| OCR Text |
Show ORIGINAL CONTRIBUTION Neuroanatomic Evidence to Explain Why Bilateral Internuclear Ophthalmoplegia May Result from Occlusion of a Unilateral Pontine Branch Artery C M Fisher, MD Abstract: Microscopic study of serial sections of the pons in two autopsy cases without known neurologic disease showed that the terminal portion of paramedian tegmental pontine arteries may divide to supply the region of the medial longitudinal fasciculus bilaterally. This finding provides the basis for the occurrence of bilateral internuclear ophthalmoplegia with unilateral basilar branch artery occlusion. (/ Neuro- Ophthalmol 2004; 24: 39- 41) Unilateral pontine infarction resulting from occlusion of a penetrating branch of the basilar artery often is accompanied by bilateral internuclear ophthalmoplegia ( INO). Thus far, there has been no anatomic evidence to explain why the INO should be bilateral. The study of two autopsy cases without known neurologic disease has disclosed that the terminal portion of a pontine tegmental paramedian artery may split into two branches to supply the medial longitudinal fasciculus ( MLF) on both sides. Occlusion of a unilateral basilar branch artery could thus explain infarction of the MLF bilaterally. METHODS Serial sections were made of the pons and medulla from the brains of two cases that had come to autopsy and in which the pons and medulla were not diseased according to the history and gross examination. The sections, cut in a horizontal plane, were 8 urn in thickness and were stained with a phosphotungstic acid hematoxylin stain. One case entailed 2100 sections, the second 1700 sections. RESULTS The paramedian tegmental arterial supply to the MLF region on each side could be traced distally down to twigs 20- 30 urn in diameter ( Fig. 1). The source of these arteries From the Neurology Service, Massachusetts General Hospital, Boston Massachusetts. Address correspondence to CM. Fisher, MD, Massachusetts General Hospital, Fruit Street, Boston, MA 02114, VBK 910. could be traced backwards to larger arteries in the basis pontis, but their ultimate origin from arteries in the subarachnoid space could not be identified. The paramedian arteries on each side ran ventrodor-sally in the tegmentum of the pons, slightly removed from the midline, to reach the region of the MLF. They were generally spaced alternately on each side of the midline, at intervals of about 1.5 mm. Most of the paramedian arteries remained strictly lateralized, running to the region of the ipsilateral MLF without crossing the midline. Rarely, a small artery running to the region of the MLF on one side and lying just beneath the ependyma arose from a tegmental branch of a long circumferential artery ( Fig. IB). None of these reached the midline. In two instances in each of the two cases, however, a paramedian artery on one side, on reaching the region of the MLF on its side, divided into two almost equal- sized branches, one of which crossed the midline to run to the FIGURE 1. Schematic illustration of alternative forms of blood supply of medial longitudinal fasciculus ( MLF) regions. A. Usual arrangement of pontine tegmental paramedian arteries supplying the region of the ipsilateral MLF. B. Magnified view of tegmental region showing two paramedian arteries, each supplying the ipsilateral MLF. b = dorsal tegmental branch of a circumferential artery. C. Paramedian artery divides to supply the region of the MLF on both sides of the midline. J Neuro- Ophthalmol, Vol. 24, No. 1, 2004 39 JNeuro- Ophthalmol, Vol. 24, No. 1, 2004 Fisher region of the MLF on the contralateral side, the other branch staying on its own side and running to the ipsilateral MLF ( Fig. 1C, Figs. 2- 4). The ratio of such dividing to non-dividing paramedian tegmental pontine branches is estimated at 1: 7. FIGURE 2. Case 1. Photomicrographs of three adjacent horizontal sections spanning 80 | jm. A. Paramedian tegmental pontine artery ( PA) reaches region of MLF. B. Paramedian artery dilates and begins to bifurcate ( arrow). C. Paramedian artery has formed two separate distended vessels on either side of midline ( arrows). IV = fourth ventricle. FIGURE 3. Case 1. Photomicrograph of two adjacent horizontal sections of pons spanning 24 um at a different level from Figure 2. A. Paramedian artery ( PA) ascends near midline and divides into two branches ( arrows). B. Two branches seen as a single vessel ( arrow) traveling laterally toward both MLF regions. FIGURE 4. Case 2. Photomicrograph of horizontal section of pons of eight micron thickness. Single vessel ( arrow) distal to the bifurcation of a single paramedian tegmental pontine branch artery supplying both MLF regions. 40 © 2004 Lippincott Williams & Wilkins Bilateral Internuclear Ophthalmoplegia JNeuro- Ophthalmol, Vol. 24, No. 1, 2004 DISCUSSION If terminal branching of a paramedian tegmental pontine perforator, as shown in these dissections, were the most common pattern, bilateral INO should be the rule in pontine stroke. Given that unilateral INO is common in pontine stroke, we must assume that bilateral supply of the MLFs is more common than unilateral supply unless a unilateral INO results from blockage of a tiny terminal paramedian branch, an unlikely event. This dissection has shown that terminal branching of the paramedian tegmental pontine perforator is an anatomic variant. This phenomenon could explain the uncommon occurrence of bilateral INO with single perforator occlusion. This finding is of clinical importance. In the past, acute bilateral INO led to the suspicion of occlusion of the basilar artery and bilateral pontine perforating vessels. Because occlusion of the basilar artery represents a threat of more extensive stroke, urgent vascular imaging and anticoagulation often has been considered. The anatomic elucidation presented here should temper that presumption. The terminal branching pattern also could be an explanation for the " one- and- a- half syndrome" of unilateral gaze palsy and ipsilateral INO in cases of pontine hemiplegia resulting from occlusion of a penetrating basilar branch ( 1). Strictly unilateral tegmental pontine infarction as a cause of gaze palsy and ipsilateral adduction deficit has required the hypothesis that the MLF crosses the midline at the level of the sixth nerve nucleus. The crossed fibers then would be included in the infarct. If the tegmental pontine artery bifurcates at the level of the pontine paramedian reticular formation or the sixth nerve nucleus to supply the MLF on both sides, the adduction palsy could result from infarction of contralateral MLF fibers near their origin from the sixth nerve nucleus. Further detailed anatomic study of such cases would be needed to verify this point. REFERENCE 1. Fisher CM, Caplan LR. Basilar artery branch collision: a cause of pontine infarction. Neurology 1971; 21: 900- 5. 41 |
