Concussions may afflict as many as 4 million Americans every year, from the swimmer kicked in the head by another swimmer to an elderly woman who trips over a bunched throw-rug.

Toddlers experience them falling down stairs. Football players can get them even while wearing helmets, so can a bare-headed house cleaner who carelessly stands up under a shelf.

The effects of concussion are as wide-ranging as the list of possible causes, according to two medical experts who spoke on the subject last week in Livermore.

A concussion can produce dizziness, memory loss, confusion and nausea, the experts agreed — or it can seem to generate no consequences at all.

The effects might not show up for days, and sometimes they linger for weeks, according to the medical authorities, Dr. Paulomi Khadakia Bhalla, a former Stanford Health Care neurologist, and Shelby McGinnis, a Stanford physical therapist.

Both Bhalla and McGinnis have extensive experience dealing with concussions and other injuries of the brain.

They spoke at Livermore’s Bankhead Theater. Their presentation kicked off a new Stanford ValleyCare Speaker Series, which will continue into 2020 at the Bankhead.

People often hold misconceptions about concussion, McGinnis said. Among the most common is believing that losing consciousness indicates the seriousness of the injury.

“The reality,” she said, “is that the loss of consciousness has no real impact on your long-term recovery and is more a result of the way your brain has been hit and the location of the hit than the severity of it.”

On the other hand, concussion almost always affects vision, and a simple test of whether someone’s eyes can smoothly track left to right motion of the finger can help make it clear whether concussion has occurred.

A video study of football players who were slow to get up after a hard hit showed that every one of them was later diagnosed as having experienced a concussion.

“You guys know what I’m talking about,” McGinnis said. There is bouncing up right after a hit and “then there is the opposite of that.” A player stays on the ground, holds or shakes his head, “tries to figure out what is going on — that is something you want to pay attention to.”

In soccer, supposedly a non-contact sport, but one in which players collide and headers are common, female players are more prone to concussion than male players because their necks tend to be weaker.

A weak neck can lead to sudden, violent movement of the head and brain. McGinnis once advised a young female athlete to take up wrestlers’ neck-strengthening exercises for greater stability and a smaller chance of concussion.

McGinnis thinks the brain is best pictured as a tree supported by a trunk — the brainstem, extending upwards from the spine — with branches vulnerable to sudden, sharp motion as if they were shaken by the wind.

In a concussion, “The tree is being whipped around inside the skull.”

The U.S. Centers for Disease Control and Prevention defines concussion as “a type of traumatic brain injury caused by a bump, blow, or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth.

“This sudden movement can cause the brain to bounce around or twist in the skull, creating chemical changes in the brain and sometimes stretching and damaging brain cells.”

Concussions happen to every age group and can occur in the most unlikely of situations.

McGinnis once treated a concussed young woman who had been struck in the head by a full bottle of Gatorade thrown by another spectator.

For two reasons, young children are considered most vulnerable to concussion. Learning to walk, they are prone to falls that shake or strike their heads, and they have yet to develop protective myelin coatings that will grow around brain tissue through about age 25.

Next after young children, in McGinnis’s view, are adolescents, a group likely to engage in sports, but also prone to risk-taking in cars and elsewhere. A third group of concern is the elderly, who are often susceptible because of declining muscle strength, reduced sensation in their feet and poor eyesight.

Also, in the elderly, medications may contribute to loss of balance. The use of blood thinners can make head injuries more dangerous.

Knowing what symptoms to look for from a possible concussion is important, but the results are often ambiguous at best.

Sometimes, immediate dizziness, nausea or memory loss can point clearly to concussion, but symptoms are often delayed and hard to distinguish from everyday ups and downs. The description of a teenager who acts moody or does not sleep well for three or four days after suffering a hard knock in a football game was provided as an example.

McGinnis described an elderly female patient, fictional but typical, who developed lasting symptoms of concussion from a fall without striking her head.

A medical check of “Mrs. Jones,” as the patient was called, found no fractures, but as the days passed, subtler symptoms emerged.

She started to forget things, slept more than usual, turned down social invitations and basically didn’t feel good. “Her husband agrees that she is not herself.”

With the elderly, McGinnis said, it’s important to be aware of the possible side effects of medications they may be taking.

In high school and collegiate sports, she noted, it is vital to remove players who are even suspected of having experienced a concussion. A new concussion on top of an unhealed older one is considered medically very dangerous.

Not only in the U.S. but in athletic programs around the world, the general rule has become, “When in doubt, sit them out.” In other words, if there is any uncertainty, get the player off the field and to a physician for evaluation.

Baseline screening programs are commonly available today for athletes before the start of a competitive season.

Athletes are tested for performance in vision, balance, reaction times and other functions so that following some future head injury, medical personnel will be able to repeat the tests to look for any deterioration.

Stanford ValleyCare has been conducting baseline testing for local school districts for several years using a range of tests with names like ImPact, King-Devick and COBALT.

McGinnis considers them highly valuable. As Bhalla pointed out in an email, the brains of young athletes are constantly developing and changing, so the tests should be done annually to make results valid.

The ValleyCare Speaker Series is sponsored by ValleyCare Charitable Foundation. In January, Dr. Marion Buckwalter will speak on the aftermath of stroke. In May, Dr. Jake Scott will discuss the rise and threat of antibiotic resistant bacteria.