Send Correspondence to journal:
Re: Acute motor conduction block neuropathy Another Guillain–Barré syndrome variant

We read with interest the article by Capasso et al [1] and the accompanying Editorial [2] concerning two patients with pure motor Guillain-Barré syndrome (GBS), early nerve conduction block (CB), preserved motor conduction velocity (MCV) and high titers of anti- ganglioside antibodies. They conclude that these cases may represent a new axonal GBS variant with nerve CB induced by anti-ganglioside antibodies. We would like to offer an alternative explanation.

Starting from current criteria for electrophysiological classification of GBS, it seems that there is evidence of demyelination at initial electrophysiological examinations:

1) In both patients, MCV of ulnar nerves (above elbow-below elbow segment) less than 85% of lower limit of normal; and

2) In patient 2, distal motor latencies (DML) of three nerves greater than 120% of upper limit of normal. [1]

It is worth noting that DML prolongation for left median nerve is not slight, as stated by the authors, but marked (according to Figure 1D, about 6.5 ms representing 150% of upper limit of normal). Therefore these cases should be categorized as examples of demyelinating GBS.

The mechanism of nerve CB in GBS has not been positively established. In vitro and in vivo experimental studies indicate that physiological action of the anti-ganglioside antibody is insufficient to explain observed CB in human diseases. [3] In demyelinating GBS, early and potentially reversible attenuation of CMAP with preserved MCV may be due to an increase of endoneurial fluid pressure in nerve trunks causing variable degrees of nerve ischemia. [4] In experimental allergic neuritis, initial symptoms do not correlate with demyelination but with the appearance of endoneurial edema and inflammatory infiltrates. [5]

In these cases, [1] it is conceivable that early in the clinical course, severe focal inflammatory lesions could induce ischemic CB in certain segments of intermediate and distal motor nerves. Nerve ischemia in EAN or GBS mainly involves centrofascicular areas, but substantially preserved endoneurial transverse areas remain eventually guaranteeing maximal MCV, [4] as observed in most but not all nerve segments examined by the authors. We agree that their cases represent an “arrested” type of GBS possibly because of early immunomodulatory therapy. The mechanism of selective involvement of the motor system in GBS is puzzling as GM1 epitopes are present in motor and sensory axons, and nerve ischemia involves motor and sensory fibers. [3,4]

References

1. Capasso M, Caporale CM, Pomilio F, Gandolfi P, Lugaresi A, Uncini A. Acute motor conduction block neuropathy. Another Guillain-Barré syndrome variant. Neurology 2003; 61: 617-622.

2. Yuki N, Saperstein DS. Axonal Guillain-Barré syndrome subtypes. Do we need more splitting? Neurology 2003; 61: 598-599.

3. Kaji R, Kimura J. Facts and fallacies on anti-GM1 antibodies: physiology of motor neuropathies. Brain 1999; 122: 797-798.

4. Berciano J, García A, Figols J, Muñoz R, Berciano MT, Lafarga M. Perineurium contributes to axonal damage in acute inflammatory demyelinating polyneuropathy. Neurology 2000; 55: 552-559.

Send Correspondence to journal:
Re: Reply to Berciano

We thank Dr. Berciano for giving us the opportunity to clarify the pathophysiological basis of conduction slowing in the patients we reported. [1] Dr. Berciano correctly pointed out that in both patients motor conduction velocity in the above-below elbow segment of ulnar nerves was slowed in the range usually considered demyelinating. The slowing was present from the very first recording when conduction block (CB) was at maximum and the duration of proximal compound muscle action potential (CMAP) was also decreased (Figure 1 E and B). Conduction velocities increased with the decrease of CB and returned to normal range when CB had disappeared without development of excessive temporal dispersion of proximal CMAP (Figure 1 E and B).

The above findings suggest that the conduction slowing is a factitious phenomenon due to preferential, possibly functional, block of large diameter, fastest conducting fibers and it is not due to de-remyelination. Thus, the fibers responsible for the onset latency from distal stimulation are not the same responsible for onset from proximal stimulation. Conduction velocity is slow because different population of fibers are considered for calculation. Similar findings have been described in acute compressive neuropathies with conduction block. [2]

Regarding the alleged increased distal motor latency (DML) of median nerve in patient 2, the actual value is 6.1 ms and it refers, according to the legend of figure 1, to the left peroneal nerve as the symbol is a filled rhombus. The left median is represented by filled circles with DML of 4.2 ms. Dr Berciano my have erred because the print in the final reproduction of the picture is very small. In our laboratory, the upper limit of normal for peroneal DML is 5.5 ms. Therefore the recorded DML does not reach the 120% limit indicative of demyelination. The same reasoning applies to DML of left ulnar nerve in patient 1 and right ulnar nerve in patient 2 which were 3.8 ms with upper limit of normal of 3.2 ms. Sequential recordings in figure 3 demonstrated that DML improved in parallel with the increasing of distal CMAP area (see also figure 1 C and D) and similar to the interpretation of conduction slowing, we think that the slightly prolonged DMLs are due to preferential distal conduction block of large diameter fibers.

For these reasons, we disagree with Dr. Berciano’s statement that our patients “should be categorized as examples of demyelinating GBS”. Regarding Dr. Berciano’s suggestion that CB in our patients may be due to inflammatory endoneurial edema causing nerve ischemia, there is currently no proof supporting this hypothesis versus an immune-mediated conduction failure at the Ranvier nodes without demyelination as we suggested.

References

1. Capasso M, Caporale CM, Pomilio F, Gandolfi P, Lugaresi A, Uncini A. Acute motor conduction block neuropathy: another Guillain-Barré syndrome variant. Neurology 61: 617-622, 2003