Hello, dear readers, and welcome to our monthly letters column. Flu season in the United States peaks during December, January and February, so please take precautions. We recently wrote about several common respiratory illnesses, including influenza, whooping cough, COVID-19 and pneumonia. We also discussed how to protect those around you when you’re ill. If you missed any of these, or would like a refresher, check out our archives. And now, your letters.

-- In a column about whooping cough, we wrote about the importance of vaccines. These are DTaP and Tdap, each of which protects against whooping cough, diphtheria and tetanus. DTaP is recommended for children and infants under age 7. Tdap, a booster, is for children 7 years and older, adolescents and adults. As a reader pointed out in his letter, it is important for an infant’s caregivers to be current on the Tdap booster. “You could have mentioned that pregnant women should be vaccinated with Tdap to help protect newborns,” he wrote. “And new dads and grandparents who are potential babysitters for a newborn should also have a Tdap booster.” We appreciate the chance to expand on vaccine recommendations. Because whooping cough is a particular danger to infants and young children, everyone who spends time with an infant should be current on whooping cough vaccines and boosters.

-- We recently wrote about the importance of good-quality sleep and shared strategies that some people find helpful. One is the use of sound machines, which can create a soothing environment and mask disruptive noise. We heard from a reader who wondered about safety. “I read that listening to the same sound for very long could be bad for your hearing,” he wrote. “Do you have some information about that?” The primary safety concern when using a sound machine is volume. The repetition of the same sound, when played at a safe volume, does not pose a danger. Although an official standard for volume has not been set, the general consensus is that for infants and children, a sound machine should never exceed 50 decibels. For adults, below 60 decibels is considered optimal. The machines should be located at least 7 feet away from the sleeper.

-- A reader concerned with the effects of an enlarged prostate wondered about a stent as a possible treatment. “As men get older and the prostate enlarges, it causes urinary flow and other problems,” he wrote. “I always wondered why it can't be dealt with by the insertion of a stent at the point of restriction.” A stent is a small tube used to hold open a narrow or unstable anatomical passageway. Although the idea of a stent to prop open the urethra makes sense, in practice, a number of complications can occur. They can be uncomfortable, contribute to incontinence, cause an obstruction and become infected. Prostatic stents are used in specialized circumstances, but at this time, they are not considered a primary treatment.

Thank you, as always, for taking the time to read and respond to these columns. We appreciate your feedback and look forward to hearing from more of you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I am 72 years old and have started to have problems with dry eyes. It’s not all the time, but at some point each day, my eyes will suddenly get very dry. It’s even hard to blink. What causes that? Those lubricating drops help, but only for a little while.

Dear Reader: You are experiencing what is commonly known as dry eye syndrome, or dry eye disease. The medical term is (deep breath) keratoconjunctivitis sicca. It’s a daunting name for a common condition that affects one-fifth of all adults. An estimated 3 million new cases occur each year. Although dry eye syndrome can happen to anyone and at any age, it becomes more common as people grow older. After age 40, the risk of developing dry eye syndrome increases with each passing decade. The condition is also seen more often in women who have entered menopause.

The exposed surface of the eye is covered by a triple layer of liquid known as the tear film. The components are secreted by various glands and tissues, and each plays a role in lubricating, protecting and nourishing the tissues of the eye. The inner layer, made up of mucus, provides stability and lubrication. Mucus has antibacterial properties, so it also helps protect against infection. The middle layer, known as the aqueous layer, is made up primarily of water. It provides hydration and washes away contaminants. It also contains proteins that nourish the cornea and electrolytes that help manage surface tension. The topmost layer is made up of lipids that act as a seal to prevent evaporation.

If something goes wrong with any of the three layers of tear film, the ability to nourish, hydrate, lubricate and protect the eyeball can be compromised. This can occur due to changes that take place as we age, some autoimmune diseases, a side effect of some medications, and hormonal changes. Environmental factors such as a dry climate, dry air from a heat source, air pollution, wood smoke, allergens and too much screen time can also play a role.

The most common form of dry eye is a deficiency in the lipid layer of tear film. Known as evaporative dry eye, it allows the tears that keep the eyeball moist to dissipate more quickly than they can be replaced. Lubricating drops with lipid-based ingredients, which can thicken the outer layer of tear film, can ease symptoms of evaporative dry eye. When the watery layer of tear film is compromised, it is known as aqueous-deficient dry eye. This is less common than evaporative dry eye. In this case, drops labeled as “hypotonic” can improve tear volume and help restore moisture to the exposed surface of the eye. For more serious cases of dry eye, prescription medications may become necessary.

Redness is common in dry eye, but avoid drops that promise “redness relief.” Most contain vasoconstrictors, which narrow blood vessels and can worsen dry eye symptoms. When the condition is severe, see your doctor. Complications can include chronic inflammation, damage to the surface of the eye, being susceptible to infection, and even vision loss.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My husband and I enjoy a glass of wine with dinner. I prefer red wine, but it always gives me a bad headache. It happens from even half a glass, so I know it’s not a hangover. I thought it was the sulfites, but I hear there’s a new theory. Do you know what that is?

Dear Reader: For some people, a glass of red wine is one of life’s small pleasures. For many others, it’s the trigger for a painful headache. An analysis of responses to health questionnaires found that while alcohol in general was associated with non-hangover headaches, red wine was cited most often as the specific cause. Research into the question found that up to one-third of people say that they get a pulsing, pounding headache after drinking red wine, sometimes as soon as within 30 minutes of consumption.

While the existence of red wine headaches has proven easy to pinpoint, the reason why has remained elusive. One theory focused on a sensitivity to histamines, an organic compound whose chemical structure is similar to ammonia. The histamines produced by our bodies are used in intracellular communication. They aid in sleep regulation, are important to brain function and play a role in allergic response. Histamines are also abundant in some fermented foods, including red wine. Sulfites, a preservative commonly used in wine, were also seen as a potential culprit. But when studies found that red wines without sulfites, and those low in histamines, also triggered the dreaded headache, the search for an answer continued.

Last year, researchers from the University of California’s Davis and San Francisco campuses suggested a potential new cause for the red wine headache: a class of organic compounds known as flavanols. These occur naturally in a wide range of plants, and their antioxidant properties play a role in cardiovascular health. Foods rich in flavanols include berries, walnuts, dark chocolate, tea, coffee and, yes, red wine.

When it comes to a link to red wine headaches, the researchers pointed to a specific flavanol called quercetin. It is produced in red grapes as they are exposed to sunlight. Quercetin impedes the body’s ability to completely metabolize alcohol. This results in the buildup of a compound known as acetaldehyde. Although small amounts of acetaldehyde are manageable, accumulations can become toxic. The researchers suspect this may be the mystery compound behind the red wine headache.

Until this new theory is proven, red wine drinkers susceptible to getting headaches will have to take evasive measures. These can include taking a defensive dose of an over-the-counter anti-inflammatory medication before drinking, or choosing a white or rose wine, each of which is low in quercetin. If opting for a painkiller, it is important to know that both ibuprofen and acetaminophen can raise the risk of liver damage and gastrointestinal bleeding when mixed with several glasses of wine. And considering that the health risks of drinking alcohol extend well beyond a headache, a glass of sparkling water can be the wisest alternative.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)