出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/06/23 14:45:05」(JST)
| Orthostatic hypotension | |
|---|---|
| Classification and external resources | |
| Pronunciation | /ˌɔːrθəˈstætɪkˌhaɪpəˈtɛnʃən/ |
| Specialty | Cardiology |
| ICD-10 | I95.1 |
| ICD-9-CM | 458.0 |
| DiseasesDB | 10470 |
| MeSH | D007024 |
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[edit on Wikidata]
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Orthostatic hypotension, also known as postural hypotension,[1] orthostasis, and colloquially as head rush or dizzy spell, is a form of low blood pressure in which a person's blood pressure falls when suddenly standing up or stretching. In medical terms, it is defined as a fall in systolic blood pressure of at least 20 mm Hg or diastolic blood pressure of at least 10 mm Hg when a person assumes a standing position.
The symptom is caused by blood pooling in the lower extremities upon a change in body position. It is quite common and can occur briefly in anyone, although it is prevalent in particular among the elderly, and those with low blood pressure.
When orthostatic hypotension is present, the following symptoms can occur after sudden standing or stretching (after sitting):
They are consequences of insufficient blood pressure and cerebral perfusion (blood supply). Occasionally, there may be a feeling of warmth in the head and shoulders for a few seconds after the dizziness subsides. The drop in blood pressure may cause a vasovagal episode to occur.
Orthostatic hypotension is caused primarily by gravity-induced blood-pooling in the lower extremities, which in turn compromises venous return, resulting in decreased cardiac output and subsequent lowering of arterial pressure. For example, changing from a lying position to standing loses about 700 ml of blood from the thorax, with a decrease in systolic and diastolic blood pressures.[2] The overall effect is an insufficient blood perfusion in the upper part of the body.
Still, the blood pressure does not normally fall very much, because it immediately triggers a vasoconstriction (baroreceptor reflex), pressing the blood up into the body again. (Often, this mechanism is exaggerated and is why diastolic blood pressure is a bit higher when a person is standing up, compared to a person in horizontal position.) Therefore, a secondary factor that causes a greater than normal fall in blood pressure is often required. Such factors include low blood volume, diseases, and medications.
Orthostatic hypotension may be caused by low blood volume, resulting from bleeding, the excessive use of diuretics, vasodilators, or other types of drugs, dehydration, or prolonged bed rest. It also occurs in people with anemia.
The disorder may be associated with Addison's disease, atherosclerosis (build-up of fatty deposits in the arteries), diabetes, pheochromocytoma, and certain neurological disorders, including multiple system atrophy and other forms of dysautonomia. It is also associated with Ehlers-Danlos syndrome and Anorexia Nervosa. It is also present in many patients with Parkinson's disease resulting from sympathetic denervation of the heart or as a side-effect of dopaminomimetic therapy. This rarely leads to fainting unless the person has developed true autonomic failure or has an unrelated heart problem.
Another disease, dopamine beta hydroxylase deficiency, also thought to be underdiagnosed, causes loss of sympathetic noradrenergic function and is characterized by a low or extremely low levels of norepinephrine, but an excess of dopamine.[3]
Quadriplegics and paraplegics also might experience these symptoms due to multiple systems' inability to maintain a normal blood pressure and blood flow to the upper part of the body.
A study by a Harvard Medical School team found the two sacs in the inner ear, the utricle and the saccule, affect brain blood flow; thus, inner ear problems, which increase with old age, may be involved in orthostatic hypotension.[4]
Orthostatic hypotension can be a side-effect of certain antidepressants,[clarification needed][further explanation needed] such as tricyclics[5] or monoamine oxidase inhibitors (MAOIs).[6] Marijuana and tetrahydrocannabinol can on occasion produce marked orthostatic hypotension.[7] Orthostatic hypotension can also be a side effect of Alpha-1 blockers (alpha1 adrenergic blocking agents). Alpha1 blockers inhibit vasoconstriction normally initiated by the baroreceptor reflex upon postural change and the subsequent drop in pressure.[8]
Orthostatic hypotension sometimes is a reversible neurological complication of vitamin B12 deficiency.[9]
The use of a safety harness does not contribute to orthostatic hypotension in the event of a fall. This notion is a hypothetical risk which has all but been eliminated due to modern design and safety regulations.[10] If worn properly, a safety harness may safely rescue its user from a fall without any further complications so long as the individual does not remain suspended for prolonged periods of time.[10]
Patients prone to orthostatic hypotension are the elderly, post partum mothers, and those having been on bedrest. People suffering from anorexia nervosa and bulimia nervosa often suffer from orthostatic hypotension as a common side-effect. Consuming alcohol may also lead to orthostatic hypotension due to its dehydrating effects.[citation needed]
There is a simple test for OH that measures the person's blood pressure after lying flat for 5 minutes, then 1 minute after standing, and 3 minutes after standing.[11] Orthostatic hypotension is defined as a fall in systolic blood pressure of at least 20 mmHg and/or in the diastolic blood pressure of at least 10 mmHg between the supine reading and the upright reading. In addition, the heart rate should also be measured for both positions. A significant increase in heart rate from supine to standing may indicate a compensatory effort by the heart to maintain cardiac output or Postural Orthostatic Tachycardia Syndrome (POTS). A tilt table test may also be performed.
The evidence to support treatment is poor. A number of measures have slight evidence to support their use including compression bandages, midodrine, indomethacin, fluoxetine, dopamine antagonists, metoclopramide, domperidone, monoamine oxidase inhibitors with tyramine (can produce severe hypertension), oxilofrine, potassium chloride, and yohimbine.[12] Midodrine can reduce dizzy spells and faints by about a third, although the quality of the evidence underlying this finding is limited.[13] The main side-effect is piloerection ("goose bumps").[13]
Orthostatic hypotension may cause accidental falls.[14] It is also linked to an increased risk of death, cardiovascular disease, heart failure, and stroke.[15]
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| リンク元 | 「眩暈」「めまい発作」 |
| 関連記事 | 「spell」「dizzy」 |
| 末梢性めまい | 中枢性めまい | |
| 眼振 | 水平性 | 水平性 |
| 回転性 | 回転性 | |
| 垂直性 | ||
| 耳鳴、難聴 | ○ | 稀 |
| 頭位 | 増強 | 軽度変化 |
| カロリックテスト | 陰性/低下 | 正常 |
| 意識消失 | なし | あり |
| 神経症状 | 稀 | あり |
| 末梢前庭性めまい | 中枢性めまい | |
| 性状 | 回転性が多い | 回転性は少ない |
| 強さ | 強い | 軽度 |
| 持続時間 | 数日まで | 数日以上 |
| 眼振の方向 | 一方向性 | 注視方向性 |
| 自発眼振の性状 | 水平回旋性が多 | 純回旋性、垂直性 |
| 固視の影響 | 抑制される | 抑制されない |
| 注視眼振の増強する方向 | 健側 | 患側 |
| 蝸牛症状 | 多い | 稀 |
| 中枢神経症候 | なし | あり |
| 悪心・嘔吐 | 軽度～重度 | ない or 軽度 |
| 前庭症状 | 蝸牛症状 | 特徴 | |||
| 眩暈 | 難聴 | 耳鳴 | |||
| 薬剤性 | ループ利尿薬 | ○ | ○ | ○ | 投与歴 |
| アミノグリコシド系抗菌薬 | ○ | ○ | ○ | 投与歴 | |
| 感染症 | 内耳炎 | ○ | ○ | ○ | |
| 新生物 | 小脳橋角部腫瘍 | ○ | ○ | ○ | CT, MRI異常 |
| 特発性 | 突発性難聴 | △ | ○ | ○ | 単発 |
| 特発性 | メニエール病 | ○ | ○ | ○ | 発作性、反復性 |
| 特発性 | 良性発作性頭位眩暈症 | ○ | × | 特定の頭位で発生。眩暈頭位の反復で減衰 | |
| 炎症 | 前庭神経炎 | ○ | × | × | 単発 |