Characteristics of Abnormalities in Somatosensory Submodalities Observed in Residents Exposed to Methylmercury

Abstract

Hundreds of thousands of people living along the Yatsushiro Sea coast have been exposed to methylmercury from the contaminated water of the Chisso factory in Minamata. The most common neurological disorder caused by methylmercury is somatosensory disturbance, but very few studies have been conducted in the world to determine its pathophysiology and origin, including the Japanese cases, which have produced numerous intoxicated individuals. We have already shown in previous studies the body part where the disorder occurs and that its cause is not peripheral nerve damage but damage to the parietal lobes of the cerebrum. We reanalyzed the results of subjective symptoms, neurological findings, and quantitative sensory measurements in 197 residents (63.2 ± 10.7 years old) from contaminated areas exposed to methylmercury from seafood and 130 residents (63.7 ± 9.3 years old) from control areas, the same subjects as in previous studies, to determine the characteristics of somatosensory disturbance in detail. The most commonly affected sensory modalities were superficial peripheral touch and pain in the extremities, followed by two-point discrimination and deep senses, and in the most severe cases, full-body sensory dysfunction and impairment of all sensory submodalities. The severity of sensory submodalities correlated with each other but not with peripheral nerve conduction test indices, further confirming the correctness of our assertion about the responsible foci of sensory disturbance. The health effects of chronic methylmercury toxicosis can be elucidated by a detailed examination of sensory deficits.

Keywords: methylmercury; nerve conduction study; somatosensory disturbance; somatosensory quantification; somatosensory submodalities.

Figure 1

Examples of somatosensory disturbance types due to methylmercury exposure.

Figure 2

Threshold of “minimal tactile sense” in each touch disturbance type by examination. Lower lip: n.s.: V/IV, II/I, II/Control, p < 0.05: I/Control, p < 0.01: all others. Chest: n.s.: V/IV, II/I, p < 0.01: all others. Index finger (R): n.s.: V/IV, II/I, p < 0.01: all others. Index finger (L): n.s.: V/IV, II/I, p < 0.01: all others. Great toe (R): n.s.: V/IV, p < 0.05: II/I, p < 0.01: all others. Great toe (L): n.s.: V/IV, II/I, p < 0.01: all others.

Figure 3

Threshold of “vibration sense” in each touch disturbance type by examination. Chest: n.s.: V/IV, II/I, p < 0.01: all others. Wrist (R): n.s.: V/IV, II/I, p < 0.05: V/III, p < 0.01: all others. Wrist (L): n.s.: V/IV, II/I, p < 0.05: III/I, p < 0.01: all others. Ankle (R): n.s.: V/IV, II/I, p < 0.05: III/I, p < 0.01: all others. Ankle (L): n.s.: V/IV, II/I, p < 0.05: III/II, III/II, p < 0.01: all others.

Figure 4

Threshold of “position sense” in each touch disturbance type by examination. Index finger (R Upper): n.s.: V/IV, II/I, II/Control, p < 0.05: IV/III, p < 0.01: all others. Index finger (R Lower): n.s.: V/IV, II/I, II/Control, I/Control, p < 0.05: V/III, p < 0.01: all others. Index finger (L Upper): n.s.: V/IV, II/I, II/Control, p < 0.01: all others. Index finger (L Lower): n.s.: V/IV, II/I, p < 0.05: II/Control, I/Control, p < 0.01: all others. Great toe (R Upper): n.s.: V/IV, II/I, p < 0.05: III/I, II/Control, I/Control, p < 0.01: all others. Great toe (R Lower): n.s.: V/IV, II/I, p < 0.05: III/I, p < 0.01: all others. Great toe (L Upper): n.s.: V/IV, II/I, I/Control, p < 0.05: III/I, p < 0.01: all others. Great toe (L Lower): n.s.: V/IV, II/I, p < 0.05: I/Control, p < 0.01: all others.

Figure 5

Threshold of “two-point discrimination sense” in each touch disturbance type by examination. Lower lip: n.s.: II/I, p < 0.01: all others. Index finger (R): p < 0.05: V/IV, II/I, p < 0.01: all others. Index finger (L): n.s.: II/I, p < 0.05: V/IV, I/Control, p < 0.01: all others.

Figure 6

Threshold of visual field by Goldmann’s perimeter in each touch disturbance type by examination. Right otolateral: p < 0.01: IV/III, IV/II, IV/I, p < 0.05: V/III, V/II, V/I, n.s.: all others. Right nasal: p < 0.01: IV/III, IV/II, p < 0.05: V/III, IV/I, n.s.: all others. Left otolateral: p < 0.01: p < 0.01: IV/III, IV/II, IV/I, n.s.: all others. Left nasal: p < 0.01: IV/I, p < 0.05: IV/III, IV/II, n.s.: all others.

Figure 7

Auditory acuity by audiometer in each touch disturbance type by examination. Right ear: n.s.: V/III, III/I, II/I, p < 0.05: V/IV, V/I, p < 0.01: all others. Left ear: n.s.: V/IV, III/I, II/I, p < 0.01: all others.