Internuclear ophthalmoplegia plus ataxia indicates a dorsomedial tegmental lesion at the pontomesencephalic junction.

Internuclear ophthalmoplegia (INO) indicates a lesion involving the medial longitudinal fasciculus (MLF) that interconnects the abducens nucleus and medial rectus subnucleus of the oculomotor nuclear complex. In fact, rostral-caudal localization value of the INO is often limited except when it accompanies symptoms and signs owing to involvement of nearby structures. Ataxia is often observed in lesions involving the cerebellum or the fibers to and from it anywhere in the brainstem. Herein, we sought to determine the localizing value of INO plus ataxia in the rostrocaudal axis of the brainstem. Thirty patients with INO plus limb or truncal ataxia were subjected to analyses. For comparison, 20 patients with isolated INO without any ataxia served as the control. We determined the lesion extent in the MRIs responsible for INO plus ataxia using a probabilistic lesion mapping and subtraction analysis and analyzed the neuro-otologic findings using video-oculography. In patients with INO with limb or truncal ataxia, the responsible lesions were mostly restricted to the paramedian tegmentum at the pontomesencephalic junction. In contrast, the lesions causing isolated INO without ataxia were mostly located in the caudal or mid-pontine area. The rostro-caudal distribution of the lesions was similar among the patients with only limb ataxia (n = 3), both limb and truncal ataxia (n = 10), and only truncal ataxia (n = 17). INO plus ataxia indicates a lesion involving the MLF at the pontomesencephalic junction. Damage to the brachium conjunctivum or mesencephalic locomotor region may explain the ataxia in association with INO in lesions involving this area.