Oguchi disease-1 is a rare inherited eye condition that mainly affects how the retina adapts to darkness. People with Oguchi disease-1 often notice night blindness from childhood or the teen years, and early signs of Oguchi disease-1 may include trouble seeing in dim rooms or outside after sunset. Not everyone will have the same experience. Oguchi disease-1 is usually lifelong, but it does not typically shorten life, and treatment focuses on regular eye care, protecting vision, and managing daily activities in low light.
How many people live with Oguchi disease-1?
Oguchi disease-1 is so rare that there is no widely accepted estimate of how many people live with it worldwide or in the United States. Global and U.S. prevalence figures are not consistently reported by a single authoritative source, and major rare-disease references describe it as extremely rare rather than giving a reliable count.
Overview
Oguchi disease-1Symptoms
Oguchi disease-1 mainly causes lifelong night blindness, so people with Oguchi disease-1 may struggle in dim rooms or outdoors after sunset. Vision in daylight is usually normal. Early signs of Oguchi disease-1 often begin in childhood, with very slow adjustment to darkness.
Outlook and prognosis
For many with Oguchi disease-1, night vision problems start early and usually stay fairly stable over time rather than steadily worsening. Daytime sight is often near normal, so many people can manage school, work, and daily life well with practical low-light adjustments and regular eye care.
Causes and risk factors
Oguchi disease-1 is caused by changes in a gene involved in how the retina resets after light exposure, usually inherited from both parents. Risk is highest with a family history, related parents, or a known carrier background.
Genetic influences
Genetics are central to Oguchi disease-1. It is usually caused by inherited changes in the SAG gene, and the exact variant can affect testing, family planning, and whether relatives may also carry the condition.
Diagnosis
Diagnosis of Oguchi disease-1 usually starts with eye findings and a history of night vision problems. An eye exam often shows characteristic retinal changes. Genetic tests can confirm the diagnosis of Oguchi disease-1.
Treatment and drugs
Treatment for Oguchi disease-1 focuses on making low-light vision problems easier to manage. Many living with Oguchi disease-1 benefit from regular eye checkups, good lighting at home, and extra caution with night driving. Genetic counseling can also help families understand inheritance.
Symptoms
Oguchi disease-1 often affects everyday tasks in low light, such as walking outside at dusk or finding things in a dark room. For many people with Oguchi disease-1, the biggest problem is trouble seeing at night, while daytime vision stays fairly good. This inherited eye condition is usually present from childhood, and the early features of Oguchi disease-1 can be easy to miss if someone mainly struggles after dark.
Seeing in dim light can be hard, especially outdoors at night or in poorly lit rooms. Many living with Oguchi disease-1 may pause on stairs, move more slowly, or need extra light to feel sure of their footing.
After bright light, the eyes can take much longer than usual to adapt to darkness. You might notice small changes at first, such as needing far longer than others to see clearly after stepping into a dim hallway, theater, or parking garage.
Vision in daylight is often normal or close to normal. Reading, recognizing faces, and color vision may be little affected, which can make the condition less obvious outside low-light settings.
On an eye exam, the back of the eye may look yellow-gray or metallic under regular light. After a long period in the dark, this unusual color can fade, which is a classic clue for Oguchi disease-1.
How people usually first notice Oguchi disease-1?
The first signs of Oguchi disease-1 are usually noticed when a child has much more trouble seeing in dim light or at night than other children, even though daytime vision may seem mostly normal. Families may first pick up on it when a child hesitates in dark rooms, struggles outdoors after sunset, or takes an unusually long time for their eyes to adjust after the lights go down. How Oguchi disease-1 is first noticed is often through these early night-vision problems, with an eye doctor later spotting a distinctive change in the retina during an exam that helps confirm what is going on.
Types of Oguchi disease-1
No distinct types of Oguchi disease-1 are widely recognized.
Can I slow down the onset of symptoms?
No, there is currently no proven lifestyle change or medical treatment that can meaningfully delay the start of symptoms in Oguchi disease-1, because it is an inherited problem affecting the light-sensing part of the eye and night vision trouble often begins early in life. General healthy habits like not smoking and managing overall eye health are good for you, but they have not been shown to prevent this condition or push back when it starts. Research is ongoing, including work on inherited eye diseases, so regular follow-up with an eye specialist can still be helpful even though it cannot promise a delay in symptoms.
What genetics can tell you about symptoms
In Oguchi disease-1, genetics can help explain why night blindness starts early and tends to be the main symptom, while daytime vision usually stays fairly normal. Changes in the gene linked to this condition disrupt how the retina resets after exposure to light, so many people living with it have slow adjustment to darkness and a shiny, unusual appearance of the retina on eye exam. Genetics can confirm the diagnosis, but based on what is known so far, it does not reliably predict exactly how severe symptoms will be for each person or whether additional vision problems will develop later.
Causes and risk factors
For many living with Oguchi disease-1, the main cause is an inherited change in the SAG gene. It usually happens when a child inherits two changed copies of this gene, one from each parent. A family history of Oguchi disease-1 and closely related parents can raise the chance of it happening. Lifestyle, diet, screen use, and sunlight exposure do not cause Oguchi disease-1, but other eye problems can affect how noticeable the vision changes are. Genetic testing can sometimes clarify your personal risk.
What are the most common causes or risk factors?
Oguchi disease-1 is almost always caused by inherited changes in a gene called SAG, and it usually happens when a child gets one changed copy from each parent. The main risk factors are having a family history of this condition or having parents who both carry the changed gene, which is more likely if the parents are related by blood. There are no known lifestyle, diet, or environmental causes.
Environmental and biological risk factors
The known risk factors for Oguchi disease-1 are unusual because experts have not found clear environmental or non-genetic biological triggers. Current evidence suggests that common exposures and general health conditions do not appear to make Oguchi disease-1 more likely. Because of that, this section is brief and focuses on what has not been clearly linked rather than listing risks that are not proven.
No clear link has been established between Oguchi disease-1 and outside exposures such as air pollution, radiation, heavy metals, or everyday chemicals. Current evidence does not show that these factors increase the chance of the condition occurring.
Maternal illnesses, infections, medications, or pregnancy complications have not been identified as established risk factors for Oguchi disease-1. In the same way, birth-related events have not been shown to cause it.
Advanced maternal or paternal age is not a recognized risk factor for Oguchi disease-1. Unlike some congenital conditions, there is no well-established age-related pattern here.
Oguchi disease-1 can affect people of any sex. There is no strong evidence that being male or female changes the likelihood of having the condition.
Genetic risk factors
For many living with Oguchi disease-1, the cause is an inherited gene change, not something they did. When people ask about the genetic causes of Oguchi disease-1, the answer usually points to changes in the SAG gene, which helps the light-sensing cells in the retina recover after light exposure. Some risk factors are inherited through our genes, and the main one here is receiving a nonworking SAG copy from each parent.
SAG gene: Oguchi disease-1 is caused by harmful changes in the SAG gene. This gene helps rod cells in the retina switch off properly after they respond to light.
Two changed copies: The condition usually develops only when both copies of the SAG gene carry a harmful change. If just one copy is changed, someone is usually a carrier and typically does not have symptoms.
Carrier parents: A child is at risk when both parents carry one changed SAG copy. In each pregnancy, there is a 25% chance of Oguchi disease-1, a 50% chance of being a carrier, and a 25% chance of inheriting two unchanged copies.
Family history: Having relatives with Oguchi disease-1 can raise the chance that the same SAG change is present in a family. Brothers, sisters, and children of known carriers may have a higher chance of being affected or being carriers themselves.
Related parents: The chance of Oguchi disease-1 can be higher when parents are biologically related or come from a small community where the same rare SAG change is more common. This does not cause the gene change, but it makes it more likely that both parents carry the same one.
Lifestyle risk factors
Oguchi disease-1 is an inherited retinal condition, so lifestyle habits do not cause it. Still, how lifestyle affects Oguchi disease-1 can be important because daily choices may change how much trouble a person has with night vision, glare, and safety in dim settings. The main lifestyle risk factors for Oguchi disease-1 are activities and habits that increase visual demands in low light or add avoidable eye strain and injury risk. Thoughtful adjustments can help reduce accidents and make symptoms easier to manage.
Driving at dusk, night, or in poorly lit weather can be especially difficult because Oguchi disease-1 slows dark adaptation. This can raise the risk of missing hazards, pedestrians, or road signs when vision has not adjusted well.
Dim rooms, dark stairways, and low-light work or leisure activities can make the condition more limiting in everyday life. Brighter, even lighting may improve function because it reduces reliance on impaired night vision.
Bright headlights, harsh contrast, and glare from screens or reflective surfaces can make visual transitions harder for people with Oguchi disease-1. Reducing glare with screen settings, matte surfaces, or careful lighting placement may lessen discomfort and improve usable vision.
Sports, cycling, hiking, or swimming in low light can become less safe when night vision is reduced. Planning these activities for daylight hours can lower injury risk related to delayed adaptation in the dark.
Risk prevention
Oguchi disease-1 is an inherited eye condition, so there is no known way to prevent the condition itself if the gene change is present. For many living with Oguchi disease-1, prevention is mostly about lowering the chance of accidents in dim light, protecting eye health, and planning ahead for daily activities after dark. Different people need different prevention strategies—there’s no single formula. Family planning support and regular follow-up can also help people understand risks and manage life with Oguchi disease-1.
Bright, even lighting at home can make moving around much easier when night vision is poor. Motion lights, stair lighting, and carrying a flashlight may help prevent falls or injuries in dim settings.
Many people with Oguchi disease-1 need extra caution with night driving because dark adaptation is impaired. Avoiding driving after sunset, in tunnels, or during bad weather can reduce risk for crashes.
Regular visits with an eye specialist can help confirm that vision changes are due to Oguchi disease-1 and not another eye problem developing alongside it. This is also a good time to discuss the early symptoms of Oguchi disease-1 and practical safety steps for school, work, and travel.
Sunglasses that block UV light may help protect overall eye health, even though they do not prevent Oguchi disease-1 itself. Alongside medical care, everyday habits also matter.
Can you prevent Oguchi disease-1?
No, Oguchi disease-1 cannot usually be prevented, because it is an inherited eye condition that people are born with. Nothing in daily life, such as diet, exercise, or screen use, is known to stop it from happening. If it runs in your family, genetic counseling may help you understand the chances for future children, but it cannot prevent the condition in someone who has the changed gene.
Transmission
Oguchi disease-1 is not infectious, so it cannot spread from one person to another through touch, coughing, sex, food, or blood. Instead, the genetic transmission of Oguchi disease-1 happens when a child inherits two changed copies of the related gene, usually one from each parent. Parents are often healthy carriers and may have no symptoms, so the condition can seem to appear unexpectedly in a family. If both parents are carriers, each pregnancy has a 25% chance of Oguchi disease-1, a 50% chance that the child will be a carrier, and a 25% chance that the child will inherit neither changed copy.
When to test your genes
Consider genetic testing if you have lifelong night blindness that began in childhood, especially if an eye specialist suspects Oguchi disease or there is a family history of inherited retinal conditions. Testing can confirm the diagnosis, clarify the gene involved, guide family planning, and help you access appropriate retinal follow-up and research studies.
Diagnosis
People with Oguchi disease-1 often come to medical attention because they struggle to see in the dark, such as taking much longer than others to adjust after walking into a dim room. This rare inherited eye condition is usually suspected from the pattern of night blindness and confirmed with specialized eye testing. Getting a diagnosis is often a turning point toward answers and support. The diagnosis of Oguchi disease-1 usually combines an eye exam, retina imaging, and genetic testing.
Doctors ask about lifelong or early-onset night blindness and whether vision is much worse in dim light than in daylight. A detailed family and health history can help show whether similar symptoms run in the family.
An eye specialist examines the back of the eye after dilating the pupils to look for the characteristic retinal appearance seen in Oguchi disease-1. In many people, the retina has a metallic or golden-gray sheen that becomes less noticeable after long periods of dark adaptation.
Doctors may perform tests that measure how the eyes adjust to darkness over time. People with Oguchi disease-1 often recover night vision very slowly, which is one of the key clues.
Electroretinography, a test that records the retina's response to light, can show a pattern typical of this condition. These findings help support the genetic diagnosis of Oguchi disease-1 and distinguish it from other causes of congenital night blindness.
How is Oguchi disease-1 diagnosed?
Oguchi disease-1 is usually diagnosed by an eye doctor based on a history of lifelong night blindness, a careful eye exam, and tests that check how the back of the eye responds after spending time in the dark. Doctors may also look for a distinctive change in the color and appearance of the back of the eye that often fades after a few hours in darkness. A genetic test can help confirm the diagnosis by finding a change in a gene linked to this condition, especially if the exam results are not fully clear.
Stages of Oguchi disease-1
Oguchi disease-1 does not have defined progression stages. This is because it is usually a stationary eye condition, meaning vision problems tend to stay fairly stable over time rather than steadily worsening in clear steps. Many living with Oguchi disease-1 mainly notice trouble seeing in dim light or at night, and doctors usually diagnose it with an eye exam, a review of symptoms, and tests that measure how the retina responds to light. If a hereditary factor is suspected, referral to a genetics specialist may follow, especially when confirming how Oguchi disease-1 is diagnosed or discussing family risk.
Outlook and prognosis
The outlook for people with Oguchi disease-1 is generally very good, because it is usually a nonprogressive retinal condition and most affected individuals keep normal or near-normal daytime vision throughout life. The main long-term issue is lifelong night blindness with markedly delayed dark adaptation, which can continue to affect driving, mobility, and safety in dim environments. Central visual acuity is often preserved, although some patients may develop mild retinal changes over time, so periodic ophthalmic follow-up is reasonable. Life expectancy is not known to be reduced, and Oguchi disease-1 is not considered a fatal disorder. Overall, prognosis depends more on adapting to low-light visual limitations than on loss of general health or progressive blindness.
Long-term effects
For many living with Oguchi disease-1, the main long-term effect is lifelong trouble seeing in dim light or after moving from a bright place into darkness. It’s important to remember that long-term doesn’t always mean progressive, and this condition is usually considered stable rather than steadily worsening. Many people first notice the early symptoms of Oguchi disease-1 in childhood, while daytime sight often stays fairly normal.
The hallmark long-term feature is persistent night blindness, so dark rooms, evening walks, or driving at night may remain difficult over time. Dark adaptation is also unusually slow, which means vision can take much longer than expected to adjust after lights go down.
Many people with Oguchi disease-1 keep good vision in daylight and in well-lit settings for years. Reading, recognizing faces, and other daytime tasks are often much less affected than seeing in the dark.
A distinctive change in the back of the eye can be present, where the retina looks unusually shiny or discolored in ordinary light and then changes after prolonged time in the dark. This is a recognized feature of Oguchi disease-1, but it does not usually mean the condition is rapidly damaging vision.
The condition often begins early in life and then remains fairly similar across adolescence and adulthood. In most reported cases, there is not major ongoing loss of central vision or a broad decline in overall eyesight.
How is it to live with Oguchi disease-1?
Living with Oguchi disease-1 often means that the hardest part of the day begins after sunset, because vision in dim light is unusually poor and the eyes can take a very long time to adapt to darkness. For many people, this affects everyday things like walking outdoors at night, driving in low light, moving safely through dark rooms, or adjusting when going from a bright place into a darker one, while daytime vision is often much better. This can mean relying on brighter lighting, extra planning, and help from others in unfamiliar or poorly lit settings, so family and friends may need to understand that the challenge is real even when someone seems to see normally in daylight.
Treatment and drugs
Oguchi disease-1 does not have a cure that restores the eye’s night vision, so treatment focuses on helping people manage symptoms and stay safe in dim light. Many living with Oguchi disease-1 do best by planning ahead, such as using brighter indoor lighting, carrying a flashlight, and taking extra care when driving or walking at night. Regular follow-up with an eye specialist can help confirm that the condition remains stable and check for other eye problems that may affect vision. Supportive care can make a real difference in how you feel day to day, and this is often the most practical approach for people looking for treatment for Oguchi disease-1 night blindness. Because Oguchi disease-1 is inherited, some families also choose genetic counseling to better understand the condition and what it may mean for relatives.
Can you cure Oguchi disease-1?
No, there is no cure right now for Oguchi disease-1. Care focuses on helping people manage poor night vision, such as using brighter lighting, avoiding activities like driving in the dark, and having regular eye checkups; for many living with this condition, vision in daylight stays fairly normal and the problem often does not get much worse over time. Researchers are studying new treatments, but none are proven or widely available yet.
Non-drug treatment
Oguchi disease-1 mainly affects seeing in dim light, so day-to-day care centers on safety, practical adaptations, and vision support rather than medication. Many living with Oguchi disease-1 do fairly well in daylight but may struggle in dark hallways, movie theaters, or while walking outside at dusk. Supportive therapies can make daily tasks easier, and regular eye care helps people adjust as needs change.
The main non-drug approach is reducing time in poorly lit settings and improving lighting at home, school, and work. Bright entry lights, night-lights in hallways, and a flashlight or phone light can help prevent falls and make evening routines safer.
Many people with Oguchi disease-1 need to avoid driving at night or in other low-light conditions because delayed dark adaptation can make hazards hard to spot. This is often a key part of how to manage night blindness in Oguchi disease-1 in everyday life.
Low vision specialists can suggest practical tools and strategies for moving around safely and using vision more effectively in dim settings. This may include orientation training, contrast-enhancing tips, and advice on the best lighting for reading or household tasks.
Children and adults may benefit from simple adjustments such as sitting in well-lit areas, allowing extra time to adapt when entering dark rooms, and avoiding unnecessary low-light activities. These changes can ease stress and help with learning, work, and social events.
Your genes may have an impact on how drugs work for you
For people living with Oguchi disease-1, there are no standard medicines that treat the condition itself, so there is not much known about genes changing how well a usual treatment works. This condition mainly affects how the eye responds to low light, and at this time there are no well-established examples of special gene-based drug choices for it. Even so, it is still smart to tell your eye doctor and pharmacist about the diagnosis, because any new treatment plans should take your vision needs and overall health into account.
Pharmacological treatments
For Oguchi disease-1, there is currently no approved medicine that restores normal night vision or changes the course of the condition. Treatment is usually centered on monitoring, practical low-light adaptations, and checking for other eye problems that may need their own care. If you are searching for drug treatment for Oguchi disease-1, the key point is that routine medication is not a standard part of care at this time.
There is no established prescription drug for Oguchi disease-1 itself. Medicines do not currently reverse the night blindness or the characteristic retinal changes seen with this condition.
Vitamin A is not a routine treatment for Oguchi disease-1, because this condition is not usually caused by a simple vitamin shortage. Doctors may still check for vitamin A deficiency if symptoms or diet history suggest it, since that would be treated separately.
Some people with Oguchi disease-1 may still need medication for other eye or general health problems, but those drugs do not treat the underlying retinal condition. For example, lubricating eye drops can help dry eye symptoms if they are present for another reason.
Beyond standard options, research is exploring treatments aimed at the gene and light-sensing pathway involved in Oguchi disease-1. These approaches are still investigational and are not part of usual care yet.
Genetic influences
For many living with Oguchi disease-1, trouble seeing when driving at dusk or moving through a dark room links back to a change in the SAG gene. This gene helps the retina switch off light signals and recover after light exposure, so when it does not work properly, the eyes can struggle to adapt in dim settings. Oguchi disease-1 is inherited in an autosomal recessive pattern, which means a child usually develops it only after inheriting one changed copy of SAG from each parent. A carrier means you hold the gene change but may not show symptoms. This helps explain why early symptoms of Oguchi disease-1, especially poor night vision, may appear in one child even when both parents have normal eyesight. Genetic testing can help confirm the diagnosis and clarify the chance that brothers, sisters, or future children could also be affected.
Is Oguchi disease-1 genetic?
Genetics are central to Oguchi disease-1. It is usually caused by inherited changes in the SAG gene, and the exact variant can affect testing, family planning, and whether relatives may also carry the condition.
Having a genetic risk is not necessarily a diagnosis
Genetic variants are common across the general population and are a normal part of human biology. Most do not have a direct impact on health and simply reflect natural differences among people. For many genes, having a variant connected with a condition points to a possible link rather than a certain diagnosis, especially when several genes and environmental factors are involved. In some situations, though, a single well-characterized variant in a particular gene can be enough to confirm a diagnosis.
Pharmacogenetics—how genetics influence drug effects
For many living with Oguchi disease-1, the main day-to-day challenge is seeing poorly in dim light, and genetics matter because they help doctors confirm the exact cause. Oguchi disease-1 is usually linked to changes in the SAG gene, and that result can help separate it from other retinal conditions that may need different care. At this point, there is no approved drug that corrects the underlying gene change, so the current treatment options for Oguchi disease-1 are mostly supportive, such as using brighter lighting, being cautious with night driving, and having regular eye follow-up. Doctors can use your genetic information to guide counseling about what to expect and whether you may be eligible for research studies or future gene-based treatments. Genetic testing can also help avoid unnecessary medicines if the problem is inherited rather than caused by inflammation or another eye disease that is treated differently. In other words, for Oguchi disease-1, genetics currently influences treatment planning more by improving diagnosis and guiding future options than by choosing among standard medications.
Interactions with other conditions
People with Oguchi disease-1 usually notice problems in dim light rather than broader health issues, so it does not typically interact with heart, lung, or other body diseases in the way some genetic conditions do. Another important aspect is how it may link with other eye diseases. Because Oguchi disease-1 is an inherited retinal condition, its symptoms can sometimes resemble other causes of lifelong night blindness, and this can make diagnosis harder, especially when doctors are sorting out the early symptoms of Oguchi disease-1 from related retinal disorders. In some families, more than one inherited eye condition may be considered during testing, but Oguchi disease-1 itself is generally not known to cause disease outside the eyes. If someone living with Oguchi disease-1 also has another eye problem, such as cataracts or a different retinal disorder, their vision difficulties may feel more noticeable and may need follow-up from an eye specialist with genetics experience.
How genes can cause diseases
Humans have more than 20,000 genes, each carrying out one or a few specific functions in the body. One gene instructs the body to digest lactose from milk, another tells the body how to build strong bones and another prevents the body's cells from beginning to multiply uncontrollably and developing into cancer. As all of these genes combined are the building instructions for our body, a defect in one of these genes can have severe health consequences.
Through decades of genetic research, we know the genetic code of any healthy/functional human gene. We have also identified that in certain positions on a gene, some individuals may have a different genetic letter from the one you have. We call these hotspots "Genetic Variations" or "Variants" in short. In many cases, studies have been able to show that having the genetic letter "G" in the position makes you healthy, but having the letter "A" in the same position disrupts the gene function and causes a disease. Genopedia allows you to view these variants in genes and summarizes all that we know from scientific research, which genetic letters (Genotype) have good or bad consequences on your health or on your traits.
What other conditions can cause similar symptoms to Oguchi disease-1?
Oguchi disease-1 can look a lot like other conditions that cause lifelong trouble seeing in dim light, especially other inherited forms of night blindness. It may also be confused with retinitis pigmentosa, an eye disease that usually gets worse over time and can slowly affect side vision, as well as less common but important causes such as low vitamin A or other problems in the retina, the light-sensing layer at the back of the eye. An eye specialist can tell these apart with a careful exam and tests that check how the eye responds to light.
History
People living with Oguchi disease-1 often notice the same practical problem that first drew doctors’ attention many years ago: seeing poorly in dim light, then seeing much better again in daylight. A child might move confidently outside at noon but hesitate in a dark movie theater or on an evening walk. That pattern made this condition easier to recognize than many other rare eye disorders.
Oguchi disease-1 was first described in the early 1900s by the Japanese eye doctor Chuta Oguchi, whose name is now linked to the condition. He reported an unusual form of night blindness along with a striking change in the back of the eye that could shift after long periods in the dark. At the time, doctors mainly relied on symptoms, eye exams, and careful observation over time, because modern imaging and genetic testing did not yet exist.
As medical science evolved, specialists learned that Oguchi disease-1 belongs to a group of inherited retinal conditions that mainly affect how the eye adapts to low light. Over the years, doctors recognized that the day-to-day course is often quite stable. Many living with Oguchi disease-1 do not develop the kind of steady vision loss seen in some other retinal diseases, although the severity of night vision problems can vary from one person to another.
Later advances in retinal testing helped confirm what earlier clinicians had observed. Tests that measure how the retina responds to light showed that the rod cells, which are most important for vision in the dark, recover and function differently in this condition. This helped separate Oguchi disease-1 from other causes of congenital night blindness and gave doctors a clearer way to explain why symptoms tend to center on dim-light vision rather than sharp daytime sight.
In more recent decades, researchers also identified a genetic cause for Oguchi disease-1, which confirmed that the condition begins at a molecular level in the retina rather than from injury or eye strain. Knowing this history helps explain why diagnosis today usually combines a person’s symptoms, a detailed eye examination, and sometimes specialized retinal tests or genetic testing. For many families, that clearer picture brings reassurance: the condition has been recognized for more than a century, and doctors now understand it far better than they once did.