A special joint session of the Italian parliament today voted to drop impeachment-type proceedings against Premier Francesco Cossiga that had threatened the political survival of the current three-party government and the Sardinian Christian Democrat.

The parliament, meeting for the first time in its history to consider remanding a prime minister to the constitutional court, voted 507 to 416 to defeat a motion brought by the Communist Party to reopen an investigation of charges that Cossiga illegally revealed official secrets and thereby helped the terrorist son of a fellow Christian Democrat to avoid arrest.

Closing five days of debate, often conducted before a half-empty chamber, the country's deputies and senators also voted 535 to 370 to drop the charges and close the case.

A third vote on another charge of aiding and abetting a crime, was canceled by joint agreement when it became clear that it had weak support.

Today's vote brings to a close a case that at times threatened to become a full-scale scandal and that refueled an old argument here about the origins of political violence. The case broke in mid-May when it became known that Roberto Sandalo, a young terrorist arrested in Turin, had told interrogators that help form the top had enabled the 28-year-old son of ranking Christian Democrat Carlo Donat-Cattin to flee the country and elude arrest.

The "Donat-Cattin affair," as the case was dubbed, has been steadily exploited by the Communists. Italy's largest opposition party first used the case as an issue in the campaign for important regional elections in early June.

The Communists then decided to discount an initial acquittal of Cossiga by the 20-member parliamentary investigating commission by using a parlimentary rule allowing a case to be reported when acquittal is based only on a simple majority.

Despite their strong law-and-order position, the Communists have often been blamed for the origin and spread of political violence here. The accusations against Cossiga and Donat-Cattin enable them to emphasize the fact that some terrorists come from Christian Democratic backgrounds as well as to beef up a vigorous opposition designed to eventually overthrow the present government.

Following his arrest in April, Sandalo told investigators that Donat-Cattin had asked him to tell his son that he had been named in another terrorist's confession and an arrest warrant was in the works.Furthermore, Sandalo said Donat-Cattin had cited Cossiga as the source of his information.

Cossiga repeatedly denied the charges although he admitted seeing the elder Donat-Cattin, and this week the government introduced new evidence from antoher jailed terrorist that Donat-Cattin's son Marco learned that he was wanted by the police earlier from leaks printed in the Italian press.

The current three-party government controls a majority in the parliament and Cossiga's acquittal had been prediceted. Nevertheless, the former law professor did not wait for the final results. Cossiga quietly left the parliament after this morning's first vote.