Test Your Knowledge - Vertical Saccadic Palsy Due to Bilateral riMLF Infarctions

This is a 30-year-old who was found minimally responsive on the lounge floor of an ice skating rink. He was brought to the ED, where he had a GCS score of 8 (where 15 is normal) for poor responsiveness. His ocular motor exam is shown in the video. Regarding Finding #1, which of the following is false? A. When slow saccades are seen in a patient with myasthenia gravis (MG) or a peripheral ocular motor palsy, they should be proportional to the amount of ophthalmoparesis and cannot be overcome by the vestibulo-ocular reflex (VOR). B. Bilateral upgaze paresis can result from a unilateral oculomotor nucleus lesion. C. Unilateral downgaze paresis can result from a unilateral oculomotor nucleus lesion. D. The most likely localization is the interstitial nucleus of Cajal (INC). E. A unilateral rostral interstitial medial longitudinal fasciculus (riMLF) lesion should not abolish upward saccades. Answers A. True. Slow saccades result from lesions involving the ocular motor nuclei, fascicle, peripheral course of the nerve, or involving the neuromuscular junction (MG) or extraocular muscle (myopathy). Patients whose motility deficits are due to restrictive processes (e.g., thyroid eye disease) usually have saccades with normal velocity that abruptly stop. Since the VOR pathways project to their respective ocular motor nuclei to move the eyes in the plane(s) of the stimulated semicircular canal, slowing and limitation of eye movements will be equal with saccades, VOR, and smooth pursuit with a nuclear/infranuclear process or in a condition like MG. However, with a supranuclear vertical saccade palsy due to bilateral riMLF injury, vertical VOR and smooth pursuit may be spared, as in this patient. B. True. A lesion involving one oculomotor nucleus can cause bilateral superior rectus (SR) paresis. Ex) the fibers destined for the right SR originate in the contralateral (left) SR subnucleus, exit the left oculomotor nucleus, decussate to the right side and then travel through the ipsilateral (right) oculomotor nucleus to the right SR. With a lesion involving the left oculomotor nucleus, the left SR subnucleus can be affected (causing right SR paresis) and the fibers that have already crossed from the right SR subnucleus and are traveling through the left oculomotor nucleus can also be affected (causing left SR paresis). This combination can result in bilateral upgaze paresis from a unilateral midbrain lesion. C. True. A unilateral lesion of the oculomotor nucleus will only cause ipsilateral inferior rectus paresis because these fibers do not decussate. D. False. The INC is located in the midbrain and is responsible for vertical and torsional gaze holding. A unilateral lesion may cause a skew deviation (contraversive ocular tilt reaction and hypertropic eye will be ipsilesional), vertical gaze-evoked nystagmus and spontaneous torsional nystagmus, with top poles beating toward the ipsilesional ear. E. True. A unilateral riMLF lesion will mainly result in poor downward saccades with relatively spared upward saccades, which relates to the dual innervation of the upward saccade pathways. When there are complete, bilateral riMLF lesions, as in this patient, downward and upward saccades will be abolished. Regarding Finding #2, which of the following is true?A. Given Finding #1, the most likely localization for Finding #2 is the pons. B. The abducting saccades seen with a horizontal optokinetic stimulus are consistent with a lesion involving the fascicle of the 6th nerve. C. Eyelid retraction, 3rd nerve palsy, and/or pupillary light-near dissociation may be associated features. D. Abduction will not improve with the horizontal vestibulo-ocular reflex (VOR). E. With alternate covering testing, the patient will have an exotropia. Answers: A. False. The patient has a complete vertical saccadic palsy, which localizes to bilateral riMLF (rostral midbrain). Instead of having separate pontine ischemia, it is more likely that this patient has pseudoabducens palsies related to disrupted descending inhibitory convergence pathways that travel through bilateral thalalmus and decussate in the mesodiencephalic region. When these inhibitory pathways are damaged, there's disinhibition of the medial rectus (MR) muscles, causing increased MR tone and an excess of convergence (the most accepted theory1). B. False. Abducting saccades should be slow with lateral rectus (LR) weakness related to injury to the 6th nucleus or infranuclear segments. In this patient, with an optokinetic stimulus the abducting saccades appear normal, or better than would be expected given abduction deficits related to lateral rectus weakness. C. True. Patients with unilateral or bilateral pseudoabducens palsies often have other associated neuro-ophthalmic features relating to midbrain injury including the dorsal midbrain (Parinaud's) syndrome (light-near dissociation, eyelid retraction [Collier's sign], convergence-retraction nystagmus, upgaze palsy) or nuclear/fascicular 3rd nerve palsies. D. False. Because pseudo-abducens palsies are "supranuclear" - i.e., 6th nucleus and nerves are normal and vestibular pathways are spared - commonly, the abduction deficit can be overcome with vestibular stimuli such as the bedside VOR, rapid impulses of the head, or even calorics. In some cases, deficits cannot be overcome at the bedside, and it may be that the stimulus is just not strong enough1 - e.g., no change with bedside VOR testing, but normal abduction can be seen with cold water calorics (demonstrated as the slow phase toward the ear irrigated with ice water). E. False. Since the inhibitory convergence pathways are injured, this results in convergence excess and an esotropia. Final diagnosis: Head trauma related to a fall while ice skating causing left vertebral artery dissection leading artery-to-artery embolus and top of the basilar syndrome. His vertical saccadic palsy resulted from bilateral riMLF ischemia (also had no ipsitorsional slow phases when tilting his head to the right and left, which is another feature of riMLF injury) while bilateral pseudoabducens palsies resulted from bilateral damage to the descending inhibitory convergence pathways in the mesodiencephalic region. His bilateral strokes were in the distribution of the artery of Percheron, which is a rare anatomic variant consisting of a single trunk arising from one posterior cerebral artery that supplies bilateral midbrain/thalamic structures. 1. Kaas BM, Zee DS, Kheradmand A, Gold DR. Midbrain Infarction Resulting in Bilateral Pseudoabducens Palsies. Neurologist 2017;22:72-76