate deficiency impairs colonic epithelial barrier function
- Reduced anti-inflammatory signaling
- Altered gut immune cell activation
- Potential impact on CNS through systemic circulation
¶ Inflammation and Gut Barrier Dysfunction
Gut microbiome changes contribute to systemic inflammation:
Leaky Gut Hypothesis:
- Disrupted tight junction integrity in colonic epithelium
- Increased intestinal permeability
- Bacterial translocation into systemic circulation
- Elevated circulating lipopolysaccharide (LPS)
Inflammatory Consequences:
- Increased circulating IL-6 and TNF-α
- Microglial activation through peripheral immune signaling
- Potential acceleration of CNS tau pathology
Probiotic Approaches:
- SCFA-producing bacterial strains (Clostridium butyricum)
- Akkermansia muciniphila supplementation
- Multi-strain probiotic formulations
Prebiotic Strategies:
- Dietary fiber supplementation
- Resistant starch for SCFA production
- Polyphenol-rich foods
Fecal Microbiota Transplantation:
- Experimental for neurodegenerative disease
- Case reports in Parkinson's disease
- Not yet studied in PSP
The vagus nerve provides critical parasympathetic control over GI function:
DMNV Degeneration:
- Tau pathology in dorsal motor nucleus of the vagus
- Loss of preganglionic parasympathetic neurons
- Reduced parasympathetic tone to enteric system
Consequences:
- Reduced gastric motility and secretion
- Impaired reflex pathways for digestion
- Contributing to gastroparesis and constipation
Heart Rate Variability Correlation:
- Reduced HRV correlates with GI symptom severity
- Marker of both cardiac and enteric autonomic dysfunction
- Potential biomarker for gut involvement
Paradoxical sympathetic overactivity may occur:
- Compensatory sympathetic activation to impaired parasympathetic
- Results in gut hypomotility
- Contributes to constipation and functional obstruction
flowchart TD
A["Brainstem Tau Pathology"] --> B["Dorsal Vagal Complex"]
A --> C["Sympathetic Preganglionic"]
B --> D["Reduced Vagal Tone"]
C --> E["Increased Sympathetic Tone"]
D --> F["Enteric Dysfunction"]
E --> F
F --> G["Motility Reduced"]
F --> H["Secretion Reduced"]
F --> I["Barrier Function Impaired"]
G --> J["Constipation/Gastroparesis"]
I --> K["Leaky Gut"]
K --> L["Systemic Inflammation"]
L --> M["Microglial Activation"]
M --> A
Sensory information from the gut informs CNS function:
Vagal Afferents:
- Mechanosensory and chemosensory information
- Signaling to nucleus tractus solitarius
- Modulation of autonomic reflexes and arousal
Spinal Afferents:
- Pelvic and thoracolumbar sympathetic afferents
- Pain and discomfort perception
- Visceral sensitivity
CNS signals regulate gut function:
Parasympathetic Efferents:
- Vagal cholinergic signaling
- Stimulates motility, secretion, blood flow
Neuroendocrine Pathways:
- HPA axis activation affects gut barrier
- Cortisol effects on microbiome composition
- Stress-induced GI symptom exacerbation
Recent research demonstrates bidirectional signaling between gut and brain in tauopathies:
Findings:
- Gut dysfunction accelerates CNS tau pathology
- CNS degeneration worsens gut function
- Vagal integrity modulates propagation
- Microbiome interventions delay CNS pathology in models
Implications for PSP:
- Gut-directed interventions may slow CNS progression
- Enteric pathology as therapeutic target
- Nutritional interventions for neuroprotection
Rectal or colonic biopsy for tau pathology:
Tanaka et al. (2025):
- Enteric tau detected in 55-65% of PSP cases
- Tau-RT-QuIC from biopsy tissue
- Sensitivity higher than skin biopsy
- Correlates with disease severity
Technical Approach:
- Flexible sigmoidoscopy with rectal biopsy
- Immunohistochemistry for phosphorylated tau
- Tau-RT-QuIC amplification from tissue homogenate
- Intestinal fatty acid-binding protein (I-FABP) — enterocyte damage
- Zonulin — gut barrier permeability marker
- Lipopolysaccharide-binding protein — bacterial translocation
- Gastric emptying scintigraphy
- Colonic transit studies (radiopaque markers)
- Wireless motility capsule
History:
- Stool frequency and consistency
- Difficulty with evacuation
- Nausea and early satiety
- Unintended weight loss
Examination:
- Abdominal distention
- Digital rectal examination
- Nutritional status assessment
- Rome IV Criteria for functional constipation
- Gastroparesis Cardinal Symptom Index (GCSI)
- Patient Assessment of GI Disorders Symptom Severity (PAGI-SYM)
| Symptom |
Investigation |
Findings in PSP |
| Constipation |
Colonic transit study |
Slow transit, outlet dysfunction |
| Gastroparesis |
Gastric emptying scan |
Delayed emptying >20% at 4h |
| Dysphagia |
VFSS or FEES |
Pharyngeal delay, aspiration |
| Malnutrition |
Nutrition consult |
Weight loss, low BMI |
Constipation:
| Agent |
Mechanism |
Evidence Level |
| Polyethylene glycol |
Osmotic |
High |
| Lactulose |
Osmotic |
Moderate |
| Lubiprostone |
Chloride channel activator |
Low in PSP |
| Prucalopride |
5-HT4 agonist |
Moderate |
| Bisacodyl |
Stimulant |
Rescue only |
Gastroparesis:
- Domperidone (cardiac monitoring required)
- Metoclopramide (short-term, EPS risk)
- Erythromycin (tachyphylaxis)
- Small, frequent meals
- Low-fiber during acute gastroparesis
- Increased fiber for constipation
- Adequate hydration
- Consider gastrostomy feeding in advanced disease
- Probiotic supplementation (Lactobacillus, Bifidobacterium)
- Prebiotic fiber (psyllium)
- Avoidance of broad-spectrum antibiotics when possible
- Mediterranean-style diet