A 5-year-old boy who died in 2013 from a severe head injury would not have had a “lucid interval” after he was initially injured but would have shown symptoms immediately, a child abuse expert testified Wednesday.

Proving otherwise could become a key part of the defense for Dakota R. Black, 25, who is charged with first-degree reckless homicide for the Oct. 24, 2013, death of Brayden Turnbill, who was found injured and had sustained a severe brain injury two days earlier.

Dr. Barbara Knox, medical director for the American Family Children’s Hospital Child Protection Program, testified that Brayden would have had no “lucid interval” — a period of time between the injury event and onset of symptoms — but would have experienced symptoms of his brain injury “within minutes if not instantaneously.” Those symptoms would have included an altered mental status, loss of consciousness, confusion and lethargy, among others.

She said Brayden’s brain injury was caused by blunt force trauma, possibly from being thrown forcefully onto a bed at the Sun Prairie home where he was living with his mother, Shannon Turnbill, and Black, her boyfriend.

Although Knox said she did not know what caused the injury, being thrown with sufficient force onto the bed could have caused Brayden’s brain to move inside his skull, rupturing blood vessels and causing bleeding on his brain.

Knox said she was present when UW pediatric neurosurgeon Dr. Taryn Bragg performed emergency surgery, and saw his brain “mushroom” through an opening cut into his skull from the intense pressure inside.

Although the bleeding was mitigated, Knox said she could see that parts of his brain tissue were already dead. Bragg testified Tuesday that the pressure continued despite the surgery and was not survivable.

Under cross examination, defense lawyer John Smerlinski questioned Knox’s assertion that there had been little or no lucid interval after Brayden was injured.

Had there been a lucid interval after the initial injury, it would call into question when the injury occurred. An earlier injury would mean that it may have happened at a time before Black was the only adult alone at home with Brayden.

But Knox said that lucid intervals are exceedingly rare with the kind of subdural brain bleeding that Brayden sustained.

Lucid intervals are much more common with epidural bleeds, Knox said, of the type sustained by actress Natasha Richardson, who appeared fine but for a headache after a skiing accident but later collapsed and died.

“These children go down quickly and they stay down,” Knox said.

Knox also testified that the constellation of bruising found on Brayden’s abdomen and buttocks was indicative of abuse, though she said there is no dependable way to tell when the bruising had occurred.

Knox said that Shannon Turnbill told her that she had not noticed any bruising on Brayden’s chest when she helped him dress for school that morning.

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