Peripheral facial palsy of unknown origin (Bell's palsy) is a disease with a large number of differential diagnoses . In our study, we required evidence of intrathecally produced antibodies or preceding erythema migrans for a definite LNB diagnosis. Intrathecal antibody production is generally used for a definite diagnosis, but it has low sensitivity in the very early phase of the disease . In general, patients with LNB display intrathecally produced antibodies within two weeks of the onset of symptoms, but in some patients antibody production may be delayed for up to six weeks, which poses a diagnostic problem [16, 17]. Furthermore, both the culture of CSF for Bb and the detection of Bb in CSF by specific PCR analysis have a very low sensitivity and are not used in clinical routine . It is therefore difficult completely to rule out the possibility that some patients with LNB were classified as BP, especially since patients diagnosed as BP underwent LP earlier than patients diagnosed as LNB (definite or possible) (Table 1). However, clinical follow-up did not change the diagnosis in any patient with BP. With these diagnostic drawbacks, there will always be some patients for whom the diagnosis is difficult and who will be classified as unknown. On the other hand, using only serum antibodies against Bb as a diagnostic criterion will produce a large number of false positive cases, since the seroprevalence of antibodies against Bb is high in the general population; 7-29% in endemic areas of Sweden [19, 20].
The possible LNB group in the study (who had Bb antibodies in serum but not in CSF and no history of EM) did not differ significantly from the BP patients in terms of clinical characteristics or laboratory parameters and did not, as a median, display mononuclear pleocytosis or elevated albumin in CSF. We interpreted this as a reflection of the high seroprevalence of Bb antibodies in the population, rather than a Bb infection as the aetiology of peripheral facial palsy.
Among the 38 patients without associated neurological symptoms that were analysed as a subgroup, BP patients, as expected, constituted the majority, 66% (n = 25), and patients with possible LNB 18% (n = 7). However, 16% (n = 6) of these patients were diagnosed as definite LNB, showing that the absence of associated neurological symptoms does not exclude LNB as a cause of peripheral facial palsy. The differences in the CSF mononuclear cell count and albumin levels were also significant in the subgroup analysis, underlining the importance of CSF sampling in the diagnosis of LNB.
The observed seasonal variation in the incidence of peripheral facial palsy caused by LNB, with a peak in late summer/early autumn and no cases during late winter/early spring, confirms previous studies and co-varies with the tick-feeding season. The absence of any cases outside that season is also evidence that the incubation period for LNB is rarely more than a couple of weeks, as previously described [3, 5, 6, 21].
The differing clinical picture between peripheral facial palsy caused by LNB and BP, with a higher occurrence of neurological symptoms outside the cranial nerve area in peripheral facial palsy caused by LNB, is broadly in accordance with previous studies. Two small, early studies describe polyneuropathy and radiculitic pain as symptoms that predict LNB, whereas a larger study from 2002 describes headache, arthralgia and enlarged cervical lymph nodes [3, 4, 6]. What is not usually described, however, is the high percentage of patients with BP that describe pain from the affected area of the face (43%).
A reported history of tick bite was uncommon in the definite LNB group (29%). The same low numbers have also been seen in some earlier studies, while other studies have suggested the use of a history of tick bite in discriminating between BP and LNB [5, 22, 23]. It is therefore important to stress that the absence of a tick bite history does not exclude LNB.
The present study was performed at a department, which mainly treats adult patients. Consequently, the age of the patients in this study is not fully representative of all patients with BP or LNB. However, the median ages of the respective groups are similar to those reported previously. Patients with definite LNB were older than patients with BP, median age 46 (7-75) and 36 (15-70) respectively. This is in line with the bimodal age distribution of LNB, with a median in the fifth decade of life and with the reported peak incidence of BP around the second to fifth decade of life [14, 24].
One aim of this study was to analyse differences in CSF parameters between the definite LNB and BP patient groups. It was therefore not possible to use CSF mononuclear pleocytosis as a diagnostic criterion for LNB. In spite of this, we found that time during the year, presence of neurological symptoms outside the affected area of the face and mononuclear pleocytosis in CSF were strong predictive factors for LNB. All these data are possible to obtain within hours after admission, which can be of help in the choice of acute treatment.