CLINICAL CASE | Unilateral Taste Loss
A 25-year-old woman suddenly complained of diplopia (double vision) and impaired sense of taste. Diplopia presented as an inability to adduct the right eye on horizontal gaze to the left. On examination, taste was probed carefully by applying solutions of different qualities (salty, sweet, acidic, bitter and umami) 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 lesion in the pontine tegmentum. 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 dorsal tegmentum, where many tracts are located. On the basis of the MRI and additional tests, the patient was diagnosed with multiple sclerosis, a demyelinating disease. The lesion corresponds to focal demyelination. Here we will only consider the impact of the lesion on loss of taste. The ocular control impairments will be considered in another case in Chapter 12.
You should be able to answer the following questions based on your reading of the chapter, earlier readings, inspection of the images, and consideration of the neurological signs.
1. What are the major tracts within the lesioned/demyelinated region and what are the general functions of these tracts?
2. How might demyelination produce an impairment in the function in the key tract for taste?
3. Why is the loss of taste ipsilateral to the lesion?
4. What key pontine gustatory structure is likely to be damaged in the patient? Key neurological signs and corresponding damaged brain structures Peripheral versus central lesions and the distribution of taste loss
Although uncommon, the patient has unilateral taste loss. To understand this, 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). Critical brain stem gustatory structures
The three nerves supplying taste buds converge upon the rostral solitary nucleus. The projection from the solitary nucleus ascends in the central tegmental ...