The medial longitudinal fasciculus (MLF) is heavily myelinated composite tract that connects the nuclei of the third, fourth, sixth and eighth cranial nerves (1). This pathway provides a neural mechanism for simultaneous contraction of the lateral rectus muscle on the one side and the medial rectus muscle on the opposite side, require for conjugate lateral gaze (2, 3). We present a case showing the whole courses of the MLF pathways between abducens and oculomotor nucleus including decussation at the midbrain level on MRI in a patient with inflammatory demyelinating disorder.

A 9-year-old boy presented to the pediatric department with one-month history of ophthalmoplegia and diplopia. The physical and neurologic examination revealed inward deviation and abduction failure of the left eyeball, and weak abduction movement of right eye ball at present. The patient had any prodromal illness and had no other neurologic or laboratory abnormality. T2 weighed image of MRI (slice thickness = 4 mm, FOV = 165 × 220, Matrix =512 × 198, no gap, TR/TE= 3588/99) showed bilateral contiguous high signal intensities in the dorsal pons and midbrain, illustrating ascending fibers from the abducens nucleus to the occulomotor nucleus (Fig. 1) The bilateral fasciculi ran medially and decussated at the lower level of midbrain. The patient had conservative therapy and discharged 3 days later. The symptoms were fully recovered 3 months later.

Direct projection to the occluomotor nuclear complex arise from parts of the vestibular nuclei, the interstitial nucleus of Cajal, abducens internuclear neurons, part of the perihypoglossal nuclei, the rostral interstitial nucleus of the MLF, and the pretectal olivary nucleus (4). The MLF is a critically important conduit for many brainstem pathways, and constitutes the final common pathway for all major classes of conjugate eye movements including saccades, smooth pursuit, and vestibule-ocular reflexes, including semicircular and otolith mediated ocular motor reflexes. The paired fasciculus plays a key role in coordinating eye movements (5). Pairs of cranial nerve III, IV, VI are interconnected via the MLF, which transmits vital information for the purpose of achieving synchronization and conjugacy of eye movements to a visual target. Within this system are both the excitatory as well as reciprocal inhibitory projections that serve to precisely regulate the interplay between agonist and antagonist muscles of the eyes (3).

Injury to the MLF leads to abnormalities in eye movements that can be measured with high precision by oculography, making it an ideal eloquent pathway to study imaging/function correlates (2). Internuclear ophthalmoparesis (INO) is characterized by impaired horizontal eye movement with weak adduction of the affected eye and abduction nystagmus of the contralateral eye (6). INO is one of the most localizing brainstem syndromes, resulting from a lesion in the MLF in the dorsomedial brainstem tegmentum of either the pons or the midbrain. INO is a distinct clinical sign and seen in association with a lesion in the MLF between the third and sixth cranial nerve nuclei. Main cause of INO is stroke and multiple sclerosis, which are followed by other cause compose more than one quarter of the causes. The other causes of INO include tumor, infection, and traumatic injury (7).

Due to its high spatial resolution, magnetic resonance imaging (MRI) has allowed us to depict in vivo, the anatomic organization of the human oculomotor nerve complex, the MLF, and related structures in the brainstem. Frohman et al. (6) showed that all the patients with confirmed INO had evidence of high signal intensity abnormalities in the brainstem tegmentum in the region of MLF, which was best identified with proton density imaging. Despite the frequency and conspicuity of MLF lesions on conventional MRI in MS patients with INO, MLF is not exactly distinguished from other blended white matters tracts in pons and midbrain (8). Recently, advanced MR technique such as diffusion tensor imaging, automated brainstem coregistration and Ultra high field imaging may also provide important information for MLF, but the MLF was not specifically indicated (9, 10). In our patient, he did not have any prodromal illness and had no other neurologic or laboratory abnormality. MRI did not show any other parenchymal abnormality and MRS was normal. All the other causes of other than demyelinating pathology could be excluded. So, we considered the lesion as an isolated inflammatory demyelinating lesion of the brainstem. T2 weighted MRI showed the whole courses of the MLF pathways between abducens and oculomotor nucleus including decussation at the midbrain level.

We present a patient with isolated unilateral INO probably due to an isolated inflammatory demyelinating lesion of the brainstem. The MR finding of the illustration of the connection between 3rd and 6th nucleus by MLF can be educational. Until now, to the best of our knowledge, the connection between abducens and occulomotor nucleus by MLF has not been completely delineated on MRI.