They report in the Emergency Medicine Journal on 13 individuals who were treated for such injuries.

"Although most of our patients showed improvement in visual acuity, there were severe injuries, with the potential for severe ocular morbidity," write Dr. Jon Durnian, of Royal Liverpool University Hospital, UK, and colleagues.

"We conclude that there is sufficient injury caused by this prank to warrant a public health message," they add.

The researchers note that in the United States, five cases of assault with thrown eggs were reported around Halloween in 1988, two of which resulted in permanent vision loss. Further, as recently as 2003, three people in Ireland experienced severe loss of vision due to injury from thrown eggs.

Eggs, which are similar in size to squash balls, are much heavier, and can act as missiles, "causing severe blunt injury even when thrown by hand," the researchers write.

To draw more attention to the topic, Durnian and colleagues analyzed eye injuries among patients that came to the St. Paul's Eye Unit at the hospital between November 1, 2004 and December 31, 2005.

Among a total of 18,651 admissions to the unit, 13 of them (0.07 percent) were due to eye injuries from thrown eggs, the investigators found.

Most (12) of these patients were men, 28 years old on average. Three patients had minor injuries, such as bruising and bloodshot eyes, which were treated with antibiotics, and two had intermediate injuries.

The remaining eight patients had severe injuries, including tears to the retina and increased eye pressure, which can lead to permanent vision loss.

One patient, a 27-year-old man, was hit by an egg while riding in a car. He experienced significantly decreased vision, which remained poor more than two months after the incident, and permanent damage to the retina. What's more, additional injuries from the thrown egg put him at lifelong risk of developing glaucoma, the report indicates.

Another man, injured when an egg was thrown from a moving vehicle, complained of "pain and mildly reduced vision," the researchers report. He showed slow improvement during subsequent visits, but upon sneezing one day more than a month after the incident, he experienced an immediate decrease in vision. His treatment ultimately required several major surgical procedures, but he eventually recovered completely.

Overall, the egg-throwing incidents occurred throughout the year, but several were clustered in October, "which is coincident with the Halloween season," the researchers note.

Throwing raw eggs at strangers is becoming increasingly common on Halloween and "mischief night," yet "the hazards of egg throwing, on and around Halloween, seem to go unnoticed," they add.

"This sort of mischief can be interpreted as innocent, but as seen in our series, can lead to severe ocular morbidity," Durnian and colleagues conclude.