How is Guillain-Barre syndrome (GBS) diagnosed?
Guillain-Barré syndrome is characterized by rapid paralysis on both sides of the body. Other unique symptoms distinguish it from other syndromes.
Guillain-Barré syndrome is called a syndrome rather than a disease because it is not clear that a single specific disease-causing agent is involved. A syndrome is a medical condition characterized by a constellation of certain symptoms (what the affected person feels) and signs (what a health care provider can observe or measure). The signs and symptoms of Guillain-Barré syndrome can be quite varied, so, on rare occasions, it may be difficult to diagnose in its earliest stages.
Several disorders have symptoms similar to those found in Guillain-Barré syndrome. Collectively, the signs and symptoms form a certain pattern that helps to differentiate Guillain-Barré syndrome from other disorders. For example, if the symptoms appear on both sides of the body (most common in Guillain-Barré syndrome) and symptoms appear rapidly (in other disorders, muscle weakness may progress over months rather than days or weeks), it is more likely to be Guillain-Barré syndrome. In addition, reflexes such as knee jerks are usually lost. Because the signals traveling along the nerve are slower, a nerve conduction velocity test can give clues to aid in the diagnosis. The cerebrospinal fluid (CSF) that bathes the spinal cord and brain contains more protein than usual in Guillain-Barré syndrome  but has a normal CSF cell count. Therefore, a spinal tap (a procedure in which the doctor inserts a needle into the patient’s lower back to draw cerebrospinal fluid from the spinal column) may need to be performed. The peripheral white blood cell count in Guillain-Barré syndrome is normal.
The two most common forms can be differentiated with specific tests. In the acute inflammatory demyelinating polyneuropathy (AIDP) form, immune system reactions result in demyelination. It is diagnosed when nerve conduction studies show slowing of nerve conduction suggestive of demyelination in two or more motor nerves. Acute motor axonal neuropathy (AMAN) is diagnosed when nerve conduction studies show a reduction of compound muscle action potential without significant conduction slowing. Testing for serum antibodies to certain nerve components may be useful for confirming the diagnosis of Miller Fisher syndrome (MFS).