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Amyotrophic lateral sclerosis (ALS) is characterised by progressive degeneration of upper and lower motor neurones. Clinical symptoms involve weakness, dysphagia, dysarthria, muscle atrophy and fasciculations, hyperreflexia, spasticity, Babinski signs, and clonus. Here we report on two patients with sporadic ALS in whom the disease initially presented with a persistent bitter “metallic” taste.
Patient 1 was a previously healthy 60 year old women. Six months before admission, she noticed a persistent bitter taste, dysarthria, and emotional liability. Several weeks later she noticed a progressive weakness in both legs which spread to both arms within four months. At the time of admission, she had bilateral bulbar weakness, episodes of pathological crying, generalised spasticity, muscle atrophy, weakness, and fasciculations. The plantar reflex was extensor on the left side. The remaining neurological examination was unremarkable.
Patient 2 was a previously healthy 64 year old women. At the time of admission, she reported a four month history of a persistent bitter “metallic” taste confined to the posterior tongue, facial weakness, and clumsiness of the left hand. Neurological examination revealed bilateral bulbar weakness, an increased jaw reflex, slow side to side tongue movement, generalised hyperreflexia, and fasciculations. Spasticity, muscle atrophy, and weakness were predominantly observed in the left arm. The remaining neurological examination was normal.
The patients had not taken prescription or non-prescription drugs during the months preceding the symptoms, or at the initiation of symptoms or at the time of admission. Oral hygiene was good in both cases and xerostomia was not evident. Family history was negative. The occupational and chemical exposure history was unremarkable.
Spatial gustatory function testing with sodium chloride (0.04 and 0.32 M), sucrose (0.07 and 0.32 M), citric acid (0.01 and 0.02 M), and quinine (0.00016 mM) was undertaken. Although both patients described the perception of a bitter taste throughout the examination, the test did not reveal hypogeusia for any quality. In both cases, routine blood chemistry and cerebrospinal fluid studies were normal. Tests for paraneoplastic autoantibodies (Hu, Yo, Ri, Ma, Ta, CV2) were negative. Cranial and spinal magnetic resonance imaging showed mild bilateral atrophy of the precentral gyrus in both patients. Motor evoked potentials revealed slowed central conduction. Peripheral electrophysiological testing showed active denervation, normal nerve conduction, and normal F wave latencies. Thus motor neuropathy with multifocal conduction block, cervical myelopathy, and paraneoplastic motor neurone disease could be excluded. A diagnosis of clinically definite ALS was made, based on the revised El Escorial criteria (http://www.wfnals.org/articles/elescorial1998.htm). Treatment with riluzole and α tocopherol was initiated in both patients.
To our knowledge, dysgeusia has not been described in this disease. The persistent perception of bitter taste developed as an early symptom of the disease in our patients. In this regard, it resembles the dysgeusia known from ciguatera food poisoning, which is thought to produce a bitter taste by blocking sodium channels.1 However, other sensory symptoms were absent, both clinically and electrophysiologically. The chorda tympani branch of the facial nerve carries taste sensations from the anterior two thirds of the tongue, whereas the glossopharyngeal nerve and the vagus nerve innervate the posterior third and the epiglottis. It has been shown experimentally and clinically that anaesthesia of the chorda tympani nerve branch results in intensified perception of bitter taste from the posterior tongue, suggesting that input by way of the chorda tympani normally inhibits the glossopharyngeal and vagus nerve input.2 In fact, spontaneous bitter taste dysgeusia (phantogeusia) similar to that perceived by our patients was observed in the posterior tongue after anaesthesia of the chorda tympani.2 Hence it may be speculated that mild sensory neuropathy of the chorda tympani branches may be responsible for our findings. Sensory signs have indeed been described in ALS. However, if at all, they develop relatively late in the disease.3 Furthermore, the spatial gustatory function test did not reveal hypogeusia confined to a localised region of the tongue in our patients, although the sensitivity of this test for mild gustatory disturbances is probably low.4 Unfortunately, neither patient was available for electrogustometry to further clarify our hypothesis.
Alternatively, the dysgeusia may be of central nervous origin. Both patients presented with bilateral nuclear facial paresis reflecting a prominent bulbar involvement in the disease. Thus it may be speculated that bilateral degeneration of the brain stem solitary tract nucleus may be responsible for the dysgeusia in our patients. Interestingly, dysfunction of the autonomic nervous system—which in part is also regulated by the solitary tract nucleus—has been described in ALS,5 supporting the view that this disease may be a multisystem disorder. Thus dysgeusia may indicate brain stem involvement in the disease. As a bulbar onset of ALS is an important predictor of the disease course,6 our finding may also be of prognostic value. We cannot provide a definite neuroanatomical basis for our observation, but we believe that future studies may be able to address these issues.
Competing interests: none declared
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