Intrinsic limits on axon regrowth

In established diabetic neuropathy, neuronal growth restraint prevents adequate replacement of lost peptidergic terminals, and relieving that restraint would materially reduce continuing net denervation.

Scope: Repair/progression exception: adults with established type 2 DPN and surviving parent neurons. Materiality means at least 20% less five-year net terminal loss in a describable subgroup comprising at least 10% of this established-DPN population. Prevention before onset remains a further extrapolation.

Causal role: failure of replacement rather than necessarily increased destruction. Unless the scope specifies an exception, materiality means at least 20% less five-year net terminal loss in an identifiable subgroup representing at least 10% of incident DPN cases in the stated population. The target remains human peptidergic C-fibers when a study measures a broader endpoint.

Source assessment

Base judgment: 19.0%; skeptical–optimistic sensitivity: 1.6%–55.3%. Subjective probabilities of the causal proposition below, not effect sizes. Decimal places only show arithmetic. Reviewed 13 September 2026. Probability method · Collection index.

Reasoning from the biological requirements

Terminal abundance changes by additions minus losses. Even an ordinary rate of terminal damage produces progressive denervation if replacement cannot keep up. A growth brake need not be abnormally expressed: a normal restraint can become consequential when replacement demand rises. However, pharmacologically forcing growth does not show that an intrinsic brake was the natural bottleneck. New branches must persist and become useful terminals; brief sprouting or altered staining is insufficient. M1 muscarinic and PTEN-related signaling are candidate branches, not jointly necessary links.

Probabilities of the necessary claims

Conditional prices are skeptical / base / optimistic. Each price after the first assumes all earlier requirements are true. Their order is an order for assessing joint belief, not necessarily a linear biological pathway. The cumulative column uses the base scenario. Temporal order, connected compartments and the causal prevention comparison are included in these requirements, without extra generic multipliers.

See the claims and probabilities table above.

R1

Human structural treatment data support some retained capacity; net shortage alone does not identify the brake.

R2

Animal neuronal manipulations support cell autonomy; topical human treatment does not identify its cellular site.

R3

A structural count at one follow-up is stronger than neurite outgrowth but does not establish durability or normal function.

R4

Ongoing destruction may erase growth; effect size, persistence and relevant subtype remain uncertain.

Evidence that moves the judgment

• Selective M1 antagonism and genetic manipulation increased sensory-neuron outgrowth and protected peripheral nerve indices in experimental neuropathies, with mitochondrial/AMPK-related effects. The findings support a neuronal growth-control route but do not prove pathological M1 overactivity in human diabetes. Calcutt et al., 2017.

• The newer pirenzepine study randomized 58 people with established type 2 DPN; 57 entered modified intention-to-treat analysis. At 24 weeks, treated-ankle IENFD changed by +1.8 fibers/mm in pooled treatment versus −0.6 with placebo, with baseline-adjusted between-group P=.012. Other efficacy outcomes did not differ in that analysis. This is a human structural signal, without exact peptidergic identification, cellular localization or five-year persistence. Sivadasan et al., published online 2025.

• The oxybutynin trial randomized 51 participants and reported within-arm IENFD improvement. The repository's reconstruction of 37 readable primary biopsy pairs gives a treatment-minus-placebo change of +0.833 fibers/mm, 95% CI −0.723 to +2.390, P=.282; a secondary sprouting contrast remains provisional. A significant change in one arm does not establish superiority. Primary paper; public-figure reanalysis (background note outside this collection).

• Combined PTEN knockdown and local insulin promoted repair in a diabetic experimental setting, but a combined intervention cannot identify which natural deficit was limiting. 2023 primary study.

Why these odds and how uncertain they are

The pirenzepine randomized structural contrast makes retained human repair capacity considerably more credible than culture outgrowth alone. I do not price the full causal hypothesis nearly as highly, because the positive trial neither identifies the cellular site nor demonstrates that a growth brake caused the original loss. The principal uncertain link is neuronal localization. A broad proposition that local treatment can increase some epidermal fibers is easier than this specific intrinsic, durable, peptidergic mechanism.

The skeptical scenario puts the treatment effect in skin/support cells or short-lived growth that does not offset continuing injury. The optimistic scenario interprets the human structural signal as lifting a neuronal maintenance restraint with durable replacement. Programme-internal convergence is not independent replication; the repository's later replication search (background note outside this collection) found no publicly reported efficacy contrast from the separate completed trial. The dynamic-trial analysis (background note outside this collection) also could not identify sprout maturation or treatment compartment from the public observations.

Conditional inputs: base [75, 60, 65, 65]%; skeptical [50, 30, 35, 30]%; optimistic [90, 85, 85, 85]%. Products give the headline prices. The span measures sensitivity to these interpretations; it is not a statistical confidence interval.

Numerical update example: Public human data separating a neuronal target effect from skin/glial exposure would raise requirement 2; credible localization entirely to extrinsic support would lower this intrinsic claim while potentially raising hypothesis 32. Changing only requirement 2 from 60% to 85% raises the whole price to 26.9%; reducing it to 20% lowers the price to 6.3%. These are hypothetical evidence updates. Even resolving that one requirement with certainty would leave a complete price of 31.7% if the others stayed unchanged.

Overlap and boundary

Complements the extrinsic skin repair environment and trophic support hypotheses. Energy and transport can constrain the growth response. The probability of this causal account is different from the probability that pirenzepine has some treatment benefit.