Axonopathy is damage to the axons of nerve cells, the long, cable-like extensions that carry electrical signals between the brain, spinal cord, and the rest of the body. Unlike conditions that attack the protective myelin sheath wrapped around nerves, axonopathy strikes the wiring itself. It can arise from diabetes, chemotherapy drugs, inherited genetic conditions, autoimmune disease, and toxic exposures, among other triggers. The resulting symptoms follow a characteristic pattern, typically starting in the longest nerves first, which means numbness or tingling in the feet and fingertips before anything else. Management today is largely about treating the underlying cause and controlling symptoms, but a wave of research into the molecular machinery of axon destruction has opened up genuinely new therapeutic targets, some already in early clinical trials.
What Happens Inside a Degenerating Axon
An axon can be thought of as a supply line. Nutrients, signaling molecules, and structural proteins travel continuously from the nerve cell body down to the axon’s tip and back again. When that supply line breaks down, the axon starves. Disruption of this internal transport system shows up early in neurodegenerative diseases and plays a central role in axon loss.1PubMed Central. Disruption of axonal transport in neurodegeneration In diabetic neuropathy specifically, high blood sugar slows the movement of cargo along the axon, reducing the delivery of structural proteins like neurofilaments and tubulin to distant nerve endings and weakening the nerve’s ability to repair after injury.2PubMed Central. Axonal Transport Failure as a Cellular Mechanism of Diabetic Neuropathy
Mitochondria, the tiny energy factories inside cells, are another early casualty. Because axons can stretch up to a meter long in humans, they depend heavily on local energy production. When mitochondria malfunction, the axon faces both an energy crisis and a buildup of damaging reactive oxygen molecules. This mitochondrial failure involves oxidative stress, energy deficiency, and problems with the transport and recycling of the mitochondria themselves.3PubMed Central. Mitochondrial Behavior in Axon Degeneration and Regeneration In progressive multiple sclerosis, for instance, deficiencies in the mitochondrial respiratory chain reduce a neuron’s ability to generate ATP, creating an energy shortfall that hits the longest nerve tracts hardest.4PubMed. Mitochondrial dysfunction and axon degeneration in progressive multiple sclerosis
Supporting glial cells also matter. In both the brain and in peripheral nerves, glial cells provide metabolic support and insulation to axons. When glial support is lost, progressive axon degeneration follows, sometimes accompanied by local inflammation.5PubMed. Axon-glial signaling and the glial support of axon function
The SARM1 Pathway and the “Self-Destruct” Program
One of the most important discoveries in recent neuroscience is that axon degeneration is not simply passive decay. It is an active self-destruction program, and a protein called SARM1 sits at its center. After an axon is injured, SARM1 triggers a rapid breakdown of NAD+, a molecule essential for energy metabolism. Once NAD+ levels collapse, the axon degenerates.6PubMed Central. SARM1 activation triggers axon degeneration locally via NAD⁺ destruction Researchers have shown that this destruction is local, happening right at the injury site rather than requiring signals from the cell body. And critically, if you can increase NAD+ production or block SARM1’s enzymatic activity, you can counteract axon destruction.
Follow-up work confirmed that SARM1’s TIR domain itself acts as an NAD+-cleaving enzyme. It chops NAD+ into smaller fragments, and one of those fragments, nicotinamide, acts as a natural brake on the enzyme, a feedback loop that normally limits the damage. In experiments with traumatic nerve injury and vincristine-induced injury (vincristine is a chemotherapy drug known to damage nerves), blocking SARM1’s enzymatic activity in the axon prevented both NAD+ depletion and degeneration.7Neuron. The SARM1 TIR Domain Is an NAD+ Cleavage Enzyme that Promotes Axonal Degeneration More recent research has shown SARM1 activation involves a two-step phase transition that unleashes its destructive activity.8PubMed. SARM1 activation promotes axonal degeneration via a two-step phase transition
This understanding has shifted how scientists think about axonopathy. Rather than a passive consequence of injury, axon loss in many conditions looks more like a triggered demolition. That reframing matters because a demolition program can, in principle, be interrupted.
Major Causes of Axonopathy
The list of things that can damage axons is long, but a handful of causes account for the vast majority of cases seen in clinical practice.
Diabetes
Diabetic neuropathy is the most common form of axonopathy worldwide. Chronic high blood sugar, abnormal lipid levels, and impaired insulin signaling converge to create oxidative and inflammatory stress that injures sensory neurons, the Schwann cells that support peripheral nerves, and the tiny blood vessels feeding those nerves.9PubMed Central. Diabetic neuropathy: cellular mechanisms as therapeutic targets Excess glucose drives several damaging pathways, principally through the polyol pathway and the formation of advanced glycation end-products, both of which generate oxidative stress. Downstream effects include disordered nerve conduction, sensory loss, and progressive axonopathy.10PubMed. Diabetic neuropathies: components of etiology The longest axons are hit first and hardest, which is why diabetic neuropathy classically begins with foot numbness.
Chemotherapy
Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect of cancer treatment. Six main classes of drugs are known to damage peripheral sensory, motor, and autonomic neurons: platinum-based agents (like cisplatin), vinca alkaloids, epothilones, taxanes, proteasome inhibitors (like bortezomib), and immunomodulatory drugs (like thalidomide).11PubMed Central. Mechanisms of Chemotherapy-Induced Peripheral Neuropathy Each class damages axons through slightly different mechanisms, but the common thread is disruption of the axon’s internal transport machinery and mitochondrial function. For many patients, the neuropathy persists long after chemotherapy ends.
Inherited Conditions
Charcot-Marie-Tooth disease (CMT) is the most common inherited neuropathy. It comes in two broad forms: CMT1, which primarily damages the myelin sheath, and CMT2, the axonal form, which directly affects the axons. Mutations in more than 17 genes and 25 chromosomal locations have been linked to CMT.12PubMed Central. Charcot-marie-tooth disease: seventeen causative genes Some of these mutations affect proteins involved in axonal transport itself. For example, mutations in the KIF5A gene, which encodes a motor protein that carries cargo along axons, have been found in patients with the axonal form of CMT as well as in hereditary spastic paraplegia.13PubMed. Mutations in the motor and stalk domains of KIF5A in spastic paraplegia type 10 and in axonal Charcot-Marie-Tooth type 2 Current care for inherited neuropathies centers on supportive therapies aimed at preserving range of motion, strength, and balance.14PubMed Central. Inherited Peripheral Neuropathies
Autoimmune and Inflammatory Conditions
Autoimmune diseases like lupus, rheumatoid arthritis, and systemic vasculitis can produce immune-mediated axonal neuropathies. The mechanisms vary. In vasculitis, inflammation of the blood vessels feeding nerves (the vasa nervorum) starves axons of oxygen. In other autoimmune conditions, antibodies or inflammatory mediators attack the nerves more directly.15PubMed Central. Immune Axonal Neuropathies Associated With Systemic Autoimmune Rheumatic Diseases Guillain-Barré syndrome also has an axonal variant, called acute motor axonal neuropathy (AMAN), which can cause rapid and severe weakness.
Toxins and Medications
Beyond chemotherapy, many other medications and environmental toxins can damage peripheral axons. Alcohol abuse is one of the most common toxic causes. Heavy metals, certain industrial solvents, and some antiretroviral and antibiotic drugs round out the list. The clinical features and electrodiagnostic patterns differ somewhat between toxins, but the fundamental process is the same: disruption of axonal metabolism or transport leading to a length-dependent dying-back pattern.
The “Dying Back” Pattern and ALS
A hallmark of axonopathy is that damage starts at the farthest tips of the axon and progresses backward toward the cell body. Researchers sometimes call this “dying back” degeneration, and it explains why the longest nerves suffer first. The pattern appears in conditions ranging from diabetic neuropathy to amyotrophic lateral sclerosis (ALS). In a widely studied mouse model of ALS, about 40% of the neuromuscular junctions at the axon tips were denervated by day 47, yet there was no evidence of motor neuron cell body loss at that stage. By day 80, roughly 60% of the motor nerve fibers in the ventral root were gone, but the motor neurons themselves were still intact. Motor neuron death did not get underway until around day 100. Inflammatory activity around the motor neurons was not detectable until after distal axon degeneration had already begun.16PubMed. Amyotrophic lateral sclerosis is a distal axonopathy: evidence in mice and man An autopsy of a human ALS patient in the same study confirmed denervation changes in muscle alongside normal-appearing motor neurons. The implication is striking: in ALS, the problem may start in the axons long before the motor neurons themselves are visibly sick.
Symptoms and How They Present
Because axonopathy follows a length-dependent pattern in most cases, the earliest symptoms tend to be sensory: numbness, tingling, burning, or prickling in the toes and soles of the feet. As damage advances, it creeps up the legs and begins to affect the fingertips. Some people describe it as feeling like they are wearing invisible socks or gloves. Pain can be a prominent feature, especially in diabetic and chemotherapy-induced neuropathies, and it often worsens at night.
Motor symptoms come later or are more prominent in certain subtypes. Weakness typically begins in the muscles of the feet and lower legs, leading to foot drop, difficulty walking, and frequent tripping. In severe cases like AMAN, one of the Guillain-Barré variants, profound limb weakness can develop rapidly over days.
Autonomic involvement is less recognized but can be disabling. Damage to the small axons that control blood pressure regulation, sweating, bladder function, and heart rate can produce symptoms like dizziness on standing (orthostatic hypotension), abnormal sweating patterns, and urinary problems. One case report described a patient with axonal neuropathy involving both myelinated and unmyelinated fibers who presented with severe orthostatic hypotension, neurogenic bladder, seizures, and sensory and motor deficits simultaneously.17PubMed. Acute autonomic sensory and motor neuropathy associated with central nervous system disturbance While that case was unusually severe, it illustrates that axonopathy is not limited to sensation and strength; autonomic nerves are vulnerable too.
Diagnosing Axonopathy
The first clue usually comes from the clinical exam: a stocking-and-glove pattern of sensory loss, reduced or absent reflexes at the ankles, and possibly weakness in the feet. But confirming that the problem is axonal (rather than demyelinating) requires additional testing.
Nerve conduction studies (NCS) and electromyography (EMG) are the standard diagnostic tools. In axonal neuropathies, the key finding is reduced amplitude of the nerve’s electrical response, reflecting a loss of functioning axons, while the conduction velocity (the speed at which signals travel) is relatively preserved or only mildly slowed. This contrasts with demyelinating neuropathies, where conduction velocity drops substantially because the insulating myelin is damaged. A study recording from both proximal and distal muscles found that in axonal neuropathies, conduction velocities were preferentially slowed when measured at distal sites but relatively normal at proximal sites, while demyelinating neuropathies showed marked slowing at both.18PubMed. Differentiation between axonal and demyelinating neuropathies: identical segments recorded from proximal and distal muscles There is also a linear correlation between action potential amplitude and conduction velocity in both types, but the degree of abnormality tends to be more pronounced in demyelinating cases.19PubMed. Correlations of nerve conduction measures in axonal and demyelinating polyneuropathies
Nerve ultrasound has emerged as a complementary tool. By measuring the cross-sectional area of nerves at various sites, ultrasound can help distinguish axonal from demyelinating patterns, with significant differences observed between the two groups in motor conduction velocity, electrical response amplitude, and nerve size.20PubMed. Ultrasound differentiation of axonal and demyelinating neuropathies
Blood biomarkers are also gaining ground. Neurofilament light chain (NfL) is a structural protein released from damaged axons into the cerebrospinal fluid and bloodstream. Elevated NfL levels have been found across a range of neurological conditions involving axonal damage and can be detected in standard blood samples.21PubMed Central. Neurofilament Light Chain as a Biomarker, and Correlation with Magnetic Resonance Imaging in Diagnosis of CNS-Related Disorders NfL is not specific to any one disease, but it is useful for tracking the overall burden of axonal injury and for monitoring whether a treatment is protecting axons over time.
Why Peripheral Nerves Recover Better Than the Brain and Spinal Cord
When an axon is damaged in the peripheral nervous system, the portion beyond the injury site degenerates through a process historically called Wallerian degeneration, first described in frogs in the 1850s. But in peripheral nerves, the story does not end there. Schwann cells, the glial cells of the peripheral nervous system, clear away the debris and lay down a supportive tube that guides the regrowing axon back toward its target. Long-distance regeneration and substantial functional recovery can occur.22PubMed Central. Axon Regeneration in the Peripheral and Central Nervous Systems
The central nervous system is a different story. After spinal cord injury, traumatic brain injury, or stroke, axon regeneration is extremely limited. A major reason is the contrasting behavior of the glial cells. In the peripheral nervous system, Schwann cells actively promote regrowth. In the central nervous system, oligodendrocytes and other cells create an environment that actively inhibits regeneration.23PubMed. Contrasting the glial response to axon injury in the central and peripheral nervous systems The injured CNS neuron itself also mounts a weaker regenerative response compared to its peripheral counterpart.24PubMed. Canadian Association of Neuroscience review: axonal regeneration in the peripheral and central nervous systems–current issues and advances
This difference has enormous practical implications. A peripheral axonopathy caused by a reversible insult, such as a drug that can be discontinued or blood sugar that can be controlled, has a real chance of at least partial recovery. Axonopathy in the brain or spinal cord, as seen in progressive multiple sclerosis or after traumatic injury, tends to produce permanent deficits. The regeneration gap is one of the biggest unsolved problems in neuroscience.
Current Management Approaches
Treating axonopathy today centers on three pillars: addressing the underlying cause, managing symptoms, and preserving physical function.
For diabetic neuropathy, tight blood sugar control is the single most important intervention. It slows progression and, in some patients with early disease, allows partial nerve recovery. For chemotherapy-induced neuropathy, dose reduction or switching agents may be necessary, though this involves difficult trade-offs with cancer treatment efficacy. For autoimmune axonopathies, immunosuppressive therapy targeting the underlying inflammatory disease can halt or slow nerve damage.
Symptom management focuses heavily on neuropathic pain, which can be one of the most debilitating features. Gabapentin and pregabalin are first-line medications that work by dampening abnormal nerve signaling. Tricyclic antidepressants like amitriptyline are also used, though evidence for their effectiveness as sole agents in chemotherapy-induced neuropathy has been mixed. In a pediatric case of refractory chemotherapy-induced neuropathy, combining high-dose gabapentin with amitriptyline produced meaningful improvement in pain scores, sleep, and activity tolerance when neither drug alone was sufficient.25PubMed Central. High-Dose Gabapentin and Amitriptyline in the Treatment of Refractory Chemotherapy-Induced Peripheral Neuropathy in a Toddler Duloxetine, another antidepressant, has the strongest evidence base for chemotherapy-induced neuropathic pain specifically. Topical lidocaine and capsaicin patches offer localized relief for some patients.
Physical rehabilitation plays an underappreciated role. For patients with severe axonal neuropathy affecting walking, structured gait training using lower-limb orthoses and body-weight-supported walkers has shown that patients can safely increase walking distance even with severe weakness.26Japanese Journal of Comprehensive Rehabilitation Science. Practice of gait training using lower-limb orthosis and body weight-supported walker for severe acute motor axonal neuropathy: a case report Balance training, ankle-foot orthoses to correct foot drop, and occupational therapy to maintain hand function all contribute to quality of life, particularly in inherited neuropathies where the disease is lifelong.
SARM1 Inhibitors and the Push Toward Neuroprotection
The discovery that SARM1 is a druggable enzyme with a clear role in axon destruction has created genuine excitement in the neuropathy field. Over the past decade, several pharmacological strategies to block SARM1 have been developed, including small-molecule inhibitors, gene therapy approaches, and antisense oligonucleotides. The most advanced approach uses a clever class of compounds called base-exchange inhibitors. These are essentially “pro-inhibitors” that SARM1 itself converts into active blocking agents, which then jam the enzyme’s active site. In laboratory experiments, these inhibitors provided striking protection of axons after nerve cutting and exposure to neurotoxins in both mouse and human neurons, as well as in mouse models of chemotherapy-induced neuropathy.27Trends in Pharmacological Sciences. Axonopathy: Causes, Symptoms, and Management
In 2024, a company developing one of these base-exchange inhibitors announced successful completion of Phase 1 clinical trials for a brain-penetrant SARM1 inhibitor, a significant milestone toward clinical application.27Trends in Pharmacological Sciences. Axonopathy: Causes, Symptoms, and Management Phase 1 trials test safety and dosing in healthy volunteers, so it remains to be seen whether these compounds will prove effective in patients with neuropathy. But the pipeline is real, and additional trials are underway.
Beyond neuropathy, SARM1 inhibition has shown promise in other contexts. In mouse models of viral encephalitis, deleting or inhibiting SARM1 preserved axonal structure, maintained mitochondrial health, reduced immune-cell infiltration in the brain, and improved survival.28PubMed Central. Targeting SARM1 as a novel neuroprotective therapy in neurotropic viral infections If SARM1 inhibition proves safe in humans, its potential applications could extend well beyond peripheral neuropathy to neurodegenerative diseases, traumatic brain injury, and viral infections that damage the nervous system. The field is moving from understanding the demolition program to figuring out how to disarm it.
Aging and Axonal Vulnerability
Even without a specific disease, aging itself erodes the resilience of axons. Research in mice lacking a protein called SIRT2, which plays a role in metabolic regulation in the nervous system, found that loss of this protein led to axonal degeneration, locomotor disability, mitochondrial depletion, energy failure, and oxidative imbalance.29PubMed Central. Loss of SIRT2 leads to axonal degeneration and locomotor disability associated with redox and energy imbalance This provides a molecular link between normal aging processes and the kind of axonal decline that can shade into clinical neuropathy. For older adults, the gradual loss of axonal integrity may explain why sensory thresholds rise, reflexes slow, and balance deteriorates even in the absence of diabetes or other recognized causes. When a diagnosed axonopathy is layered on top of age-related axonal attrition, the functional impact is compounded, which is partly why diabetic neuropathy tends to be more disabling in older patients than in younger ones with the same degree of metabolic derangement.