Source : Webster's Revised Unabridged Dictionary (1913)

Monomaniac \Mon`oma"ni*ac\, Monomaniacal \Mon`oma"ni*a*cal\, a.
   [Cf. F. monomaniaque.]
   Affected with monomania, or partial derangement of intellect;
   caused by, or resulting from, monomania; as, a monomaniacal
   delusion.

Source : WordNet®

monomaniacal
     adj : obsessed with a single subject or idea