Poyin  Huang

CASE REPORT:
This 55-year-old, right-handed man suffered from sudden decreased sensitivity of taste. He was unable to differentiate sweetness and saltiness at bilateral anterior parts of tongue. Additionally, there was numbness at the upper palate and the lips. Neurological examination revealed decreased taste sense at both sides of his anterior tongue and decreased pin-prick sensation of the left part of his lips. Brain magnetic resonance imaging (MRI) revealed acute ischemic stroke at the right ventral posteromedial nucleus (VPM). Thus, single antiplatelet therapy was administered. Two weeks later, the symptoms improved significantly and completely recovered without sequelae.

DISCUSSION:
According to contemporary knowledge, the gustatory pathway starts from the taste buds to reach the solitary nucleus, including fibers from the facial, glossopharyngeal, and vagus nerves. Relay neurons from the solitary nucleus form the medial lemniscus, which ascends ipsilaterally through the pons and midbrain, arriving at the thalamus. It then projects to the parietal operculum and insular cortex (2-4). However, whether the nerve fibers go ipsilaterally, cross to contralateral side, or walk bilaterally to the cortices, remains to be discussed. In animal studies, the pathway above the pons varied among species. Both rats’ and macaque monkeys’ central gustatory pathways ascend from the rostral part of the solitary nucleus to the pons ipsilaterally. The pathway differs after the nerve fibers passing the pons. The gustatory fibers in rats branch and ascend bilaterally to the thalami, projecting to the insular cortices. While in the macaque monkeys, the fibers ascend only to the ipsilateral thalamus and insular cortex (4-6). Therefore, whether the pathways of animals could apply to humans is questionable. As for humans, early research supports the theory of laterality that the left hemisphere is more dominant in taste sensorium than the right one. Bilateral dysgeusia occur more frequently in patients with left thalamic stroke, while unilateral dysgeusia is observed after right thalamic stroke (1). However, our reported case got bilateral hypogeusia after right thalamic stroke. It suggested that another possibility of central gustatory pathway in humans might exist. From the database in PubMed, we collected the published articles since 1990, discussing lesions causing taste dysfunction (Table 1). We found that unilateral supratentorial lesions tend to cause bilateral or unilateral taste disorders in different patients, while unilateral brainstem lesions below the midbrain or the pons tend to cause ipsilateral taste problems (Table 2). From another report in 2017 (7), we learned that the most rostral area causing ipsilateral hemiageusia is in the ipsilateral rostral midbrain, which also supports the conclusion from Table 2. Therefore, the convergent point in the gustatory pathway might locate above the rostral midbrain and adjacent to or below the thalamus. Previous research supports the theory of laterality that the left hemisphere is more dominant in taste sensorium than the right one (1). Gustatory fibers from both sides of the tongue ascend ipsilaterally to the midbrain, then going toward the left hemisphere chiefly (Figure 2). However, the gustatory pathway of our patient is different from that of most people. Our reported case, who got bilateral hypogeusia after right thalamic stroke, is one of the rarely seen cases of bilateral taste loss associated with acute right unilateral thalamic infarction (Table 1). We hypothesize that there might be variations in humans. We need to consider the possibility that the gustatory fibers ascend from bilateral sides of the tongue and reach the right thalamus mainly, rather than going to the left thalamus (Figure 2).