No specific test is available to diagnose Guillain-Barre syndrome definitively. Rather, it is necessary to consider the signs and symptoms and their time course, as well as cerebrospinal fluid findings and electromyography results. Electromyography is important both for diagnosis of the condition and as a tool to assist in predicting the long-term prognosis. The classical findings are reduced nerve conduction velocities and conduction blocks. Prolonged distal latencies may be seen, as well as absent F and H responses. Responses consistent with axonal degeneration may also be present. Patients with mean distal motor amplitudes less than 20 per cent of normal have a poor prognosis overall (MlCKh§DD...19.8.8). However, it is still not possible to make strong predictions on an individual patient basis.

The diagnostic process is complicated by the wide range of possible clinical presentations, and in view of this the National Institute of Neurological and Communicative Disorders and Stroke established a set of diagnostic criteria (Table 1) to act as a general guide (AshbM,ry..§tal: 1978).

Table 1 Diagnostic criteria for the Guillain-Barré syndrome

Differential diagnosis of Guillain-Barre syndrome includes several infectious processes, toxic exposures, and porphyria. Infections include the now uncommon disorder of poliomyelitis, which is a febrile illness associated with an asymmetrical pattern of weakness as well as meningeal irritation and a marked cerebrospinal fluid pleocytosis. Diphtheritic neuropathy is very similar to Guillain-Barre syndrome. A history of pharyngitis and a marked delay between this episode and the onset of weakness (often about 3 months) help to distinguish the two conditions. Other toxic conditions include botulism, hexane inhalation, arsenic ingestion, tick paralysis, and ingestion of marine animal toxins. Hepatic porphyria also needs to be considered; these patients will also have a history of abdominal pain, and the weakness will generally develop after exposure to barbiturates or other drugs.

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