▁used ▁to ▁reve al ▁and ▁diagn ose ▁the ▁presence ▁of ▁dis soci ated ▁vertical ▁deviation , ▁although ▁any ▁( or ▁no ) ▁amount ▁of ▁dis soci ative ▁occ l usion ▁may ▁also ▁prompt ▁it ▁to ▁occur . ▁ ▁The ▁patient ▁is ▁asked ▁to ▁look ▁at ▁a ▁light . ▁One ▁eye ▁is ▁covered ▁and ▁a ▁filter ▁is ▁placed ▁in ▁front ▁of ▁the ▁other ▁eye . ▁The ▁density ▁or ▁opacity ▁of ▁this ▁filter ▁is ▁gradually ▁increased , ▁and ▁the ▁behaviour ▁of ▁the ▁eye ▁under ▁the ▁cover ▁is ▁observed ▁not ▁of ▁the ▁eye ▁beneath ▁the ▁filter . ▁Initial ly , ▁if ▁DVD ▁is ▁present , ▁the ▁covered ▁eye ▁will ▁have ▁elev ated , ▁but ▁as ▁the ▁filter ▁opacity ▁is ▁increased ▁the ▁eye ▁under ▁the ▁cover ▁will ▁gradually ▁move ▁down wards . ▁This ▁B iel sch ow sky ▁phenomen on ▁is ▁present ▁in ▁over ▁ 5 0 % ▁of ▁persons ▁with ▁prominent ▁DVD , ▁all ▁the ▁more ▁if ▁the ▁DVD ▁is ▁asym metric ▁and ▁amb ly op ia ▁is ▁present ▁as ▁well . ▁ ▁The ▁B iel sch ow sky ▁phenomen on ▁is ▁also ▁present ▁in ▁the ▁horizontal ▁plane ▁in ▁patients ▁with ▁prominent ▁D HD ▁( dis soci ated ▁horizontal ▁deviation ). ▁ ▁D iffer ential ▁diagn osis ▁ ▁DVD ▁is ▁often ▁mistaken ▁for ▁over - action ▁of ▁the ▁inferior ▁ob lique ▁extra - oc ular ▁mus cles . ▁DVD ▁can ▁be ▁revealed ▁on ▁oc ular ▁movement ▁testing ▁when ▁one ▁eye ▁is ▁occ l ud ed ▁by ▁the ▁nose ▁on ▁later al ▁gaz e . ▁This ▁eye