CLINICAL CASE | Central Tegmental Tract Lesion and Unilateral Taste Loss
A 25-year-old woman suddenly complained of diplopia (double vision) and impaired sense of taste. On examination, taste was probed carefully by applying solutions of different qualities (salty, sweet, acidic, and bitter) to the tongue. The results indicated a loss of all tested qualities of taste on the right side of the tongue. A taste researcher in the Otolaryngology Department was contacted, and the patient was subsequently examined using an electronic device to examine taste thresholds. This confirmed loss of taste on the right half of the tongue and soft palate.
A T1-weighted MRI with gadolinium enhancement (Figure 9–1A) revealed a focal lesion in pontine tegmentum. Figure 9–1B is a closely corresponding myelin-stained section. An MRI from a healthy person (Figure 9–1C) shows the location of the pons in parts A and B, in relation to the brain in the skull. Note that the dorsal brain surface is down in all of these images. The lesion in A corresponds to the region of the central tegmental tract. The lesion also includes parts of the superior cerebellar peduncle, which transmits mostly the output of part of the cerebellum for movement control, and the medial longitudinal fasciculus, which contains axons that coordinate eye movements. Here we will only consider the loss of taste and the central tegmental tract lesion. The ocular control impairments will be considered in another case in Chapter 12. On the basis of the MRI and additional tests, the patient was diagnosed with multiple sclerosis, a demyelinating disease.
Answer the following questions.
1. Why is unilateral taste loss more likely a result of a peripheral than central lesion?
2. Why is loss of taste ipsilateral to the lesion?
3. What key pontine gustatory structure is likely to be damaged in the patient?
Key neurological signs and corresponding damaged brain structures Peripheral versus central First consider that the three nerves that supply taste buds each have a limited distribution on the tongue (see Figure 9–4). Damage to a single nerve likely would result in partial taste loss, such as only on the anterior two thirds of the tongue with damage to a branch of the facial nerve. Thus, a peripheral lesion is unlikely. Next consider that central sensory systems receive convergent input from their various peripheral components, so that a system on each side will represent completely the peripheral receptive sheet from which it receives information (eg, the homunculus, Figure 4–9; indicates a complete contralateral body representation for mechanosensations). The three nerves supplying taste buds converge upon the rostral solitary nucleus.
Ipsilateral taste loss The gustatory pathway, unlike the other sensory pathways, is ipsilateral. Thus, the loss of taste likely involves lesion somewhere along this central path.
Critical structures The projection from the solitary nucleus ascends in the central tegmental tract, and terminates in the parvocellular division of the ipsilateral ventral posterior medial nucleus ...