Why does this happen?
Every sensation in the human body is caused by nerve stimulation. The stimulus (for example: touch, heat, or smell) causes the nerve (or nerves) to fire; that is, they generate an electrical message that is sent to the brain, via the spinal cord. When the message gets to the brain, you become aware of the stimulus.
Paraesthesia is usually caused by a lack of blood supply to (and pressure on) the nerves in the arms or legs. For example, if you fall asleep on your arm, the weight of your body tends to limit the blood supply to the arm; as a result, the nerves become starved of blood, and they start to send unusual signals to the brain. At first, a tingling sensation may be felt, followed by prickling (the latter signals the resumption of pain signals to the brain). The symptoms usually appear in the hands and feet (before being felt in the rest of the limb), because the lack of blood supply affects the extremities first.
Most people have experienced this form of the condition; the paraesthesia is usually temporary, and goes away when the pressure on the limb is removed (and the blood supply returns). However, paraesthesia can also be an indication of a more serious, underlying condition. For example, if a patient suffers from atherosclerosis (a condition where the arteries become partially blocked) the bloody supply to their nerves is permanently reduced. As a result, they may experience paraesthesia for long periods of time. Other conditions, which may lead to paraesthesia, include:
- Raynaud’s Phenomenon;
- Alcohol poisoning;
- Multiple sclerosis;
- Pernicious anaemia;
- Carpal tunnel syndrome;
- Nerve root impingement;
- Pressure on the spinal cord.
The list is by no means fully inclusive.
The last two conditions are particularly relevant to us. For example, there are many conditions in the spine that can lead to nerve root impingement (also known as entrapment). If this occurs in the neck (cervical spine), then paraesthesia may be experienced in the arms. If the sciatic nerve is pinched (lumbar spine), the symptoms may be felt in the legs, a classic symptom of Sciatica.
Paraesthesia can also be caused pressure on the spinal cord itself (specifically, pressure on the dura mater, the outer-most sheath, or meninx, of the spinal cord); a number of spinal conditions can lead to this.