Somatosensory disturbance by methylmercury exposure

ARTICLE IN PRESS

Environmental Research 107 (2008) 6–19 www.elsevier.com/locate/envres

Somatosensory disturbance by methylmercury exposure Shigeru Takaokaa,, Yoshinobu Kawakamia, Tadashi Fujinoa, Fumihiro Oh-ishib, Fukuo Motokurac, Yoshio Kumagaid, Tetsu Miyaokae a

Minamata Kyoritsu Hospital, Sakurai-cho 2-2-12, Minamata 867-0045, Japan b Kuwamizu Hospital, Kuwamizu 1-14-41, Kumamoto 862-0954, Japan c Kagoshima Seikyo Hospital, Taniyama Chuo 5-20-10, Kagoshima 891-0141, Japan d Chidoribashi Hospital, Chiyo 5-18-1, Hakata-ku, Fukuoka 812-0044, Japan e Department of Computer Science, Faculty of Science and Technology, Shizuoka Institute of Science and Technology, Toyosawa 2200-2, Fukuroi 437-0032, Japan Received 31 January 2007; received in revised form 30 March 2007; accepted 22 May 2007 Available online 20 July 2007

Abstract Minamata disease is methylmercury poisoning from consuming fish and shellfish contaminated by industrial waste. The polluted seafood was widely consumed in the area around Minamata, but many individuals were never examined for or classified as having Minamata disease. Following the determination of the Supreme Court of Japan in October 2004 that the Japanese Government was responsible for spreading Minamata disease, over 13,000 residents came forward to be examined for Minamata disease. We studied 197 residents from the Minamata area who had a history of fish consumption during the polluted period to determine the importance of sensory symptoms and findings in making a diagnosis of Minamata disease. We divided the exposed subjects into non-complicated (E) and complicated (E+N) groups based on the absence or presence of other neurological or neurologically related disorders and compared them to residents in control area (C) after matching for age and sex. We quantitatively measured four somatosensory modalities (minimal tactile sense by Semmes-Weinstein monofilaments, vibration sense, position sense, and two-point discrimination) and did psychophysical tests of fine-surface-texture discrimination. Subjective complaints were higher in groups E and E+N than C. Over 90% of E+N and E subjects displayed a sensory disturbance on conventional neurological examination and 28% had visual constriction. About 50% of the E and E +N groups had upper and lower extremity ataxia and about 70% had truncal ataxia. The prevalence of these neurological findings was significantly higher in exposed subjects than controls. All sensory modalities were impaired in the E and E+N groups. All four quantitatively measured sensory modalities were correlated. The prevalence of complaints, neurological findings, and sensory impairment was similar or a little worse in group E+N than in group E. We conclude that sensory symptoms and findings are important in making the diagnosis of Minamata disease and that they can be determined even in the presence of neurological or neurologically related diseases. r 2007 Elsevier Inc. All rights reserved. Keywords: Minamata disease; Complaints; Symptoms; Somatosensory disturbance; Somatosensory modalities; Mercury; Methylmercury

1. Introduction Minamata disease is methylmercury poisoning from consuming contaminated fish and shellfish. The Chisso Company in Minamata began to use mercury as a catalyst in the production of acetaldehyde in 1932. They discharged mercury and methylmercury contaminated waste directly Corresponding author. Fax: +81 966 62 2044.

E-mail address: [email protected] (S. Takaoka). 0013-9351/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.envres.2007.05.012

into Minamata bay and the Yatsushiro Sea. Minamata disease was first recognized in 1956 and determined to be methylmercury poisoning by 1959 (Harada, 1995). However, the Chisso Company continued to discharge mercury contaminated waste until 1968. Several months after they stopped acetaldehyde production, the Japanese Government formally stated for the first time that Minamata disease was methylmercury poisoning. During this extended period of pollution residents living along the coast of the Yatsushiro Sea continued eating contaminated fish

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and shellfish. There are about 470 thousands people now living in this area. The most severe signs and symptoms of Minamata disease are known as the Hunter-Russell syndrome consisting of somatosensory disturbance, motor ataxia, visual constriction, auditory disturbance, and dysarthria. The official Japanese government criteria for certification as a victim of Minamata disease included the following: (1) the patient must apply for certification; (2) the patient must have lived in a polluted area and must have a history compatible with methylmercury exposure; and (3) neurological damage must be confirmed by a medical examination (Environmental Agency, 1986). Since the mid-1970s, the Japanese Environmental Agency required that somatosensory disturbance be accompanied by other neurological deficits such as ataxia, visual field constriction, or dysarthria in order to be certified as a Minamata disease patient. Consequently, numerous residents with physical complaints and sensory disturbance were not certified officially as having Minamata disease and were not treated as poisoned by methylmercury. In addition, many residents were never examined because of the regional social discrimination against victims of the poisoning and the general lack of information on mercury poisoning. In addition to the problem with the formal criteria, there was also an issue with the practical application of these criteria. Many patients were not certified as Minamata disease in spite of the fact that they fulfilled all the official criteria (Miyai, 1997). In our clinical work, we also believe this to be true. More than 17,000 people applied for Minamata disease certification prior to 1999 (Takizawa and Osame, 2001). However, only 2264 patients were certified as having Minamata disease in the Minamata area (Minamata City, 2000). In 1995, over 10,000 individuals were partially compensated for health problem including somatosensory disturbance, but they were not certified as having Minamata disease (Tsubaki and Takahashi, 1986). Many residents with health problems did not seek diagnosis or support because of the social discrimination against those with Minamata disease. However, between October 2004 following the judgment of the Supreme Court of Japan and December 2006, an additional 4845 residents sought compensation for Minamata disease and another 8642 residents who were examined and diagnosed as having somatosensory disturbance started receiving compensation for medical expenses on the condition that they never seek official certification. Since October 2004, we have examined nearly 3000 Minamata area residents. Most of them had never been examined for Minamata disease. We studied subgroups of these subjects to determine the importance of sensory symptoms and signs in making the diagnosis and to determine if the presence of Minamata disease could be distinguished in the presence of other neurological issues.

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2. Materials and methods 2.1. Subjects The study cohort was selected from among 629 residents from the polluted area who were examined at the Minamata Kyoritsu Hospital or the Neurology & Rehabilitation Kyoritsu Clinic between November 2004 and April 2005. Among them, there were 610 residents who consented to participate and on whom we collected demographic data, did a detailed questionnaire about epidemiological conditions and complaints, and performed a neurological examination that included four kinds of quantitative sensory measurements (minimal tactile sense by SemmesWeinstein monofilaments, vibration sense, position sense, and two-point discrimination). Among these subjects there were 197 who received the examination on weekdays and had additional tests including psychophysical test of fine-surface-texture discrimination, neurophysiological, neuroradiological, and other laboratory tests. Full data were not available on the 413 residents who were evaluated on holidays. We analyzed only the 197 subjects with complete data. Only about 10% of the subjects in groups E and E+N were born before 1932 and no subjects were born after 1968. Control subjects were obtained from residents living in a non-polluted area. Among the 227 control subjects enrolled, 13 were excluded because they had lived around Minamata City or suffered from a neurological disease or a neurologically related illness. Their neurologically related diseases included cerebrovascular diseases, diabetes mellitus, lumbar spondylosis, cancer, and cubital tunnel syndrome. The control subjects were examined between February and May 2006 in Fukuoka City, Kumamoto City, and Kagoshima City. Control subjects completed the same questionnaires as study subjects and had a neurological examination and four kinds of quantitative sensory measurements.

2.2. Epidemiological conditions and questionnaire on complaints The questionnaire included information to determine the subjects’ exposure to methylmercury including where they resided, dietary habits, occupational histories, and family members’ health and histories. There were 50 specific questions related to sensory impairment (7 items), somatic pain (4), visual impairment (4), hearing impairment (3), tasting and smelling problems (3), in-coordination of the extremities (7), other movement impairment (5), vertigo and dizziness (4), general complaints (3), and mental and intellectual problems (10). For health complaints subjects were asked to select one from the four kinds of answer as follows: (1) always yes, (2) sometimes yes, (3) yes in the past and no at present, and (4) no in the past and at present. The prevalence of each complaint was calculated in each group, and compared among the three groups. Subjects completed the questionnaire before they were examined. Subject who could not complete the questionnaire alone were interviewed. Questionnaires were reviewed prior to the examination.

2.3. Standard neurological examination A standard neurological examination was performed on all subjects. Results of dysarthria, auditory disturbance, visual constriction, fingernose test, diadochokinesis, heel-shin test, gait disturbance, tandem gait, Romberg’s sign, one foot standing with eyes open and superficial sensory disturbance (touch and pain) were determined. Other neurological data were also used in order to diagnose neurological diseases other than Minamata disease. Dysarthria, auditory disturbance, visual constriction, involuntary movement, gait disturbance, and Romberg’s sign were judged as present (+) or absent (). Dysarthria, auditory disturbance, and visual field were judged by the examining physician without using special instruments. Visual disturbance was considered present when the confrontation test showed 801 or less of lateral vision.

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Limb and truncal ataxia were judged as absent (), mildly abnormal (+), or severely or moderately abnormal (++). Finger-nose test and heelshin test were judged ++ if there was constant dysmetria or decomposition and + if there was uncertain dysmetria, decomposition, or slow reaching to the destination. Dysdiadochokinesis was ++ if there was a constant abnormality and + if an uncertain abnormality or slow movement. Tandem gait disturbance was ++ if the subject could not walk more than five steps and + if they could walk five steps but were unstable. One foot standing was ++ if it was impossible to stand more than 3 s with eyes open and + if they could stand more than 3 s with eyes open. We calculated the summed percentage of ++ and + for this study. All the physicians participating in the study were trained by document, direct instruction, or videotapes. Physicians who performed the neurological examination differed between the control and polluted area.

2.4. Quantitative sensory measurements Vibration and position sense were measured by each physician. Minimal tactile sense and two-point discrimination were measured by two trained doctors from the Minamata Kyoritsu Hospital. Fine-surfacetexture discrimination was measured by a single trained examiner using previously published criteria (Takaoka et al., 2004). The temperature of the laboratory was maintained between 23 and 27 1C during sensory measurement except for the measurement of the fine-surface-texture discrimination. 2.4.1. Minimal tactile sense by Semmes Weinstein monofilaments After completing the conventional superficial sensory examination, the minimal tactile sense was measured by Semmes-Weinstein monofilaments. We used 20 kinds of filaments from 0.008 to 300 g. Subjects were tested with eyes closed after receiving clear instructions on the tested site. Each filament was pushed until it bent about 901 for about a second. The threshold was the smallest size filament which a subject could feel as touch. The trial was performed starting with the smaller size. Each trial was once with each filament, except when the subject was unsure in which case the examiner provided an odd number of trials with the same filament and selected the answer given to over 50% of the trials. If a subject could not detect the maximum filament (300 g), we defined the threshold as 400 g for calculation. Testing was performed on the lower lip, upper chest, and ventral sides of both index fingers and first toes. Two doctors from the Minamata Kyoritsu Hospital performed all of these tests. 2.4.2. Vibration sense Vibration sense was measured by using a 128 Hz tuning fork. The examiner fully knocked the tuning fork and started the stopwatch at the same time. Subjects were instructed to immediately state when they could not feel the vibration at all and the time recorded. Vibration was determined on the middle or upper sternum, the radial side of both wrists, and the fibular side of both ankles. Vibration sense was confirmed on all subjects by two examiners from the Minamata Kyoritsu Hospital. 2.4.3. Position sense Position sense was measured with the subject’s eyes closed and using a ruler with millimeter lines. Each examiner set a zero point on the horizontal position of the lateral side of the nail and held the lateral side of the finger or toe when it was moved up or down for about 1 s. A distance interval of 5 mm was considered the minimal threshold that a subject could detect the direction. Each trial was once at each distance except when the subject’s answer was uncertain in which case the examiner would give an odd number of trials and consider the correct answer as the one over 50%. If a subject could not feel the maximum movement, the threshold was defined as the maximum distance plus 5 mm. Trials were performed on both index fingers and first toes. All the physicians performed this test and there was minimal examiner variation.

2.4.4. Two-point discrimination Two-point discrimination threshold was determined with the subject’s eyes closed and using a drafting divider. The divider was applied to the skin at a 30–451 angle with a depth of 1–2 mm for about 1 s. The twoalternative, forced-choice technique was used. Tested distances were 1–6, 8, 10, 12, 15, 20, 25, 30, and 36 mm. The threshold was the lowest distance at which a subject answered correctly on three successive trials. The starting point distance was estimated by each physician after observing the whole state of sensory impairment, in order to diminish the testing time and to avoid fatigue. This method was performed on the lower lip and ventral side of each index finger. If a subject could not answer the maximum distance (36 mm) the threshold was defined as 40 mm for calculation. Only the two doctors from Minamata Kyoritsu Hospital performed this test. 2.4.5. Fine-surface-texture discrimination In order to measure fine-surface-texture discrimination, six aluminumoxide abrasive papers (Sumitomo 3-M) were used. The grit values assigned by the manufacturer were 600, 1200, 2000, 3000, 4000, and 8000, corresponding to an average particle size of 30, 12, 9, 5, 3, and 1 mm, respectively. This method was as described by Takaoka et al. (2004). The control data for fine-surface-texture discrimination were not obtained from group C, but from the former study.

2.5. Neurophysiological, neuroradiological, and other laboratory tests Neurophysiological tests included Goldmann’s perimeter, audiometry, and nerve conduction of the median and ulnar nerves. Neuroradiological testing consisted of head computed tomography and X-rays of the cervical (six directions) and lumbar (four directions) spine. Biochemical tests included blood sugar, HTLV-I antibody, and anti-nuclear antibodies for Lupus Erythematosis (FA method). These tests were not performed on control subjects. We used only the median sensory and motor nerve velocities for comparison with sensory measurements. Other data were used to diagnose other neurological and neurologically related diseases in subjects from the polluted area. Sensory nerve conduction velocity was measured by the antidromic method. Stimulation was on the wrist and recording was 14 cm proximal from the distal interphalangeal joint of the index finger. Stimulation was given at 1 Hz interval, 0.2 ms duration, with the strength below or above the maximum muscle response. Sensitivity was 20 mV. Filters were 20 Hz low cut and 2 kHz high cut. Motor nerve conduction velocity was measured by stimulating at the wrist and elbow. The electromyogram was recorded on the thenar muscle. Stimulation was given at 1 Hz intervals, 0.2 ms duration, with the strength above the maximum muscle response. Sensitivity was 5 mV. Filters were 20 Hz low cut and 3 kHz high cut. Control data for the nerve conduction velocity tests were taken from non-exposed residents around Kumamoto City. For the controls the maximum stimulation strength was lower than threshold for the sensory nerve conduction velocity.

2.6. Statistical methods We used the presence of other neurological disorders to divide the exposed subjects into complicated (E+N) and non-complicated (E) groups. One hundred and seventeen subjects had at least one complication. Complications were defined as follows: diabetes mellitus (34); cervical spondylosis (54); lumbar radiculopathy (15); carpal tunnel disturbance (42); cerebrovascular disease (29); and other diseases (14). Criteria for diagnosing diabetes mellitus were either a past diagnosis of diabetes mellitus, a fasting blood sugar X110 with a HbA1c45.8%, or a blood sugar at any time X160. Cervical spondylosis was diagnosed when there was bilateral foraminal stenosis on cervical X-rays. Lumbar radiculopathy was diagnosed when there was an abnormality on lumbar X-rays or consistent focal neurological signs. Carpal tunnel disturbance was diagnosed when there was a distal latency of the median nerve

ARTICLE IN PRESS S. Takaoka et al. / Environmental Research 107 (2008) 6–19 X4.3 ms present on both sides. Cerebrovascular disease was diagnosed with consistent findings on a head CAT. Other neurological abnormalities included chronic psychiatric drug users (3), mental retardation (3), hypothyroidism (3), cubital tunnel syndrome (2), other polyneuropathies (2), HTLV-I associated melopathy (1), spinocerebellar degeneration (1), and epilepsy (1). In order to compare these groups with controls (C), we matched them by age and sex. One hundred and eleven subjects from among the 214 of group C, 74 subjects from the E+N group, and 74 subjects from group E were selected by non-intentional method, respectively (Table 1). Most of the calculations were performed using MS Excel and SPSS software. w2 was used in MS Excel when the prevalence was compared, and t-test was used in the MS Excel when the average was compared. The correlations were calculated by SPSS. The psychophysical analysis was performed using MS Excel. 2.6.1. Questionnaire and neurological examination To analyze the questionnaire, data percentages of the answer ‘‘always yes’’ and ‘‘always or sometimes yes’’ were summed and the results compared among the three groups, C, E, and E+N. The correlations between the three groups were calculated. For components of the standard neurological examination and superficial sensory disturbance (touch and pain), we calculated the correlations including milder disturbances. 2.6.2. Quantitative sensory measurements For the minimal tactile sense by Semmes-Weinstein monofilaments, vibration sense, position sense, and two-point discrimination, the results of the three groups were calculated and compared. We did not use the gram weight to calculate the minimal tactile sense, but instead converted it to the evaluator size using the equation: Evaluator size ¼ log([gram])+4. Methods of data processing and threshold calculation for fine-surfacetexture discrimination were the same as the former study (Takaoka et al., 2004). 2.6.3. Correlations between quantitative sensory measurements and between sensory measurements and nerve conduction velocities Correlations between minimal tactile sense, position sense, and twopoint discrimination of the right index finger and vibration sense at the right radial wrist were calculated. Three kinds of senses of the right index finger are conducted by way of the right median sensory nerve. They were calculated only for group E because other neurological diseases might affect these modalities and nerve conduction velocity. We did not calculate the individual threshold in fine-surface-texture discrimination, because it would have required several hundred trials with each subject. Instead, we compared only the probability of 3 mm vs. 5 mm, 3 mm vs. 9 mm, and 3 mm vs. 12 mm roughness discrimination. Correlations between each sensory measurement and nerve conduction velocities were also calculated.

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3. Results 3.1. Backgrounds of subjects (Table 1) The backgrounds of the subjects are shown in Table 1. The percentage of those who fished themselves or belonged to a family that fished was significantly higher in groups E and E+N than C. But the percentage of those who fished in groups E (12%) and E+N (7%) was not very high. About two-thirds of the groups E and E+N had at least one family member who received compensation for Minamata disease. However, 70–80% of subjects in groups E and E+N had never applied for certification as being affected by Minamata disease. 3.2. Questionnaire on complaints (Tables 2–4) The results of the questionnaire on complaints are shown in Table 2. When groups C and E were compared on the answers ‘‘always,’’ 45 symptoms were significantly more prevalent in group E than in C. There was no significant difference between groups E and E+N on 47 questions (Table 3). When we compared the percentage of each complaint between groups E and E+N, the correlation was stronger than the correlation of complaint percentage between groups C and E (Table 4). When groups C and E on the answers ‘‘always’’ and ‘‘sometimes’’ were compared, all of the symptoms were significantly more prevalent in group E than C, but there was no significant difference between groups E and E+N on 46 questions (Table 3). When we compared the percentage of each complaint between groups E and E+N, the correlation was stronger than the correlation between groups C and E (Table 4). 3.3. Neurological examination (Tables 5 and 6) The results of the superficial sensory examination are presented in Table 5. Only one subject in group C showed a disturbance in all four limbs. In contrast, 75% of group E

Table 1 Age, sex, and backgrounds of each group (data after age and sex matching)

Group C Group E Group E+N

Group E Group E+N a

Age

n (M/F)

Fishermena

Fishermen’s Familyb

61.979.9 61.4710.6 62.478.6

111 (36/75) 74 (24/50) 74 (24/50)

0 (0%) 9 (12%) 5 (7%)

1 (1%) 34 (46%) 17 (23%)

Certification of a family member

Compensation of a family member (including certification)

Experience of certification application

33 (45%) 24 (32%)

49 (66%) 53 (72%)

14 (19%) 18 (24%)

For groups C and E, po0.01; groups C and E+N, po0.05. For groups C and E, po0.01; groups C and E+N, po0.01.

b

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10 Table 2 Percentages of complaints No

Complaint

‘‘Always’’ yes Group C (%)

1 2 3 4 5 6 7

Sensory numbness in both hands Sensory numbness in both legs Hot sensation in the hands Hot sensation in the legs No pain when burned or wounded Difficulty in judging the correct temperature of bath water Carrying a bag on the elbow or shoulder instead of holding it in the hand

8 9 10 11

Headache Shoulder stiffness Lower back pain Muscle cramps

12 13 14

Disturbed vision Limited peripheral vision Difficulty in visually recognizing objects when you continue to stare at them Difficulty in finding objects in shops

15 16 17

‘‘Always’’+‘‘Sometimes’’ yes Group E (%)

Group E+N (%)

3 1 0 0 0 0

44 42 13 17 11 9

56 44 11 16 17 19

2

34

0 11 4 3

Group C (%)

Group E (%)

Group E+N (%)

7 8 0 1 0 0

89 90 46 57 40 41

95 85 44 54 49 38

40

5

66

77

37 64 52 26

35 74 66 33

25 55 48 36

84 96 87 97

84 93 92 86

3 0 0

46 29 20

56 34 21

18 4 1

80 66 60

89 66 58

0

40

30

9

79

77

8 1

26 7

38 14

17 7

61 49

76 54

18 19 20 21

Difficulty in hearing Difficulty in understanding a word or a sentence even if you can hear it Tinnitus Difficulty in smelling Difficulty in tasting Difficulty in judging the taste of your own cooking

6 0 0 1

28 14 20 14

34 26 21 17

17 5 2 2

75 49 47 46

84 48 44 42

22 23 24 25 26 27 28

Stumbling on flat ground Difficulty in wearing slippers Losing your slippers or sandals while walking Difficulty in fine finger tasks Difficulty in buttoning Dropping things held in the hand Dropping chopsticks while eating

0 0 0 0 0 0 0

6 24 18 56 14 14 4

12 32 28 61 41 30 14

1 1 2 8 0 8 0

63 62 69 86 54 76 63

75 81 78 86 68 81 66

29 30 31 32 33

Difficulty in speaking words or sentences well Hand weakness Leg weakness Hand tremor while reaching Hand tremor at rest

0 3 3 2 1

6 60 51 18 13

19 63 57 31 19

3 5 5 4 1

51 84 87 75 55

67 86 81 74 51

34 35 36 37

Vertigo (feeling of spinning around) Swaying or dizziness Fainting (syncope like) dizziness Dizziness when standing up

0 0 0 0

10 7 4 19

10 8 4 15

5 4 2 15

68 58 49 88

60 58 42 80

38 39 40

General fatigue Difficulty in sleeping Loss of Appetite

1 4 0

42 31 9

39 43 7

22 18 4

88 86 44

86 83 45

41 42 43 44 45 46 47 48 49 50

Lack of motivation to do things Inability to persevere or keep working Feeling as if your mind is blank or empty Difficulty thinking about anything Losing your train of thought during conversations Forgetfulness Feeling as if you are not yourself Irritation Feeling sad Difficulty in resuming when interrupted

2 0 0 0 0 1 0 0 0 2

21 28 4 3 13 33 7 31 14 24

31 42 10 8 14 39 14 33 17 30

23 14 7 2 9 57 1 34 21 14

83 74 54 50 69 97 39 93 75 82

90 78 60 58 77 96 52 81 68 82

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Table 3 p-Value on complaints between groups po0.01

po0.05

n.s.

‘‘Always’’ yes

Group C vs. E Group C vs. E+N Group E vs. E+N

All others All others No. 26

No. 22, 29, 35, 47 No. 40 No. 27, 29

No. 17, 28, 36, 43, 44 No. 36 All others

‘‘Always’’+‘‘Sometimes’’ yes

Group C vs. E Group C vs. E+N Group E vs. E+N

All All –

– – No. 11, 23, 29, 48

– – All others

Table 4 Correlation of prevalence of complaints between groups Correlation coefficient

p-Value

‘‘Always’’ yes

Group C vs. E Group C vs. E+N Group E vs. E+N

0.558 0.619 0.934

o0.001 o0.001 o0.001

‘‘Always’’+‘‘Sometimes’’ yes

Group C vs. E Group C vs. E+N Group E vs. E+N

0.702 0.630 0.908

o0.001 o0.001 o0.001

Table 5 Superficial somatosensory disturbance by standard testing methods Type of sensory disturbance

Touch Group C (%)

General (limbs ¼ chest) General (limbs4chest) Four limbs Two limbs None Total

Pain Group E (%)

Group E+N (%)

Group C (%)

Group E (%)

Group E+N (%)

0 0 1 2 97

9 12 55 15 9

7 7 70 8 8

0 0 1 2 97

11 30 53 3 4

11 30 54 3 3

100

100

100

100

100

100

C vs. E: po0.01, C vs. E+N: po0.01, and E vs. E+N: n.s. (both of touch and pain).

subjects had a tactile disturbance and 93% of group E had a pain disturbance in all four limbs. There was no significant difference between groups E and E+N on the percentage of tactile and pain disturbance. Dysarthria, auditory disturbance, and visual constriction were all significantly higher in group E than in C. Comparing groups E and E+N, there was no significant difference in symptoms except for the auditory disturbance. Finger-nose test and heel-shin test were disturbed in about 50% of the subjects in groups E and E+N. Dysdiadochokinesis was observed in about 30% of group E and 50% of group E+N. Tandem gait and one foot standing were disturbed in about 70% of the subjects. Motor abnormalities were all significantly higher in groups E and E+N than in C. Comparing groups E and E+N, there was no significant difference in overall symptoms except for dysdiadochokinesis (Table 6).

3.4. Quantitative sensory measurements 3.4.1. Minimal tactile sense by Semmes-Weinstein monofilaments Thresholds of the minimal tactile sense by SemmesWeinstein monofilaments were higher in groups E and E+N than C for all body sites tested (Fig. 1). The thresholds were also statistically different between groups E and E+N at all sites. Table 9 shows that the threshold for minimal tactile sensation of the right index finger increased with age and similar tendencies were observed at other sites. The thresholds were higher at all sites tested, but the ratios of the thresholds in the fingers and toes per chest were lower in groups E and E+N than in C (Table 7). We did not calculate the ratio of lower lip to chest threshold because 71% of the control subjects had a

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Table 6 Prevalence of abnormal findings on neurological examination Neurological examination

Dysarthria Auditory disturbance Visual constriction Finger-nose test Dysdiadochokinnesis Heel-shin test Imvolentary movement Normal gait Tandem gait Romberg’s sign One foot standing

Group C (%)

Group E (%)

Group E+N (%)

C vs. E

C vs. E+N

E vs. E+N

2 6 0 0 1 2 3 0 6 1 8

23 33 28 50 33 48 28 41 73 6 66

34 55 28 50 51 52 24 43 76 8 72

o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.05 o0.01

o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

n.s. o0.01 n.s. n.s. o0.05 n.s. n.s. n.s. n.s. n.s. n.s.

25

7.00 6.00

20

5.00 4.00

second

Evaluator size = log([gram]) + 4

p-Value

Prevalence of abnormality

3.00 2.00

15 10

1.00

5

0.00 Lower lip

Chest

Index Index Great toe Great toe finger (R) finger (L) (L) (R)

Group C

Group E

0 Chest

Wrist (R)

Wrist (L)

Ankle (R)

Group C

Group E

Group E+N

Ankle (L)

Group E+N

Fig. 1. Threshold for minimal tactile sense by Semmes-Wiestein monofilaments. C vs. E: po0.01 (all sites), C vs. E+N: po0.01 (all sites), and E vs. E+N: po0.01 (all sites).

Table 7 Ratio of threshold of minimal tactile sense of fingers and toes per chest Group C

Group E

Group E+N

Index finger (R)/chest Index finger (L)/chest Great toe (R)/chest Great toe (L)/chest

1.1570.31 1.1370.30 1.4370.37 1.4370.36

1.0270.14 1.0170.15 1.2070.19 1.1970.18

1.0270.09 1.0170.10 1.2070.11 1.2170.12

n

111

71

69

C vs. E: po0.01 (all ratios), C vs. E+N: po0.01 (all ratios), and E vs. E+N: n.s. (all ratios).

minimal threshold of the least filament (0.008 g) and the true thresholds of these subjects was presumably below 0.008 g. 3.4.2. Vibration sense The threshold time for vibration sense was shorter in groups E and E+N than in C for all the body sites tested (Fig. 2). Subjects with shorter times had higher thresholds of vibration sense. The time was also statistically shorter in

Fig. 2. Threshold hold for vibration sense by tuning fork. C vs. E: po0.01 (all sites), C vs. E+N: po0.01 (all sites) and E vs. E+N: po0.01 (all sites). n.s. (left ankle), po0.05 (all sites other than left ankel).

group E+N than in E at all sites. Vibration sense was disturbed at all sites for groups E and E+N. The ratios of the vibration sense thresholds in the wrists and ankles per chest were higher in groups E and E+N than in C (Table 8). Thresholds of the vibration sense at the right wrist (radial side) were higher as subjects aged (Table 9) and similar tendencies were observed at the other tested sites. 3.4.3. Position sense The threshold of position sense was higher in groups E and E+N than in C at all body sites tested (Fig. 3). There was no statistical difference between groups E and E+N. Control subjects had thresholds of position sense that were 5 mm except for three subjects (one had a disturbed lower direction of the right index finger; one had an upper direction of the right great toe; and the other had an upper direction of the right great toe and lower and upper directions of the left great toe). No correlations with aging were observed for the threshold of position sense using this testing method (Table 9).

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3.4.4. Two-point discrimination The threshold of two-point discrimination was higher in groups E and E+N than in C at the lower lip and both index fingers (Fig. 4). There was no statistical difference

between groups E and E+N. Table 9 shows that two-point discrimination at the right index finger declined with age and similar tendencies were observed at other tested sites. However, aging did not account for the abnormalities found in groups E and E+N.

Table 8 Ratio of threshold time of vibration sense of wrists and ankles per chest

3.4.5. Fine-surface-texture discrimination Fine-surface-texture discrimination was analyzed by age groups, but subjects over age 80 were excluded. One group was aged 40–59 and the other 60–79. In groups E and E+N, 67 and 64 subjects, respectively, received this test. Six of group E (age 40–59) and 11 of group E (age 60–79) were excluded from the total calculation of psychometric function because the probability of 12 mm vs. 3 mm was lower than 5 mm vs. 3 mm. In the same way, six of group E+N (age 40–59) and seven of group E+N (age 60–79) were excluded from the calculation. As a result, the

Group C

Group E

Group E+N

Wrist (R)/Chest Wrist (L)/Chest Ankle (R)/Chest Ankle (L)/Chest

1.1770.19 1.2070.22 0.9670.22 0.9670.23

0.9870.24 1.0370.24 0.7070.20 0.7070.24

0.9170.26 0.9670.28 0.6570.23 0.6570.25

n

111

74

73

C vs. E: po0.01 (all ratios), C vs. E+N: po0.01 (all ratios), E vs. E+N: po0.05 (left wrist/chest), and n.s. (all ratios other than left wrist/chest).

Table 9 Correlation between age and sensory value in each somatosensory modality at the right index finger or at the right wrist Somatosensory modality

Group

Correlation between age (x) and sensory value (y)

Correlation coefficient

p-Value

Comparison

p-Value Value

Slope

Minimal tactile sense (y ¼ [evaluation size])

C E E+N

y ¼ 0.0148x+2.157 y ¼ 0.0179x+3.024 y ¼ 0.0180x+3.272

0.375 0.325 0.217

0.000 0.005 0.073

C vs. E C vs. E+N E vs. E+N

0.000 0.000 0.010

0.640 0.715 0.990

Vibration sense (y ¼ [s])

C E E+N

y ¼ 0.106x+23.53 y ¼ 0.041x+12.51 y ¼ 0.098x+14.83

0.327 0.127 0.226

0.000 0.280 0.053

C vs. E C vs. E+N E vs. E+N

0.000 0.000 0.029

0.168 0.895 0.356

Position sense (y ¼ [mm])

C E E+N

y ¼ 0.00002x+5.02 y ¼ 0.052x+5.79 y ¼ 0.047x+14.71

0.001 0.099 0.029

0.993 0.405 0.805

C vs. E C vs. E+N E vs. E+N

0.000 0.000 0.104

0.317 0.745 0.594

Two-point discrimination (y ¼ [mm])

C E E+N

y ¼ 0.050x0.36 y ¼ 0.231x+0.69 y ¼ 0.546x15.89

0.422 0.184 0.308

0.000 0.116 0.009

C vs. E C vs. E+N E vs. E+N

0.000 0.000 0.156

0.029 0.000 0.203

30

25

mm

20

15

10

5

0 Index finger Index finger Index finger Index finger (R Upper) (R Lower) (L Upper) (L Lower)

Group C

Great toe (R Upper)

Group E

Great toe (R Lower)

Great toe (L Upper)

Great toe (L Lower)

Group E+N

Fig. 3. Threshold of position sense. C vs. E: po0.01 (all sites), C vs. E+N: po0.01 (all sites), and E vs. E+N: n.s. (all sites).

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numbers from which we calculated the difference threshold were 22 of group E (age 40–59), 28 of group E (age 60–79), 16 of group E+N (age 40–59), and 35 of group E+N (age 60–79). In both age groups, difference thresholds were higher in groups E and E+N than in the controls. Probabilities of the answers that 5, 9, and 12 mm comparison stimuli were rougher than 3 mm standard stimulus were statisti40 35 30

cally lower in groups E and E+N than in C. There was no statistical difference between groups E and E+N (Tables 10 and 11). 3.4.6. Correlation between minimal tactile sense, vibration sense, position sense, two-point discrimination, and finesurface-texture discrimination There were strong correlations between minimal tactile sense, vibration sense, position sense, and two-point discrimination. There were no distinct correlations between these senses and fine-surface-texture discrimination (Table 12).

mm

25 20

3.5. Median sensory and motor nerve velocities

15 10 5 0 Lower lip

Index finger (R) Group C

Group E

Index finger (R)

Group E+N

Fig. 4. Threshold of two-point discrimination sense. C vs. E: po0.01 (all sites), C vs. E+N: po0.01 (all sites), and E vs. E+N: n.s. (all sites).

3.5.1. Correlation between median nerve velocities and quantitative sensory measurements Sensory and motor nerve conduction velocities of the right median nerve were similar in group E and controls (Table 13). Sensory nerve conduction velocity was correlated with the probability of 3 mm vs. 5 mm discrimination of fine-surface-texture, but not with other sensory modalities (Table 14).

Table 10 Probability of the fine-surface-texture discrimination (subjects age 40–59) Probability of answer ‘‘rougher than 3 mm’’

Control (n ¼ 22) Group E (n ¼ 22) Group E+N (n ¼ 16)

3 mm vs. 1 mm (%)

3 mm vs. 3 mm (%)

3 mm vs. 5 mm (%)

3 mm vs. 9 mm (%)

3 mm vs. 12 mm 3 mm vs. 30 mm (%) (%)

25 31 33

57 53 57

91 60 58

96 64 66

99 77 73

100 98 98

Difference threshold

Absolute index

2.2 8.7 8.7

0.87 0.85 0.81

Difference threshold was calculated by using from 3 mm vs. 1 mm to 3 mm vs. 9 mm data in the group C and by using from 3 mm vs. 1 mm to 3 mm vs. 12 mm data in the groups E and E+N. C vs. E: po0.01 (3 mm vs. 5, 9, 12 mm), n.s. (3 mm vs. 1, 3, 10 mm). C vs. E+N: po0.01 (3 mm vs. 5, 9, 12 mm), n.s. (3 mm vs. 1, 3, 10 mm). E vs. E+N: n.s. (3 mm vs. 1, 3, 5, 9, 12, 30 mm).

Table 11 Probability of the fine-surface-texture discrimination (subjects age 60–79) Probability of answer ‘‘rougher than 3 mm’’

Control (n ¼ 27) Group E (n ¼ 28) Group E+N (n ¼ 35)

3 mm vs. 1 mm (%)

3 mm vs. 3 mm (%)

3 mm vs. 5 mm (%)

3 mm vs. 9 mm (%)

3 mm vs. 12 mm 3 mm vs. 30 mm (%) (%)

34 39 43

56 58 55

84 63 55

94 65 59

97 78 73

100 94 99

Difference threshold

Absolute index

2.7 8.7 8.7

0.95 0.83 0.87

Difference threshold was calculated by using from 3 mm vs. 1 mm to 3 mm vs. 9 mm data in the group C and by using from 3 mm vs. 1 mm to 3 mm vs. 12 mm data in the groups E and E+N. C vs. E: po0.01 (3 mm vs. 5, 9, 12 mm), n.s. (3 mm vs. 1, 3, 10 mm). C vs. E+N: po0.01 (3 mm vs. 5, 9, 12 mm), n.s. (3 mm vs. 1, 3, 10 mm). E vs. E+N: n.s. (3 mm vs. 1, 3, 5, 9, 12, 30 mm).

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Table 12 Correlation among five kinds of sensory measurements of the right hand

Age

C.C. p-Value n

MTS

Vibration

Position

TPD

FST5

FST9

FST12

0.325 0.005 72

0.127 0.280 74

0.099 0.405 73

0.184 0.116 74

0.151 0.282 53

0.114 0.416 53

0.234 0.091 53

0.499 0.000 72

0.551 0.000 71

0.623 0.000 72

0.092 0.517 52

0.225 0.109 52

0.251 0.073 52

0.486 0.000 73

0.424 0.000 74

0.375 0.006 53

0.049 0.730 53

0.367 0.007 53

0.489 0.000 73

0.138 0.329 52

0.127 0.370 52

0.089 0.529 52

0.272 0.049 53

0.370 0.006 53

0.398 0.003 53

0.557 0.000 53

0.814 0.000 53

MTS

C.C. p-Value n

Vibration

C.C. p-Value n

Position

C.C. p-Value n

TPD

C.C. p-Value n

FST5

C.C. p-Value n

FST9

C.C. p-Value n

0.616 0.000 53

MTS: minimal tactile sense by Semmes-Weinstein monofilaments, Vibration: vibration sense, Position: position sense, TPD: two-point discrimination, FST5: comparison of 3 and 5 mm stimulation in fine-surface-texture discrimination, FST9: comparison of 3 and 9 mm stimulation in fine-surface-texture discrimination, FST12: comparison of 3 and 12 mm stimulation in fine-surface-texture discrimination, C.C.: correlation coefficient.

Table 13 Right median nerve conduction velocity Group

Age

n

Velocity (m/s)

p-Value

Sensory nerve

Control Group E

59.3710.8 61.0710.0

22 67

57.072.7 56.372.8

0.149

Motor nerve

Control Group E

59.3710.8 60.9710.0

22 68

49.874.9 49.976.3

0.469

Group

Correlation between age (x) and velocity (y)

Correlation coefficient

p-Value

p-Value Value

Slope

Sensory nerve

Control Group E

y ¼ 0.1629x+59.45 y ¼ 0.0404x+52.35

0.356 0.065

0.104 0.603

0.946

0.389

Motor nerve

Control Group E

y ¼ 0.0825x+61.93 y ¼ 0.0731x+60.75

0.334 0.257

0.129 0.034

0.286

0.883

4. Discussion 4.1. Characteristics of the subjects in the polluted and control groups We studied a consecutive series of patients from the Minamata area who had concerns about their health and compared them to controls from an area without

methylmercury pollution. We found that nearly all subjects in groups E and E+N who had a history of eating fish and shellfish during the most polluted period also had sensory disturbances and some had the constellation of findings referred to as the Hunter-Russell syndrome. Based on their dates of birth, all of these subjects could have had postnatal exposure and some may have had prenatal exposure as well.

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Table 14 Correlation between right median nerve conduction velocity and five kinds of sensory measurements of the right hand Age

MTS

Vibration

Position

TPD

FST5

FST9

FST12

MCV 0.398 0.001 67

SCV

C.C. p-value n

0.065 0.603 67

0.125 0.317 66

0.087 0.483 67

0.080 0.521 66

0.154 0.214 67

0.399 0.005 48

0.166 0.259 48

0.277 0.057 48

MCV

C.C. p-value n

0.257 0.034 68

0.059 0.633 67

0.087 0.482 68

0.090 0.468 67

0.017 0.891 68

0.135 0.354 49

0.005 0.970 49

0.262 0.069 49

MTS, Vibration: vibration sense, Position: position sense, TPD: two-point discrimination, FST5: comparison of 3 and 5 mm stimulation in fine-surfacetexture discrimination, FST9: comparison of 3 and 9 mm stimulation in fine-surface-texture discrimination, FST12: comparison of 3 and 12 mm stimulation in fine-surface-texture discrimination, C.C.: correlation coefficient, SCV: sensory nerve conduction velocity of right median nerve, MCV: motor nerve conduction velocity of right median nerve.

This high prevalence of somatosensory impairment in subjects from the polluted area suggests that somatosensory impairment is an important factor in diagnosing Minamata disease or chronic methylmercury intoxication. Tsuda and Miyai (2001) reported the relative risk of sensory disturbance of the four limbs in subjects from the polluted area at about 100. Some subjects with milder findings had not noticed their sensory disturbance. Other subjects who had noticed their sensory disturbance did not realize that it might be related to methylmercury exposure. They attributed them to aging, fatigue, or other causes. Some subjects with milder sensory changes noted that they were only apparent sporadically. In many cases, the subjects were able to manage their family and occupational life despite the sensory changes. Some attributed their difficulties to non-specific causes or a minor neurological disturbance. These results indicate that there are subjects with symptoms and findings compatible with methylmercury exposure who still reside in the Minamata area. They have not only neurological symptoms, but also non-specific complaints that range from mild to severe. On the whole, sensory impairments appear more common than motor symptoms and in some cases the motor symptoms may be related to sensory impairment. These neurological findings may be difficult for a casual observer to recognize and this may be an important factor that enhances the seclusion these subjects feel. Social discrimination against subjects with Minamata disease and the lack of precise information about its cause make these subjects feel more secluded. These feeling are enhanced because to date, neither national nor regional governments have provided precise information about methylmercury poisoning to the residents in the Minamata area. This study has some limitations too. Control subjects had to be obtained from a different area than those exposed to the pollution. Questionnaires rely upon subjective answers and those can be biased. For example, the distinction between ‘‘always yes’’ and ‘‘sometimes yes’’ likely depends upon the subject’s feelings at the time. However, we made every effort to confirm the question-

naire answers with the subjects at the time they completed the form. The high correlation of complaints between groups E and E+N and the low correlation between groups C and E suggests we were successful and that the frequency of various complaints is meaningful. The background of exposed subjects and controls was different in that exposed subjects sought an evaluation while controls did not. However, it is unlikely that the differences in complaints can be accounted for by this alone. It is known that tactile and other sensory modalities diminish with increasing age (Plumb and Meigs, 1961; Goff et al., 1965; Verrillo, 1982; Thornbury and Mistretta, 1981; Stevens, 1992; Stevens and Choo, 1996). However, this change is subtle and most normal elderly people do not notice it. Consequently, the high prevalence of somatosensory impairments in the exposed groups is not likely to be related to age. 4.2. Characteristics of somatosensory disturbance from chronic methylmercury exposure In this study, increase of the minimal tactile sense threshold in the exposed groups was greater in the chest (central portion of the body) than the index fingers (peripheral portion of the body). An increase of the vibration sense threshold (i.e., a decrease of the time threshold) in the exposed groups was greater in the index fingers (peripheral) than in the chest (central). These results suggest that vibratory threshold was more affected in the peripheral region than minimal tactile sense. Although the role of skin mechanoreceptors to the tactile modalities has not been proven, the mechanoreceptors for the minimal tactile sense and for the vibration sense are believed to be different. A somatosensory disturbance appears to be the most important symptom of chronic methylmercury exposure. However, there have been very few precise studies of somatosensory disturbance in Minamata disease. Many researchers have avoided studies of the somatosensory systems because they are difficult to quantify. In a study of somatosensory evoked potentials (SSEP) in Minamata

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disease by Tokuomi et al. (1982), the authors found a disturbance in severe cases (called ‘‘classical’’ in the paper). However, milder cases, even in the presence of the HunterRussell syndrome, showed no abnormalities. In order to ensure sensory impairment in our subjects, we performed multi-modal sensory tests and a psychophysical examination. All of the somatosensory modalities are known to be disturbed by methylmercury exposure. The four modality (minimal tactile sense, vibration sense, position sense, and two-point discrimination) thresholds we measured were correlated. Superficial pain and touch by standard methods were as sensitive as the other four modalities measured, although standard methods were not quantitative. Finesurface-texture discrimination was difficult to compare to the other four quantitative somatosensory modalities because the individual thresholds were not measured in our evaluation. Tactile sensation is divided into various modalities and there are several kinds of tactile receptors in the skin. Different receptors have different roles and different distributions on the body surface. For instance, Meissner’s corpuscles are densely distributed in the peripheral portion of the four limbs and lips and are necessary for fine sensory and motor functions of the mouth and limbs. Fine-surfacetexture discrimination is supposed to reflect Meissner’s corpuscles (Miyaoka et al., 1999). On the other hand, Pacinian corpuscles are distributed almost evenly on the whole body. Minimal tactile sense by Semmes-Weinstein monofilaments is believed to reflect the function of the Pacinian and Meissner’s corpuscles (Johansson et al., 1980). The functional disturbance of the Meissner’s corpuscle related system (FA I) by methylmercury can be easily detected than that of Pacinian corpuscle related system (FA II), because the receptive field of the FA I is far more smaller than that of FA II. This might cause the glove and stocking type sensory disturbance, and the dissociation of minimal tactile sense and vibration sense in Minamata disease. We found results similar to those of Ninomiya et al. (2005) who studied two-point discrimination and tactile threshold by Semmes-Weinstein monofilaments. They found thresholds of two-point discrimination in their controls (average age was 68.676.5) were 2.871.0 on the lip, 3.671.1 on the right index finger, and 3.671.0 in the left index finger. On our examination, thresholds of the controls (group C) were 2.171.0 on the lip, 2.771.2 on the right index finger, and 3.071.5 in the left index finger. For Ninomiya et al. thresholds in their exposed group (average age was 66.4712.5) were 5.973.7 on the lip, 7.274.0 on the right index finger, and 6.873.7 in the left index finger. In our study, the results for the exposed group (E) were 9.9711.1 on the lip, 14.9713.3 on the right index finger, and 16.3714.5 in the left index finger. Both studies found significant differences between control and exposed groups, but the thresholds are different. Such differences

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may be related to the subject’s age, other neurological conditions, methods of measurements, or other factors. These reasons may account for why the thresholds of the exposed groups differ. The biggest difference was discriminating pressure on the skin. Ninomiya et al. might have attached greater importance to differentiating two-point discrimination from other kinds of tactile sense, but we attached greater importance to the unity of methods including how pressure was applied. We believe the differentiation of superficial tactile sense from combined sensation like two-point discrimination is very difficult and providing constant supra-threshold stimulation to different individuals is even more difficult. Our results using the threshold by Semmes-Weinstein monofilaments indicate that whole body minimal tactile sense was disturbed in many subjects. These results are similar to those of Ninomiya et al. Threshold differences between control and exposed groups were almost the same. However, thresholds in our study were about 0.6 (by evaluator size) lower than those of Ninomiya et al. both in control and exposed groups. These threshold differences were probably related to the subject’s age, neurological complications, and methods of measurements. Average ages in our study were younger than those of Ninomiya et al. and it is not clear if they excluded neurologically complicated subjects. In addition, the criteria for determining the threshold might have differed. They required ‘‘three replies at the same limit’’ for the determination of thresholds, but how this was interpreted was not clear. Increase of the minimal tactile sense threshold in the exposed group in the chest more than in the fingers was not observed by Ninomiya et al. These different results might have been caused by a difference in subjects, difference of methods including test time, criteria determining the threshold, or other factors. Differences of minimal tactile sense threshold of the central and peripheral sites of the body are not greater and might easily be influenced by some factors, when comparing to the increase of the whole body threshold. We analyzed the exposed groups by dividing subjects with and without complications. Somatosensory disturbance was worse in the subjects with complications, but the characteristics of the sensory disturbances were similar. This suggests that in the polluted area, the cause of the sensory disturbance may be exposure to methylmercury. Uchino et al. (2001) reported sensory impairment in 38 officially certified cases of chronic Minamata disease. Fourteen of the 38 cases had no disturbance of sensation in any limb. Among those cases, 10 had a disturbance of two-point discrimination, 5 had a disturbance of vibration sense, and none had any disturbance of position sense. Our results indicate that peripheral or whole body superficial sensory disturbance is present in almost all of the certified Minamata disease cases and visual constriction is present in over half of them. The subjects in this paper had milder sensory involvement than their cases. However, in the

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Uchino et al. paper sensation was not measured quantitatively and the criteria for defining sensory disturbance were not clear. There is a possibility that whole-body superficial sensory disturbance was present in their cases. In order to detect milder impairments, quantitative methods may be necessary since their detection may depend on the method of evaluation. We found almost no correlations between sensory disturbance and sensory nerve conduction velocity of the median nerve. The normal median nerve conduction velocities suggests that demyelination of peripheral nerves is not the cause of sensory disturbance in chronic methylmercury exposure. Sensory nerve conduction velocity provides information on the function of Aa fibers. These are thick and conduct proprioceptive information from muscle spindle. The absence of any correlation between sensory nerve conduction velocity and position sense is thus important. We did not calculate the amplitude of the sensory conduction test, because the strength of the stimulation was not constant among subjects in this study. The sensory disturbance in Minamata disease has been thought to be caused by injury to the parietal cortex and not related to peripheral nerve impairment (Ninomiya et al., 2005). Small neurons in the central nervous system are known to be vulnerable to methylmercury (Berlin, 1987). Small neurons in the internal granular layer of the cerebral cortex are known to receive input from sensory fibers from the thalamus. If the parietal lobe cortex is responsible for somatosensory disturbance following methylmercury exposure, then fluctuation of the sensory disturbance present in some patients with Minamata disease might be explained. We agree that the cerebral cortex injury is the main cause of the somatosensory disturbance in Minamata disease. But, interestingly, this study suggests that a glove and stocking type superficial sensory disturbance such as occurs with peripheral neuropathies can occur in Minamata disease. This is different from the so-called cortical (complex) sensations like two-point discrimination that are known to occur in Minamata disease and thought to be secondary to involvement of the cerebral cortex. Subjects in this study with and without neurological or neurologically related complications had almost the same complaints and symptoms. Recent papers on Minamata disease from Japan have suggested that aging and health complications may make the differentiation of Minamata disease difficult (Uchino et al., 2005; Futatsuka et al., 2005). Our data suggest that Minamata disease can be diagnosed using quantitative sensory testing in the presence of neurological or other health complications. Some researchers have recently emphasized the presence of psychological and social factors in subjects living in the methylmercury polluted area (Futatsuka et al., 2005; Ushijima et al., 2004a, b). We agree that it is important to consider mental and social factors in order to determine the subjects’ health status. However, we believe it is very important to measure sensation quantitatively in order to

detect the effects of chronic exposure to methylmercury and thus determine the person’s true health status. 5. Conclusions Using a questionnaire, conventional neurological examination, and quantitative sensory measurements, we showed that many subjects exposed to methylmercury during the period of pollution have health disturbances consistent with methylmercury exposure. We also found that symptoms and signs consistent with methylmercury exposure can be diagnosed in the presence of aging and neurological complications. These findings suggest that there may be a large number of individuals who were affected by the mercury pollution who have not been diagnosed previously. We believe that the current health status of residents who lived near Minamata at the time of the pollution should be explored more extensively using sophisticated testing methods including quantitative sensory testing. Information on funding sources and approval The study was conducted without specific funding. This research was carried out at Minamata Kyoritsu Hospital, Neurology & Rehabilitation Kyoritsu Clinic, Kuwamizu Hospital, Kagoshima Seikyo Hospital, and Chidoribashi Hospital. These hospitals and the clinic are privately funded. Subjects were informed orally and in writing about the examination method, how the data would be used, and that their confidentiality would be protected. Each participant gave written informed consent. Acknowledgments The authors thank Hiroshi Suzuki and Kazuhisa Okuda of Minamata Kyroritsu Hospital, Akira Fukuhara, Takamaru Mitsunaga, Takehide Seki, Kazuya Taniguchi, and Takako Fujimoto of Kuwamizu Hospital, Aya Andou, Tomofumi Fujisaki, Takesi Kamei, Hisae Kasida, Hirofumi Kida, Yoshihisa Kitajima, Takasi Kodani, Masanari Komatu, Yosihiro Kubo, Shuzou Maruo, Koushi Mawatari, Kazufumi Minowa, Kouji Nagatani, Akira Oono, Eiji Saeki, Naoko Sasaki, Shuiti Satou, Masumi Suzuki, Shoukan Tanoue, Kiyosi Tokuda, Shin-ichi Tsuzurahara, Hirokazu Uemura, and Yosihito Yamasita of Kagoshima Seikyo Hospital, Yasuharu Arima, Mitsuhiko Funakoshi, Fumiko Higashi, Takahiko Inagaki, Kazufumi Kataoka, Satomi Katsuki, Jun Konishi, Yoshihiro Miyamura, Takahiro Nakashi, Shinko Ootani Kiriko Takahashi, Takaaki Tanaka, Yoshihiro Umeno, and Kazuko Viano of Chidoribashi Hospital for cooperating the examination of subjects in the polluted and control area. They also thank Yuzo Kashiwagi of Minamata Kyoritsu Hospital for having performed psychophysical tests and electromyogram, and Gary J. Myers of the Department of Neurology,

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