Abstract
BackgroundThe existence of a prenuclear abduction paresis is still debated.MethodsIn a retrospective design, we identified 22 patients with isolated unilateral (n = 20) or bilateral (n = 2) abduction paresis and electrophysiologic abnormalities indicating rostral pontine and/or mesencephalic lesions. Another 11 patients had unilateral abduction paresis with additional ocular motor abnormalities indicating midbrain dysfunction. Eight of these 11 patients also had electrophysiological abnormalities supporting this location. Electrophysiological examinations in all patients included masseter and blink reflexes (MassR, BlinkR), brainstem auditory evoked potentials (BAEP), and direct current elctro-oculography (EOG).ResultsUnilateral MassR abnormalities in patients with unilateral abduction paresis were seen in 17 patients and were almost always (in 16 of 17 patients) on the side of the abduction paresis. Another 11 patients had bilateral MassR abnormalities. BlinkR was always normal. EOG disclosed slowed abduction saccades in the non-paretic eye in 6 patients and slowed saccades to the side opposite to the abduction paresis in another 5 patients. Re-examinations were done in 27 patients showing normalization or improvement of masseter reflex abnormalities in 18 of 20 patients and in all patients with EOG abnormalities. This was always associated with clinical improvement.ConclusionsElectrophysiologically documented or clinically evident rostral pontine and/or mesencephalic lesions in our patients exclude an infranuclear intrapontine 6th nerve lesion and indicate the existence of an abduction paresis of prenuclear origin. An increased tone of the antagonistic medial rectus muscle during lateral gaze either by abnormal convergence or impaired medial rectus inhibition seems most likely.
Highlights
The existence of a prenuclear abduction paresis is still debated
Lutz's basic neuroanatomical assumption was wrong, the existence of a PINO remained controversial, as a PINO was repeatedly discussed in patients, whose abduction paresis was thought to differ from a 6th nerve palsy
We retrospectively identified 22 patients with isolated unilateral (n = 20) or bilateral (n = 2) abduction paresis, who had electrophysiological abnormalities indicating rostral pontine and/or mesencephalic lesions
Summary
The existence of a prenuclear abduction paresis is still debated. In 1921, Anton Lutz postulated the existence of a prenuclear abduction paresis, the so-called "ophthalmoplegia internuclearis posterior" (posterior internuclear ophthalmoplegia, PINO) [1]. Lutz's basic neuroanatomical assumption was wrong, the existence of a PINO remained controversial, as a PINO was repeatedly discussed in patients, whose abduction paresis was thought to differ from a 6th nerve palsy This included absence of strabismus and diplopia in the primary position [2,3,4,5], adduction nystagmus of the contralateral, i.e. non-paretic eye on lateral gaze [3,4,6], isolated impairment of abduction saccades, i.e. unrestricted abduction with following eye movements [7], preserved abduction sacccades with caloric testing, i.e. with vestibular (caloric) nystagmus [8,9], or preserved abduction with the vestibulo-ocular reflex [9]. Some authors attributed such cases to a lesion of the intrapontine segment of the 6th nerve thereby rejecting a pre- or supranuclear origin of the abduction paresis [11,12,13,14]
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