The report, aired on the PBS show "Frontline," said Adam Lanza, 20, had been diagnosed with sensory processing disorder (SPD), ABC News reported Wednesday.
The disorder can cause people to over-respond to stimuli and clothing, physical contact, light, sound or food can be unbearable.
Another version of SPD can cause weakness, clumsiness or delayed development of motor skills.
The disorder made Lanza unable to recognize pain and he would shut down if faced with loud noises, confusion or change, the report said.
Show producer Frank Koughan described Lanza as "someone that was afraid of the world."
No one in the medical community knows whether SPD is a distinct disorder or a collection of symptoms of other neurological problems such as attention-deficit hyperactivity disorder.
Neither does anyone know why Lanza shot his mother four times before shooting 20 children and six adults at Sandy Hook Elementary School on Dec. 14.
SPD is a controversial condition. The American Psychiatric Association earlier this year refused to recognize the ailment because there are no distinct criteria for diagnosing it.
The report was a collaboration between "Frontline" and the Hartford (Conn.) Courant.
Ray Liotta sues skin care company over use of likeness
NBC reportedly holds celebs hostage to Jimmy Fallon's show