A 20-year-old Fairfax County man who is serving a 50-year sentence for shooting his teen-aged girlfriend filed a $10 million lawsuit yesterday against two psychiatrists who treated him, claiming they failed to diagnose his potentially violent behavior.

The suit, filed in Fairfax Circuit Court, alleges that Charles R. Brewer Jr. shot 17-year-old Joy Keo because doctors did not identify and treat a life-long organic brain disease that subjected him to violent fits.

"The doctors . . . knew that he (Brewer) was likely to commit acts of violence in the future if not properly hospitalized, tested and treated," the suit claims. "Instead, they negligently discharged him and directed that he take Lithium," a drug which the suit contends exacerbated Brewer's explosive behavior.

Brewer was convicted of three felony charges and one misdemeanor in the Aug. 28, 1982, shooting of Keo. One of the four bullets that Brewer fired into her body, severed Keo's spinal cord and left her a quadriplegic.

In June, Keo filed a $30 million civil lawsuit against Brewer, two psychiatrists who had treated him and the manufacturer of the handgun that he used in the shooting.

The two psychiatrists named in both suits are Dr. David L. Charney of Alexandria and Dr. Martin H. Stein of the Dominion Psychiatric Treatment Center in Falls Church. Brewer's suit also names the center as a defendant.

Neither of the doctors could be reached yesterday for comment.

Charles Brewer said his son has been examined by "literally dozens of doctors" since he was 2 years old, and no certain diagnosis evolved.

Throughout his life, the younger Brewer "for no apparent reason . . . would become impulsive and explosive," the suit alleges.

Doctors at one point diagnosed the youth as a manic-depressive, according to his father. Brewer said he placed his son in the Dominion center several weeks prior to the Keo shooting and that tests there "revealed for the first time significantly abnormal electrical patterns indicating Brewer suffered from an underlying organic brain disorder."

The suit alleges that Stein and Charney ignored those tests and failed to inform the Brewer family of their importance.

Keo, who is confined to a wheelchair, is scheduled to attend George Mason University this fall.