Essential criteria for a positive diagnosis of CIDP are:
- progressive weakness in two or more limbs due to a polyradiculoneuropathy;
- loss or diminution of tendon reflexes;
- progression for more than eight weeks or recurrence or relapse and
- evidence of damage to peripheral nerve myelin from nerve conduction tests.
Investigations will include blood tests; usually a lumbar puncture and nerve conduction tests with an electromyogram (EMG) machine, and possibly a Magnetic Resonance Image (MRI) scan. A nerve biopsy may also be performed. The lumbar puncture involves lying on one side and having a needle inserted under local anaesthesia between the vertebrae into the sac of cerebrospinal fluid which surrounds the nerve roots. The idea is worse than the procedure really and it does not usually hurt. The cerebrospinal fluid often contains much more protein than usual while the cell content remains normal. If different changes are found your doctor will have to review the diagnosis with even more care.
The EMG is an electrical recording of the muscle activity. If a nerve is stimulated with a brief electrical pulse (felt like a sharp tap or jolt) muscle activity can be recorded and the speed of nerve conduction worked out. Usually in CIDP nerve conduction is markedly slowed or even blocked. The test lasts about half an hour. It can be uncomfortable but is quite harmless.
The Magnetic Resonance Image (MRI) Scanner is a more recent diagnostic tool and takes pictures, similar to X-rays, of the brain and spinal cord (ie of the central nervous system). The procedure involves your upper body being slid into the tunnel-like scanner and remaining absolutely still during the scanning process which lasts about half an hour. It is entirely painless. MRI scans are used to eliminate the possibility that damage has occurred to the central nervous system that could alter the diagnosis.
Sometimes a nerve biopsy may be performed. This involves a small piece of nerve being removed, usually from the side of the heel of the foot, to be examined in the laboratory. This allows the doctor to see any inflammation and the type of nerve damage. Having the biopsy test involves the use of local anaesthetic and the area may be sore for a week or two afterwards. You may be left with some loss of sensation in a very small area of the foot.
It is helpful to subdivide CIDP into four subcategories that are characterised by the pattern of the disease. These are:
- 'subacute' where symptoms continue to progress and worsen for at least four weeks but not more than eight weeks before leveling off or improving;
- 'chronic progressive' where symptoms continue to progress and worsen for a period exceeding eight weeks;
- 'chronic relapsing' where more than one episode in which symptoms progress and worsen for a period greater than four weeks and
- 'recurrent GBS' where each bout has a progressive phase of less than four weeks.