Images from Brazil of babies with undersized heads, a condition called microcephaly, are unsettling, and the surge in cases raises many questions about what the future may hold for these children, whose mothers were infected with the Zika virus while pregnant.
There's a broad spectrum of health issues children with microcephaly may face. Some may be in otherwise good health, while some will suffer crippling mental and physical disabilities. The U.S. Centers for Disease Control and Prevention says people with microcephaly may have problems including seizures, intellectual disability, trouble walking or standing, or difficulty speaking or swallowing. Some also have hearing or vision problems.
The authors of a study out this week in JAMA Ophthalmology identified eye problems in a number of infants with Zika-related microcephaly. The study looked at 29 newborns and found that 10 of them had defects in the retina or the optic nerve. Seven of the babies had problems in both eyes. "These type of defects cannot be fixed. They may lead to blindness," CBS News medical contributor Dr. Tara Narula told "CBS This Morning."
Babies whose brain development is stunted before birth will not grow out of it and may require life-long care.
Dr. Nassim Zecavati, an assistant professor of pediatrics and neurology at Georgetown University School of Medicine has treated many children born with microcephaly, though none related to the current Zika outbreak.
Zecavati said that while media reports are using the term "microcephaly" as if it's a condition, it would be more accurate to describe it as a catchall term.
"Microcephaly is a very general description of a very small head," she said.
It's a reflection of a lack of brain growth, but it can occur due to a variety of causes, said Zecavati, who is also director of pediatric neurology education at MedStar Georgetown University Hospital.
"The prognosis is entirely dependent on the cause," she said.
Zecavati said she divides cases into three "buckets" when she's teaching medical residents about the condition. But even her buckets have lots of variability.
About a third of babies with microcephaly are born with genetic or chromosomal issues, such as Down syndrome.
Infants exposed in utero to substance abuse or an infection, or other womb-related problems, fall into the second group. They include those born with fetal alcohol syndrome or whose mothers used methamphetamine or opioids, for example.
A baby's brain development can be impacted if the mother suffers an infection such as toxoplasmosis, cytomegalovirus (also called CMV, a type of herpes virus), German measles (rubella), chickenpox (varicella), herpes, or syphilis during pregnancy. Zika virus also appears to have this effect, although the link is still under investigation.
"Anything that slows down brain growth in the womb" is a problem, Zecavati told CBS News, as is "severe deprivation of blood flow and oxygen" before, during or after delivery. Or sometimes the placenta is deficient and nutrients don't reach the baby and promote normal brain growth.
"The brain is developing at a rapid pace in infants. But it's even more rapid in utero. The neurons are increasing at an exponential rate, compared even to the first year of life," she said.
And then there's the third group, which Zecavati labels "other."
"Sometimes we can't even identify something, where the cause is unknown, including some microcephaly babies with normal brain function. A person may have a small head because of family genetics but they have normal intelligence," she said.
Dr. Edward McCabe, senior vice president and chief medical officer of the March of Dimes, told CBS News, "About 10 to 15 percent of all babies in the U.S. with microcephaly do not have abnormal brain development. They are a normal variation. It's not a birth defect."
The prognosis for Zika-linked cases of microcephaly appears ominous, but it's still too early to know the full picture. Health officials in Brazil report some 4,000 babies have been born with microcephaly in recent months, and doctors there are still trying to understand the scope of the problem.
Dr. Angela Rocha heads up the effort to understand and manage the crisis at the Oswaldo Cruz Hospital in Recife, Brazil, at the epicenter of the Zika outbreak. She told CBS News chief medical correspondent Dr. Jon LaPook, she's never seen anything like it. "It's a generation of babies with disability, which is a huge social, economic and public health problem," she said.
The Zika outbreak withstanding, microcephaly is not common. In the U.S., the CDC says estimates of its frequency range from two to 12 babies per 10,000 live births.
Zecavati said microcephaly is diagnosed based on guidelines for normal head growth at certain ages. At birth, the average baby's head circumference is 13.7 inches (35 centimeters), and then increases about 0.4 inches (1 centimeter) per month during the first year of life.
In a healthy infant, the most rapid growth occurs in the first six months after birth. Brain weight doubles by four to six months and triples by a baby's first birthday. By age two, the brain is about 80 percent of adult size, said Zecavati. If a child doesn't meet those benchmarks, microcephaly is a concern.
Developmental delays -- falling short of standard milestones like sucking and swallowing early on, visually fixating on the mother's face by one month, head control starting to develop in the first three months, sitting up and cooing by six months, for example -- signal how severe a child's disability may be, Zecavati explained.
"I'm hesitant to give a timeline," she said, because there's developmental variation even in healthy babies.
Lots of questions still remain when it comes to Zika-linked microcephaly, but right now experts believe many babies whose moms were infected with the virus will face serious health challenges, possibly including disabling nerve damage.
Zecavati said the stage of pregnancy that the mother was infected could affect how severe the impact is on the child. Exposure to infection in the first trimester of pregnancy -- when the baby's brain, spinal cord, heart and other organs are beginning to form, and when the neural tube along a baby's back is closing -- may lead to more severe problems.
"If exposure to the Zika virus occurs later in pregnancy when major brain structures have been formed, the findings may be different," Zecavati said.
Magnetic resonance imaging (MRI scans) may give clues to when the virus was introduced and which parts of the baby's brain are most affected, she added.
The March of Dimes' McCabe said the CDC is now recommending that women who are pregnant and who have visited one of the countries where Zika virus is spreading should get a medical work-up. "Even if they're trying to get pregnant," he added, seeing a doctor is a good idea.