Numbness is a term understood by everyone. Whether talking about emotional numbing or a numb finger, numbness means the same thing: poor or reduced sensation.
Numbness may be caused by damage to any part of the nervous system, whether due to a nerve injury in the arms and legs, or damage to the spinal cord or brain.
Numbness in the limbs is usually caused by nerve compression - a nearly universal experience - experienced when your leg or arm goes to sleep. Typical features of nerve compression are numbness, pins and needles, and some degree of weakness. Common causes of nerve compression would be conditions such as carpal tunnel syndrome, as well as slipped discs, in which case pain is usually also present.
However, very similar symptoms may arise in the event of spinal cord or brain disease. In general, the vast majority of patients with numbness caused by problems in these regions will have significant other complaints, such as weakness, spasms or seizures. For example, although multiple sclerosis will present with numbness in various parts of the body, there will frequently be associated features, such as poor vision, increased passing of urine, spasms in the legs and weakness and poor balance.
When to see a doctor
It's probably a good idea to get numbness checked out if it persists for more than a week, and especially if it seems to be getting worse. Of course, if it is accompanied by more severe complaints, such as weakness, this would make a visit to the doctor even more urgent.
How is it assessed?
Given that numbness is a very subjective experience, it is often difficult for both the doctor and the patient to assess how significant it may be. Numbness that comes and goes is likely to be less important than numbness that is always there, or more worrying still, numbness that is becoming steadily worse. Numbness accompanied by pain and pins and needles is typically due to peripheral nerve compression, and may require attention to avoid permanent nerve injury. On the other hand, numbness may often be one of a series of vague complaints, which fortunately do not indicate a serious problem, but may be related to non-specific factors and stress.
Assessment of numbness is therefore difficult: at the one extreme, numbness may be a feature of nothing more than stress, whereas at the other it may indicate something as serious as a brain tumour or multiple sclerosis. Although the latter problems usually have many other features that point to them as the probable cause of numbness, this is not invariably the case. In this respect, numbness resembles headache, which is usually not a symptom of severe brain disease on its own, but does cause worry for both patient and doctor.
Generally speaking, the best qualified person to investigate most types of numbness would probably be a neurologist. The importance of numbness is critically affected by the company it keeps and the risk profile of the patient. Patients with high risk of stroke who suffer from numbness might therefore require more intense investigation than those who do not.
It is highly likely that if numbness is the only symptom, and the neurological examination is normal, further tests are unlikely to yield a cause. This is however a generalisation and each case needs to be judged on its own merits.
Depending on the nature of the complaint and accompanying features, investigations may focus on the peripheral nerve, on the roots and vertebral disc, on the spinal cord, or on the brain itself.