The commonest causes of such numbness are:
Lumbar disc lesion: This may be associated with pain in the back and weakness of the leg.
Peripheral neuropathy: common conditions causing this are diabetes, Vit. B deficiency, etc.
Vascular insufficiency: Atherosclerosis of arteries, spasm, etc, may cause vascular insufficiency. This might lead to claudication with pain in the calf muscles on walking which is relieved on rest.
Pain around the back going down the leg, numbness of the leg or foot and tingling or itchiness are all symptoms of a disc lesion. Most disc lesions are minor and can be handled with simple exercises, lumbar belts, steroid injections, non steroidal anti-inflammatory injections, etc. Some cases may need surgery.
Peripheral neuropathies are most commonly caused by diabetes mellitus, but there are many other diseases as well and it needs to be well investigated to find out the cause.
Vascular insufficiency is another common cause, and is more common in those above 60 and in smokers.
But these causes need to be considered when the symptoms are chronic and have occured for several weeks. For you, it might be something simple like a bad fitting shoe, etc. SInce it is only two days, you can try initially with Vit. B12 supplements and rest. If the symptoms persist for more than 3 weeks, then these causes need to be thought of.
If they persist, you should consult your health provider or a neurologist if possible and get yourself checked up; you may need a CT or MRI of the brain and tests for muscle conductivity and nerve velocity, etc. You may also needa Doppler study of the blood flow in your leg if vascular insufficiency is suspected.
You may consult these sites for more info: http://www.ninds.nih.gov/disorders/peripheralneuropathy/detail_peripheralneuropathy.htm
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