Laboratory Studies
Laboratory tests help to support the diagnosis of acute inflammatory demyelinating polyneuropathy (AIDP) and monitor patients with the syndrome. No associated hematologic or urinary findings are characteristic of the diagnosis. The erythrocyte sedimentation rate is normal. Serum protein electrophoresis does not show an abnormality. Hyponatremia due to inappropriate ADH secretion may occur. [26]
Cerebrospinal fluid
Increased CSF protein without an increased WBC count (albuminocytologic dissociation) is observed classically in AIDP. However, this finding is not specific to AIDP.
About two-thirds of patients have this CSF finding during the first week of symptoms and 82% have it by 2 weeks after symptom onset.
Although protein values can be elevated by 10-fold or more, no association exists between protein level and clinical severity.
Some patients have oligoclonal banding of the CSF.
Myelin basic protein also is increased in some patients.
More than 90% of patients have fewer than 10 WBC/µL, with a mean of 3 WBC/µL. If more than 50 WBC/µL are present, an alternative diagnosis should be considered, including HIV, Lyme disease, polio, or other infections. Patients with HIV-associated AIDP often have >50 WBC/µL (mean, 23 WBC/µL).
In non-HIV cases, the cells are overwhelmingly lymphocytes, whereas a nonlymphocytic pleocytosis is seen in patients with HIV.
Studies have shown sphingomyelin (SM) levels in CSF are elevated in AIDP and typical CIDP, and interestingly the SM levels are related to the disease activity. [27, 28] . Further studies are needed to confirm this, and if results are favorable CSF SM level might become the new biomarker for AIDP diagnosis and prediction of severity and prognosis.
Blood tests
Blood tests have little role in the diagnosis of AIDP but may help to exclude other conditions and to serially monitor patients with AIDP in the hospital (especially those who are critically ill).
Recently, an association has been found between acute axonal motor variants and immunoglobulin G (IgG) directed against ganglioside GM1 and/or GD1a. Furthermore, most patients with the Miller-Fisher variant of AIDP have antibodies directed against ganglioside GQ1b. Some patients with pure sensory variants have antiganglioside GD1b antibodies. These tests are seldom beneficial in classic AIDP, but can help when patients present with variants.
Rabbit ataxic neuropathy and several case reports have suggested a close association of IgG anti-GD1b antibodies with ataxia in Guillain-Barré syndrome. However, about half of the patients with Guillain-Barré syndrome having IgG anti-GD1b antibodies with no reactivities against other gangliosides (GD1b-mono IgG) do not exhibit ataxia. Antibodies specific to ganglioside complexes (GSCs) containing GD1b have been found in sera from some patients with Guillain-Barré syndrome. IgG antibodies highly specific for GD1b may induce ataxia in Guillain-Barré syndrome. [29]
Although not necessary for diagnosis, measurement of antiviral or antibacterial antibodies may confirm an association.
Measurement of potassium, phosphate, and porphyrin metabolism products may help exclude alternative diagnoses in atypical cases.
Some critically ill patients with AIDP develop the syndrome of inappropriate antidiuretic hormone (SIADH) with associated hyponatremia and reduced serum osmolarity.
Additionally, liver enzymes sometimes are elevated in AIDP.
If intravenous immunoglobulin (IVIg) therapy is anticipated in noncritical cases, immunoglobulin A (IgA) levels should be drawn before treatment. [30]
Urine tests
Urine tests to exclude heavy metal intoxication may be necessary in some patients.
Stool cultures
Stool cultures may confirm C jejuni enteritis. Patients with this condition may have a more aggressive course and a slightly worse prognosis.
Imaging Studies
Imaging is seldom necessary for diagnosing acute inflammatory demyelinating polyneuropathy, but it may be necessary to exclude alternative diagnoses and to monitor critically ill patients.
MRI of the spine is sometimes necessary to rule out spinal cord and/or nerve root processes that mimic AIDP.
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Nerve root, cauda equina, or cranial nerve enhancement is observed sometimes on T1-weighted, gadolinium-contrasted scans. This can help diagnose some atypical cases.
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Cytomegalovirus radiculitis, meningeal carcinomatosis, lymphomatosis, and sarcoidosis may have similar MRI findings.
Chest radiography in children may reveal a pattern that is consistent with mycoplasmal pneumonia. Additionally, chest and abdominal radiography may be necessary in critically ill patients to evaluate for possible pneumonia and ileus.
Other Tests
Electrodiagnostic testing is always necessary to confirm the diagnosis of acute inflammatory demyelinating polyneuropathy (AIDP).
Nerve conduction studies (NCS) can document demyelination, the hallmark of acute inflammatory demyelinating polyradiculoneuropathy. Early on, findings of NCS studies are often normal. However, 90% are abnormal within 3 weeks of symptom onset.
Patients who meet 3 of the 4 NCS criteria listed below have a clear primary demyelinating neuropathy, although patients who meet fewer than 3 criteria still may have AIDP. Severe slowing of conduction velocities may be more consistent with chronic inflammatory demyelinating polyneuropathy (CIDP). Details of electrodiagnostic criteria are provided in Cornblath. [31]
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Reduced conduction velocity
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Conduction block or abnormal dispersion
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Prolonged distal latencies
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Prolonged F-waves
Matsumoto et al. provide electrophysiological evidence to show that the proximal segment of peripheral nerves is assumed to be involved in both demyelinating and axonal types of Guillain-Barré syndrome (GBS). They performed nerve conduction studies in 9 demyelinating GBS and 7 axonal GBS patients. Cauda equina conduction time (CECT) was obtained by subtracting S1-level latency from L1-level latency. CECT was prolonged in all the patients with demyelinating GBS who had leg symptoms, whereas motor conduction velocity (MCV) at the peripheral nerve trunk was normal in all the patients. In all the patients with axonal GBS having leg symptoms, CECT and MCV were normal and no conduction blocks were detected between the ankle and the neuro-foramina, suggesting that the cauda equina is much more frequently involved than the peripheral nerve trunk in demyelinating GBS. In axonal GBS, usually, CECT is normal and segmental lesions are absent between the ankle and the neuro-foramina. Therefore, the researchers believe that the CECT measurement should be very useful for directly detecting demyelinating lesions in GBS. [32]
Umapathi et al. (2015), using nerve conduction studies, confirm that the "sural-sparing pattern" of Guillain-Barré syndrome (GBS) occurs in AIDP as well as other non-demyelinating GBS-subtypes, namely acute motor axonal neuropathy (AMAN), acute motor-sensory axonal neuropathy (AMSAN) and Miller Fisher syndrome (MFS). [33]
Needle EMG can document the extent of denervation.
Several electrodiagnostic criteria have been proposed for AIDP. A review by Tansiki discussed existing criteria and found that there is agreement among existing criteria on limits of demyelination. Further studies are warranted for definition of AMAN and AMSAN and whether serial electrodiagnostic examinations are necessary for diagnosis of subtypes. [34]
Findings of other electrophysiologic tests, such as blink reflexes, phrenic nerve conduction, and somatosensory evoked responses, may be abnormal but do not offer any advantages to typical NCS studies.
Autonomic tests such as sympathetic skin responses and cardiovagal testing may be indicated in patients with autonomic failure.
Pulmonary function tests, useful in determining the timing of intensive care unit (ICU) transfers and elective intubation, should be performed in all patients. Transfer to an ICU generally is indicated when forced vital capacity (FVC) is less than 20 mL/kg. Intubation is usually warranted when FVC drops to 15 mL/kg or negative inspiratory pressure drops to less than -25 cm H2 O.
Electrocardiography (ECG) and cardiac monitoring can be helpful when arrhythmias occur. Other possible abnormalities include atrioventricular block, QRS widening, and T-wave abnormalities.
Jin et al measured the CSF tau protein levels in 26 patients with Guillain-Barré syndrome. The levels of the poor outcome group (Hughes grade at 6 months was between II and VI, n = 6) were higher than those of the good outcome group (0 or I, n = 20) (p < 0.0005). The higher levels of CSF tau may reflect axonal degeneration and could predict a poor clinical outcome in Guillain-Barré syndrome. [35]
Procedures
Lumbar puncture is performed to obtain CSF for analysis (see Lab Studies).
Histologic Findings
Nerve biopsy is seldom required to diagnose acute inflammatory demyelinating polyradiculoneuropathy. However, in patients with prolonged clinical courses, histologic examination can help to differentiate CIDP from AIDP. Nerve biopsies in AIDP show an inflammatory infiltrate during the first few days.
Later on, macrophages are seen, sometimes with myelin stripping. Axons are usually spared. Under electron microscopy, macrophages (which are stripping myelin) are seen beneath the basement membrane and are usually advancing along the minor dense line.
