A rare genetic neuro-ophthalmological disease with characteristics of congenital fourth cranial nerve palsy, manifesting with hypertropia in side gaze, unexplained head tilt, acquired vertical diplopia and progressive increase in vertical fusional vergence amplitudes with prolonged occlusion. Facial asymmetry (for example hemifacial retrusion, upward slanting of mouth on the side of the head tilt, mild enophthalmos of paretic eye) and superior oblique tendon abnormalities (such as absence, redundance, misdirection) are frequently associated. Some asymptomatic cases have been reported.