Posttraumatic vocal disarticulation
From Halopedia, the Halo wiki
Posttraumatic vocal disarticulation is a psychological condition involving a human being's ability to speak. After experiencing extreme stress or trauma, a person diagnosed with this condition is rendered completely unable to speak, even if there is no physical injury to their vocal chords. After extended periods of silence an individual suffering from posttraumatic vocal disarticulation may experience significant difficulty speaking coherently even if they overcome the condition on a psychological level.
Spartan-III supersoldier Lucy-B091 was diagnosed with this condition at age 12 after her company was killed in Operation: TORPEDO, leaving her and Tom-B292 as the only survivors out of 300. This shook her terribly and she would remain silent for years afterward. Lucy later overcame the condition as a result of an intense fit of rage, restoring her ability to speak, though she would remain relatively quiet.
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