Yet another parietal lobe syndrome, Dejerine-Mouzon, is featured by a severe impairment of the primary modalities of sensation (pain, thermal, tactile sense, but usually with relative sparing of vibratory sense) over the contralateral half of the body. Motor paralysis is variable; with partial recovery, there may be a clumsiness that resembles cerebellar ataxia. Because the sensory disorder simulates that caused by a thalamic lesion, it was called pseudothalamic by Foix and coworkers. Hyperpathia, much like that of the Dejerine-Roussy syndrome (see above), has also been observed in patients with cortical–subcortical parietal lesions. The pseudothalamic syndrome was related by Foix and colleagues to a sylvian infarct; Bogousslavsky and associates traced it to a parietal infarct caused by occlusion of the ascending parietal branch of the middle cerebral artery. In each of the aforementioned parietal lobe syndromes, if the dominant hemisphere is involved, there may be aphasia, bimanual tactile agnosia, or a
Gerstmann syndrome; with nondominant lesions, there may be anosognosia (see Chap. 21).
Often with parietal lesions, the patient’s responses to sensory stimuli are variable. A common mistake, as emphasized by Critchley, is to attribute this abnormality to hysteria (see also below under “Sensory loss due to Suggestibility and Hysteria”). A lesion confined to only a part of the parietal cortex (the best examples have been caused by glancing bullet or shrapnel wounds of the skull) may result in a circumscribed loss of superficial sensation in an opposite limb, mimicking a root, or peripheral nerve lesion.
Sensory Loss due to Suggestibility and Hysteria (See Also Chap. 47)
The possibility of suggesting sensory loss to a patient is a very real one, as has already been indicated. Hysterical patients may complain of a complete hemianesthesia—sometimes with the anatomically implausible findings of reduced hearing, sight, smell, and taste on the same side—as well as impaired vibration sense over only half the skull and sternum, most of these being anatomic impossibilities. A frequently used test to disclose this feature is performed by placing a vibrating tuning fork on each side adjacent to the middle of the forehead. The transmission of vibration through the bone ensures that loss of sensation on one side of the midline is not possible. Anesthesia of an entire limb or a sharply defined sensory loss over part of a limb, not conforming to the distribution of a root or cutaneous nerve, may also be observed. The diagnosis of hysterical hemianesthesia is best made by eliciting the other relevant symptoms of hysteria or, if this is not possible, by noting the discrepancies between the sensory loss displayed by the patient and that which occurs as part of the known, anatomically verified sensory syndromes.
Sometimes, in a patient with no other neurologic abnormality or in one with a definite neurologic syndrome, one is dismayed by sensory findings that are completely inexplicable and discordant. In such cases, one must try to reason through to the diagnosis by disregarding the sensory findings or approach the finding as revealing a second disorder such as a neurofibroma of a nerve root.
Laboratory Diagnosis of Somatosensory Syndromes