Areflexia is the complete absence of deep tendon reflexes, those involuntary muscle jerks a doctor triggers by tapping your knee, ankle, or elbow with a small rubber hammer. On the standard clinical scale used worldwide, it corresponds to a grade of 0, meaning no reflex response at all, even with special techniques to amplify weak signals.1PubMed. Physiology, Deep Tendon Reflexes While a sluggish reflex can be a minor finding, a truly absent one points to a break somewhere in the nerve circuit that loops from muscle to spinal cord and back. The list of things that can cause that break is surprisingly long, ranging from autoimmune attacks and inherited conditions to nutritional gaps and medication side effects.
How Reflexes Are Tested and What the Numbers Mean
When a clinician swings a reflex hammer against the tendon just below your kneecap, the stretch travels up a sensory nerve fiber to the spinal cord, triggers a motor nerve response, and snaps the quadriceps muscle into a brief contraction. That whole loop takes a fraction of a second and involves no input from the brain. The National Institute of Neurological Disorders and Stroke (NINDS) grades the response on a 0-to-4 scale: 0 means no reflex at all (areflexia), 1 is a faint or trace response, 2 and 3 represent the normal range, and 4 is hyperactive with possible clonus, where the joint bounces rhythmically.2PubMed. Physiology, Deep Tendon Reflexes Clinicians check multiple sites, commonly the biceps, triceps, brachioradialis, patellar, and Achilles tendons, because the pattern of which reflexes are missing gives clues about where the problem is. For example, reflexes absent only at the ankle suggest damage to a nerve segment lower in the spinal cord or in the peripheral nerve that serves the foot, while widespread absence across both arms and legs suggests a more diffuse process.3PubMed Central. Deep Tendon Reflex: The Tools and Techniques. What Surgical Neurology Residents Should Know.
Technique matters more than people realize. If you are cold, anxious, or tensing up, a reflex can look absent when it is actually just suppressed. Clinicians use a maneuver called the Jendrassik technique, asking you to clench your teeth or hook your fingers together and pull, to distract higher brain centers and “unmask” a faint reflex. A grade-0 finding is only considered genuine areflexia if it persists even with reinforcement. This distinction matters because labeling a reflex as absent can change the diagnostic path dramatically.
Guillain-Barré Syndrome and Other Acute Causes
Guillain-Barré syndrome (GBS) is probably the condition most closely associated with sudden-onset areflexia. It is an autoimmune attack on the peripheral nerves that typically starts a week or two after an infection. The immune system mistakes components of the nerve coating for foreign invaders, and the resulting inflammation strips away the myelin insulation that allows nerve signals to travel quickly. Reflexes vanish early, often before muscle weakness becomes severe, which is why absent reflexes in someone with recent tingling and progressive weakness raise an immediate red flag for GBS.
A less well-known variant, Miller Fisher syndrome, produces a distinctive triad: loss of reflexes, loss of coordination (ataxia), and paralysis of the eye muscles. A review of 243 reported cases found that the eye muscle paralysis was remarkably symmetrical at all stages and consistently appeared alongside cerebellar-type ataxia.4Acta Neurologica Scandinavica / Wiley Online Library. Neuro-ophthalmic manifestations of the syndrome of ophthalmoplegia, ataxia and areflexia: a review Miller Fisher syndrome is rarer than classic GBS but tends to have a better prognosis, with most people recovering fully over several months.
Spinal shock is another acute scenario. When the spinal cord is suddenly severed or severely injured, everything below the level of damage shuts down, including reflexes. All fundamental spinal cord functions below the injury are immediately depressed.5PubMed Central. Revisit Spinal Shock: Pattern of Reflex Evolution during Spinal Shock This initial phase of complete areflexia can last days to weeks. Over time, reflexes often return, sometimes becoming exaggerated rather than normal, because the spinal cord circuits below the injury lose their regulatory input from the brain.
Chronic and Slowly Progressive Causes
Not every case of areflexia arrives suddenly. Chronic inflammatory demyelinating polyneuropathy (CIDP) is sometimes described as the slow-motion cousin of Guillain-Barr̩. It develops over months rather than days, producing gradually worsening, fairly symmetric muscle weakness with absent or depressed deep tendon reflexes and sensory loss.6PubMed Central. Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): Overview, Treatment, and a Case Study РSection: Introduction Because the onset is insidious, some people live with it for months before anyone checks their reflexes and discovers they are gone. Treatment with immunoglobulins or corticosteroids can stabilize or improve the condition, but it often requires long-term management.
Diabetic peripheral neuropathy is far more common and often chips away at reflexes starting at the ankles. A study that stratified patients by reflex loss found that reduced Achilles reflexes alone had a sensitivity above 90% for detecting neuropathy when measured against nerve conduction studies.7PubMed Central. Diagnostic value of clinical deep tendon reflexes in diabetic peripheral neuropathy In other words, checking the ankle jerk is a surprisingly powerful screening tool. As neuropathy advances, reflex loss climbs from the feet to the knees and eventually to the hands and arms, following the classic “stocking and glove” pattern of nerve damage.
Hereditary conditions also play a role. Charcot-Marie-Tooth disease (CMT) is the most common inherited neuropathy, affecting roughly 1 in 2,500 people. It progressively damages peripheral nerves, leading to muscle wasting in the lower legs and feet. Areflexia develops gradually as nerve conduction slows. Parents sometimes first notice something is off when a child has trouble running or frequently trips, and a reflex exam turns up absent ankle jerks years before genetic testing confirms the diagnosis.
Medications and Toxins That Abolish Reflexes
Several drugs can damage peripheral nerves badly enough to eliminate reflexes. Vincristine, a chemotherapy agent commonly used in blood cancers and some solid tumors, is a well-known culprit. It works by disrupting cell division, but it also disrupts axonal transport inside nerve cells, causing numbness, pain, and tactile disturbances in the hands and feet.8PubMed Central. Vincristine-induced peripheral neuropathy: A mini-review Reflex loss from vincristine is one of the earliest objective signs that nerve damage is developing, and oncologists monitor reflexes during treatment to decide whether to adjust the dose.
Tick paralysis is a more unusual cause and one that catches people off guard because it has nothing to do with nerve inflammation or degeneration. Certain tick species secrete a neurotoxin in their saliva that blocks nerve conduction at the neuromuscular junction. The result is an ascending flaccid paralysis, meaning weakness starts in the legs and climbs upward, with reflexes dropping out along the way.9PubMed Central. Tick paralysis The remarkable thing about tick paralysis is that removing the tick usually leads to rapid and complete recovery, sometimes within hours. It is most common in children and is occasionally misdiagnosed as GBS because the clinical picture looks similar until someone finds the tick hidden in the scalp or behind an ear.
Nutritional Deficiencies That Damage Nerve Circuits
Vitamin deficiencies can mimic more dramatic neurological diseases. Vitamin E deficiency causes a neurological syndrome marked by progressive loss of coordination, impaired proprioception and vibration sense, and areflexia.10PubMed Central. Vitamin E Deficiency: An Under-Recognized Cause of Dystonia and Ataxia Syndrome. Because vitamin E is fat-soluble, deficiency is most common in people who have trouble absorbing fat, such as those with celiac disease, cystic fibrosis, or chronic liver disease. It can also occur in rare genetic conditions that affect vitamin E transport. The nerve damage from vitamin E deficiency is partially reversible if caught early, but prolonged deficiency can leave permanent deficits.
Vitamin B12 deficiency is another well-known nerve toxin, although its classic neurological presentation tends to involve the spinal cord (subacute combined degeneration) more than the peripheral nerves alone. Still, some patients present with absent reflexes and peripheral neuropathy, and case reports describe B12 deficiency coexisting with other conditions that affect reflexes, making the picture harder to untangle.11PubMed Central. Holmes-Adie syndrome and vitamin B12 – associated peripheral neuropathy: An association or coincidence? The practical takeaway is that when reflexes are absent and no obvious neurological disease explains it, checking nutrient levels is a reasonable and inexpensive step.
Areflexia in Infants and Children
In a newborn or infant, absent reflexes carry a different set of implications than in adults. Spinal muscular atrophy (SMA) is one of the most serious causes. It is a progressive neuromuscular condition usually caused by the homozygous absence of the SMN1 gene, leading to progressive muscle weakness, muscle wasting, low muscle tone, and absent or markedly decreased deep tendon reflexes.12International Journal of Contemporary Pediatrics. Spinal motor atrophy in a floppy infant: a case report The “floppy baby” presentation, where an infant lies in a limp, frog-legged posture with no reflexes, is a classic scenario that prompts urgent genetic testing. Over the past several years, new gene therapies and antisense oligonucleotide treatments have transformed the outlook for children with SMA, but only when started early, which makes reflex testing in infants an unexpectedly high-stakes clinical exercise.
It is worth noting that very young infants normally have certain reflexes that disappear over the first year (the Moro reflex, for instance, which is a startle response, not a tendon reflex). The absence of primitive reflexes is normal at certain developmental stages. What raises concern in infants is the absence of deep tendon reflexes combined with low muscle tone and poor movement, a combination that indicates motor neuron or peripheral nerve disease rather than a normal developmental transition.
Conditions Where Areflexia Is Just One Piece of the Puzzle
Some syndromes are practically defined by areflexia in combination with other specific neurological findings. CANVAS (cerebellar ataxia, neuropathy, and vestibular areflexia syndrome) is a prime example. Patients develop unsteady gait from cerebellar damage, numbness and reflex loss from peripheral neuropathy, and dizziness with balance trouble from vestibular nerve damage, all rolled into one progressive disorder. In 2019, researchers identified its genetic basis: a biallelic repeat expansion in the RFC1 gene.13PubMed Central. Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS): from clinical diagnosis towards genetic testing – Section: Molecular genetics That discovery was a breakthrough because CANVAS had been described clinically for years but couldn’t be confirmed with a lab test. Now genetic testing can distinguish it from other conditions that look similar, like certain forms of spinocerebellar ataxia.
Holmes-Adie syndrome is another quirky example. It typically presents with a dilated pupil in one eye that reacts sluggishly to light, paired with absent deep tendon reflexes. One reported case involved a 30-year-old man who came in complaining of blurry vision and was found to have a dilated pupil in the right eye that constricted when given dilute pilocarpine drops, a hallmark pharmacological test for the condition, along with absent reflexes throughout the body.14PubMed Central. Holmes-Adie syndrome and vitamin B12 – associated peripheral neuropathy: An association or coincidence? Holmes-Adie syndrome is benign in the sense that it doesn’t progress to serious disability, but it demonstrates how areflexia can show up in unexpected clinical combinations.
Can Lost Reflexes Come Back?
Whether reflexes recover depends entirely on the type and severity of the nerve injury. In animal studies of nerve crush injuries, where the nerve is damaged but not completely severed, roughly 70% of the sensory nerve fibers that drive the stretch reflex recovered function. Stretch reflex force in muscles supplied by crushed nerves actually exceeded that of the opposite, uninjured side, demonstrating a capacity for complete and even supranormal reflex recovery after crush-type injuries.15PubMed Central. Recovery of proprioceptive feedback from nerve crush That stands in sharp contrast to nerves that are completely cut: in those cases, reflex recovery is essentially absent, because regenerating axons struggle to reconnect with their original targets.
Research on nerve repair using tube grafts to bridge a gap showed that while reflexes mediated by myelinated (fast-conducting) nerve fibers recovered, sometimes to amplitudes 150 to 245% of their baseline values, reflexes dependent on unmyelinated (slow-conducting) fibers recovered much less reliably.16PubMed. Changes in crossed spinal reflexes after peripheral nerve injury and repair This mismatch matters clinically because it means the reflexes you can see at the bedside (which depend on fast-conducting fibers) may return even when more subtle nerve functions remain impaired.
For conditions like GBS, most patients recover reflexes within months as inflammation resolves and myelin regrows, though a subset are left with persistent areflexia. In CIDP, reflexes can improve with treatment but tend to fluctuate with disease activity. And in hereditary conditions like CMT or SMA, reflex loss is generally permanent because the underlying nerve or motor neuron damage is progressive and genetic. The spinal shock scenario is a special case: reflexes below the injury level typically return over weeks to months, but they often become exaggerated rather than normal, because the circuits are freed from brain-level inhibition.
How Deep Tendon Reflexes Became Central to Neurology
Given how simple the knee-jerk test looks from the patient’s perspective, it is easy to underestimate how much diagnostic weight it carries. Reflex testing became a cornerstone of the neurological examination in the late 19th century, when Wilhelm Erb and Carl Westphal independently described deep tendon reflexes and recognized their clinical significance.17PubMed. Erb and Westphal: simultaneous discovery of the deep tendon reflexes By the early 20th century, the reflex exam was already one of the cardinal features of neurological assessment. What makes it enduringly useful is that it provides an objective, instantaneous readout of nerve function that requires no equipment beyond a small hammer and no cooperation beyond sitting still. In an era of MRIs and genetic panels, the reflex hammer remains one of the most efficient screening tools in medicine. Absent reflexes do not give a diagnosis on their own, but they dramatically narrow the list of possibilities and guide clinicians toward the right tests.
The reflex exam’s staying power also comes from its ability to separate upper motor neuron disease (brain and spinal cord problems, where reflexes tend to be hyperactive) from lower motor neuron disease (peripheral nerve problems, where reflexes are diminished or absent). That single distinction, which takes less than a minute to establish, shapes the entire diagnostic approach. It is one of the few clinical tests where a negative finding, the complete absence of a response, is just as informative as an abnormal positive finding.