MLG CREW HAD ALTDEV ON ARR TO LGA IN N90 CLASS E AIRSPACE.

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INBOUND ON THE MILTON 3 ARR WE WERE GIVEN AN ATC INSTRUCTION TO RPT THE VARAZZANO NARROWS BRIDGE IN SIGHT IN ORDER TO FLY VISUALLY UP THE HUDSON RIVER FOR OUR FINAL APPROACH TO RWY 22 AT LGA. THE CAPTAIN AND I CONFERRED ON WHAT WE BELIEVED WAS THE POSITION OF THE VARAZZANO NARROWS BRIDGE AND RPTED IT IN SIGHT. WE WERE THEN CLRED DIRECT TO THE BRIDGE AND INSTRUCTED TO CONTINUE UP THE HUDSON RIVER. APPROX 15 SECONDS LATER WE WERE GIVEN AN ATC INSTRUCTION TO DSND FROM 10000 FT MSL TO 9000 FT MSL; AT WHICH TIME THE CAPT PLACED 9000 FT IN THE ALT WINDOW AND I CONFIRMED THE ALT CHANGE BY POINTING AT THE NEW CLRED ALT IN THE WINDOW. I THEN BEGAN AN AUTOPLT OFF DSCNT TO 9000 FT MSL. AT THIS TIME THE CAPT WENT TO DOUBLECHK THE POS OF THE VARAZZANO NARROWS BRIDGE AND THE HUDSON RIVER TRACK WITH AN AERO CHART WHILE I WENT HEADS DOWN TO XCHK OUR POS ON THE FMS RELATIVE TO THE POS OF THE ARPT. I DID THIS IN AN EFFORT TO FIND THE ARPT ENVIRONMENT AND PREPARE FOR SPD REDUCTIONS AND CONFIGN CHANGES. SHORTLY THEREAFTER; WE RECEIVED AN ONBOARD ALT ALERT AND I THEN MADE AN IMMEDIATE PULL TO REVERSE FLT VECTOR AND LEVEL OFF AT 9000 FT MSL. THE REMAINDER OF THE FLT WENT UNEVENTFULLY AND WE WERE NEVER ADVISED BY ATC THAT A CONFLICT HAD OCCURRED. ONE OF THE FACTORS I BELIEVE LED TO THE DEV WAS INSTRUCTIONS TO FLY A NON CHARTED VISUAL FLT PROC. UNLIKE THE SIRDY FMS RIVER VISUAL PROC TO RWY 31 AT LGA; THERE IS NO CHARTED VISUAL APCH TO RWY 22 AT LGA. IN ADDITION; THERE IS NO FIX OR ILLUSTRATION ON ANY CHARTS OR PROCS TO POSITIVELY IDENT THE POS OF THE VARAZZANO NARROWS BRIDGE. THIS; COUPLED WITH MULTIPLE INTERMEDIATE LEVELOFF INSTRUCTIONS; LED TO A BREAKDOWN IN ALT XCHK IN ORDER TO COMPLY WITH LATERAL NAV REQUIREMENTS. SUPPLEMENTAL INFO FROM ACN 466601: SINCE THERE WAS NO CHARTED VISUAL APCH FOR RWY 22 AND NO 'BRIDGE' NAMES ON ANY APCH CHART; I REACHED INTO MY FLT BAG TO TAKE OUT MY CAPT'S ATLAS TO HELP ME IDENT THE BRIDGE NAMES ALONG THE HUDSON RIVER. IT WAS AT THIS TIME I HEARD THE ALT ALERT AURAL WARNING AND NOTICED THAT THE FO HAD DSNDED BELOW THE ASSIGNED ALT OF 9000 FT MSL.