Reye's Syndrome: 15 Symptoms Worth Looking Out For

Reye’s syndrome is rare—but when it strikes, it escalates fast. Most often affecting children and teenagers recovering from viral infections like the flu or chickenpox, this condition causes dangerous swelling in the brain and liver. And while the initial symptoms might mimic common post-viral fatigue or stomach flu, the consequences can be life-threatening. Confusion, seizures, hallucinations, even persistent vomiting—these aren’t just symptoms. They’re red flags. Immediate medical intervention can make the difference between full recovery and permanent damage. One major risk factor? Aspirin (acetylsalicylic acid). Even small doses, if given during or after a viral illness, can trigger this severe reaction. That’s why understanding the warning signs matters more than ever. We’ve expanded our list to 15 critical symptoms of Reye’s Syndrome you need to watch for, breaking down what they mean and why they shouldn’t be ignored. Because when it comes to your child’s brain and liver, every second counts—and every symptom tells a story.

1. Swollen Liver

When Reye's syndrome sets in, the patient's blood sugar will usually drop. At the same time, acidity and ammonia levels in the blood rise, and the liver may swell up and develop fatty deposits. There are multiple potential liver complications of Reye's syndrome. In addition to fatty deposits, patients may have abnormal liver function tests. If the liver stops performing its usual functions, this can lead to the accumulation of toxic substances in the blood. In serious cases, the liver fails entirely. Liver failure is a condition where the liver has lost enough function that it cannot perform lifesaving functions. Patients with failing livers may exhibit bleeding and poor blood clotting. Typically, liver failure must be treated with a liver transplant.

2. Excessive Lethargy

In children over two years old and teenagers, excessive lethargy is one of the early symptoms of Reye's syndrome. The child may be unusually sleepy and have trouble focusing on the world around them. They may sleep more than usual, fall asleep at odd times, and find it difficult to concentrate on typical tasks. Unfortunately, because Reye's syndrome tends to occur after a viral infection, parents and health professionals may mistake this lethargy for a normal part of the recovery process. Lethargy can be caused by a number of other issues as well, ranging from mild to severe. It should be monitored if it follows a virus because of the seriousness of untreated Reye's syndrome. Other common causes of lethargy are sleep issues, medication side effects, chronic illnesses, anemia, heart problems, hypothyroidism, mental health issues, and certain cancers.

3. Seizures

Seizures occur when the brain becomes dangerously swollen, and they require emergency treatment. Parents should seek emergency medical help immediately if their child has convulsions or seizures, or if they lose consciousness unexpectedly. Brain swelling causes pressure inside the skull to increase. In some cases, the pressure prevents blood flow to the brain, which can lead to the death of brain tissue. If swelling blocks other fluid from naturally leaving the brain, the condition can get even worse. A seizure occurs when there is abnormal electrical activity in the brain. Some individuals with epilepsy experience seizures commonly and don't require medical treatment if the seizure doesn't last more than five minutes. But if the child doesn't have a history of seizures, a sudden seizure is a medical emergency that must be evaluated and treated to make sure serious neurological conditions aren't present.

4. Persistent Vomiting

Persistent and continuous vomiting are other early symptoms of Reye's syndrome. They may begin occurring suddenly after the affected individual has seemed to be on the mend from their viral infection. Some parents may mistake the vomiting for a continuation of the earlier virus. The vomiting tends to begin around three to five days following the initial onset of the viral infection. This is also often coupled with lethargy. It's important to get sudden and persistent vomiting evaluated by a medical professional. Even if it isn't caused by Reye's syndrome, it's vital to be sure it isn't caused by another potentially serious illness.

5. Disorientation Or Hallucinations

Disorientation, confusion, and hallucinations are neurological symptoms that may occur because of the swelling in the brain. These are severe symptoms that need emergency treatment. A child experiencing disorientation may forget where they are, what they're doing, or who they're speaking to in a manner beyond normal absentmindedness. They may be confused about what's happening around them, struggle to follow a line of conversation, or have trouble communicating. Hallucinations involve seeing or hearing things that aren't there, or experiencing other sensations that aren't happening. These experiences are caused by the brain failing to accurately interpret information about the outside world. If the affected child doesn't receive emergency treatment, neurological symptoms can lead to permanent brain injury or even death.

6. Irritability and Mood Changes

In the early stages of Reye’s syndrome, some children may display sudden mood swings or become unusually irritable. This can manifest as restlessness, frequent crying, or aggressive behavior that’s out of character. These mood changes often precede more obvious neurological symptoms and may be dismissed as crankiness during illness recovery. However, when irritability is extreme or accompanied by lethargy or confusion, it may indicate that brain swelling has begun. Since children can’t always articulate what’s wrong, behavioral shifts like these should raise concern—especially in the days following a viral infection. Early attention can prevent rapid neurological decline.

7. Hyperventilation or Irregular Breathing

As brain inflammation intensifies, the body’s automatic functions—like breathing—can become disrupted. Some children may begin hyperventilating or exhibit rapid, shallow breaths. Others may breathe irregularly, with long pauses or sudden gasps. These shifts are the body’s attempt to compensate for rising intracranial pressure and changes in blood pH caused by liver dysfunction. Parents should act fast if they notice abnormal breathing, particularly when combined with confusion, vomiting, or lethargy. Since Reye’s syndrome can progress rapidly, these changes in respiration may be one of the last signs before a medical emergency. Immediate evaluation in an emergency room is critical.

8. Enlarged Pupils or Uneven Pupil Size

Neurological changes caused by Reye’s syndrome can sometimes be seen in the eyes. Pupils may become enlarged and react slowly—or not at all—to light. In some cases, pupils may be uneven in size (anisocoria), which suggests pressure on specific areas of the brain. These signs indicate elevated intracranial pressure and are often seen in advanced stages of the syndrome. They should be treated as an emergency. If a child recovering from a viral illness begins showing unusual eye changes or unresponsiveness to light, it's time to seek urgent medical help. The eyes can tell you what words can’t.

9. Rapid Heart Rate

A racing heart may seem unrelated to brain or liver issues, but in Reye’s syndrome, it can be a significant early clue. As toxins accumulate in the bloodstream due to liver failure, the body responds with increased heart rate to maintain circulation and oxygen delivery. This compensatory mechanism may go unnoticed unless vitals are checked, but parents might feel the child’s heart pounding or see visible chest movement. Tachycardia (fast heart rate) is especially concerning when it accompanies other warning signs like vomiting, lethargy, or confusion. A fast heartbeat without fever or exertion deserves immediate attention.

10. Slurred Speech or Trouble Speaking

Speech changes, especially in older children or teens, can be one of the clearest neurological signs of Reye’s syndrome. Slurred words, delayed responses, or trouble forming coherent sentences may appear suddenly as brain function declines. These symptoms reflect swelling in areas of the brain responsible for communication. Unlike developmental delays, these changes are abrupt and disturbing. Some children may simply become nonverbal, unable to express basic needs. In this context, speech disturbance is never benign—it’s the brain’s distress call. Don’t wait for it to worsen. Immediate medical care can prevent irreversible damage.

11. Unsteady Movements or Difficulty Walking

Loss of coordination is another serious neurological symptom that may emerge as Reye’s syndrome progresses. Children who were recently mobile may suddenly have difficulty standing, walking, or performing basic movements. This unsteadiness, known as ataxia, results from brain swelling affecting motor control. Parents might notice clumsiness, staggering, or even complete inability to walk. These signs are especially troubling when they appear after an initial recovery from a virus. Unlike normal tiredness or weakness, this level of impairment reflects central nervous system involvement. It’s urgent. Time matters—especially when movement becomes a visible warning sign.

12. Personality Changes or Unusual Behavior

In some cases, Reye’s syndrome alters a child’s demeanor so drastically that loved ones hardly recognize them. A once calm, cheerful child may become agitated, withdrawn, or completely disengaged. These personality shifts often stem from brain inflammation and toxic buildup in the bloodstream. Parents may notice inappropriate reactions, odd speech, or detachment from surroundings. Because this behavior emerges subtly, it’s often mistaken for moodiness or post-illness fatigue. But when these shifts appear suddenly and escalate alongside physical symptoms, they’re a red flag. Reye’s doesn’t just change chemistry—it distorts cognition and personality. That alone demands action.

13. Loss of Consciousness

Loss of consciousness is one of the most alarming late-stage symptoms of Reye’s syndrome and requires emergency medical care. This can present as unresponsiveness, fainting, or slipping into a coma. It reflects severe swelling of the brain and typically follows earlier warning signs like vomiting, lethargy, or disorientation. By the time consciousness is lost, the brain is in critical distress, and immediate hospitalization is essential to prevent death or lasting damage. While this symptom is more common in advanced cases, it can occur rapidly—within hours of initial signs. If a child becomes unresponsive, treat it as a life-threatening emergency.

14. Visual Disturbances

Some children with Reye’s syndrome report blurry vision, double vision, or light sensitivity. These visual changes are a result of intracranial pressure affecting the optic nerves and visual centers of the brain. Because children may not always articulate visual changes clearly, parents should watch for signs like squinting, covering one eye, or complaining about “weird” vision. Light sensitivity may also trigger headaches or irritability. Though not as well-known as vomiting or seizures, visual symptoms are another subtle sign of swelling inside the skull. When combined with other neurological red flags, they should prompt immediate evaluation.

15. Low Blood Sugar (Hypoglycemia)

In the early stages of Reye’s syndrome, the liver’s inability to regulate glucose levels can result in dangerously low blood sugar. Hypoglycemia is especially common in infants and young children and may cause tremors, sweating, irritability, or seizures. If not corrected, it can progress to unconsciousness or death. Unfortunately, hypoglycemia often flies under the radar because its symptoms can mimic general illness or fatigue. Blood sugar testing is essential in any child presenting with altered mental status post-viral infection. If you suspect low blood sugar—especially in a child who’s been vomiting or unusually sleepy—don’t wait to get medical help.

Reye’s syndrome doesn’t wait. It often strikes just as a child seems to be getting better—turning a normal recovery into a race against time. What begins with vomiting or fatigue can quickly spiral into seizures, confusion, and coma. That’s why knowing the signs isn’t just helpful—it’s life-saving. With 15 symptoms now outlined, this guide goes beyond the basics to spotlight early red flags and overlooked clues that could change everything. Whether it’s a mood shift, trouble walking, or a racing heartbeat, these signs matter. And so does action. If your child is recovering from a viral illness, especially after taking aspirin, even one suspicious symptom is reason to call a doctor. Early recognition and immediate care are the best defense. Reye’s is rare—but your vigilance makes it rarer still. Don’t wait for it to look serious. Trust your instincts, act early, and give recovery the safety it deserves.