CLINICAL CASE | Lateral Medullary Syndrome and Dissociated Somatic Sensory Loss
A 69-year-old man suddenly developed vertigo and difficulty walking. He went to the emergency room and, upon examination, was found to have several additional sensory and motor deficits. Here we will only consider the somatic sensory deficits. We will revisit this patient in the case in Chapter 15, when we consider his other neurological deficits.
His neurological examination revealed a striking dissociated pattern of mechanosensory and pain/thermal sensory loss. Facial pain and thermal sensation were largely absent on the left side of his face. Remarkably, pain and thermal sensations on the arm, trunk, and leg were absent on the right side. Figure 6–1A (gray tint) shows the approximate distribution of pain and thermal sensory loss. Mechanosensation was spared bilaterally on the face, limbs, and trunk. Jaw and limb proprioception were also spared.
The patient had an MRI of the head. It was normal except for the medulla (Figure 6-1B), which showed a wedge-shaped lesion, dorsolaterally, on the left side. The corresponding myelin-stained section is shown.
You should be able to answer the following questions based on your reading of the chapter, inspection of the images, and consideration of the neurological signs.
1. What artery supplied the infarcted region in the medulla?
2. Explain why pain is lost ipsilaterally on the face and contralaterally on the limbs.
Key neurological signs and corresponding damaged brain structures Ipsilateral loss of facial pain and thermal senses The posterior inferior cerebellar artery (PICA) supplies the dorsolateral medulla. The infarcted region on the MRI in Figure 6-1B was produced by PICA occlusion, which damaged the spinal trigeminal tract and nucleus at the level of the mid-medulla. The locations of these structures are shown in Figure 6-1B, inset. Tract damage results in loss of most axons from the level of occlusion, caudally. Because damage occurred before decussation, the nociceptive and thermal innervation of the ipsilateral face was eliminated.
Contralateral loss of pain and thermal senses There was also loss of pain and temperature sensation on the contralateral limbs and trunk. This is because PICA occlusion damaged the ascending anterolateral pathway, which decussated in the spinal cord (Figure 6–1B, inset; Figure 6-12B).
Sparing of mechanical sensations and limb and jaw proprioception PICA occlusion spared the medial lemniscus, which carries ascending mechanosensory and limb proprioception information (Figure 6–1B, inset). It also spared trigeminal mechanosensations (touch, vibration sense, jaw proprioception) because the large-diameter fibers that mediate these sensations do not descend within the spinal trigeminal tract. Rather, they synapse on neurons in the main trigeminal sensory nucleus in the pons.