In the News
Biochemical Abnormalities In Fibromyalgia Syndrome
I. John Russell, M.D. (Journal of Musculoskeletal Pain, 1999).
II. SUMMARY.
Objectives: To describe new findings regarding metabolic and nociceptive processes in fibromyalgia (FS) patients.
Findings: The earlier observation of elevated substance P in the cerebrospinal fluid (CSF) of FS patients has been confirmed with the documentation of levels which approach correlation with the painful symptoms. Newly recognized biochemical abnormalities include elevated CSF kynurenine, low serum serotonin, and low red cell NADP, NADH, and ATP.
Conclusions: These findings support the hypothesis that FS is a metabolic disorder with demonstratable biochemical abnormalities potentially capable of explaining the clinical symptoms. It is hoped that the recognition of these abnormalities will facilitate the development of more specific and more effective therapies.
KEYWORDS: Fibromyalgia, pathogenesis, serotonin, substance P, kynurenine.
INTRODUCTION
Fibromyalgia syndrome (FS) is a chronic, painful, musculoskeletal disorder
of unknown etiology. A growing body of epidemiological evidence has shown
it to be relatively common, occurring in up to 2.5% of the general population.
At that rate, it would be about twice as common as rheumatoid arthritis
(RA). That comparison is relevant because the severity of the discomfort
experienced by FS and RA patients is comparable. In addition, the ability
of these two patient groups to perform specific work tasks is similarly
impaired. FS patients sleep poorly, exhibiting non-restorative sleep physiology
patterns relatively deficient in stage IV non-REM. Efforts to develop
rational therapy for FS symptoms have been hampered by a real lack of
information about its etiology and pathogenesis. For many years it was
believed that the problem was either psychological or that it resided
in the skeletal muscles of affected individuals. Carefully controlled
psychological and muscle histology studies have now largely discounted
those ideas. Such revelations have indirectly provided support for the
central metabolic hypothesis which predicts that an interpretative defect
in the central nervous system (CNS) may be responsible for abnormal perception
of pain in the absence of recognizable peripheral tissue injury. The concept
of metabolic pain enhancement in FS is not new but seems increasingly
plausible. Moklofsky and colleagues proposed that serotonin might be deficient
in FS because that neurotransmitter influences both deep restorative sleep
and pain perception. It can influence pain perception at peripheral,,
spinal and central nervous system levels. It is also known to modulate
the function of substance P, particularly with reference to the interpretation
of sensory stimuli. Studies conducted by our group over the past 10 years
in San Antonio have identified several very interesting biochemical abnormalities
in cerebrospinal fluid and blood samples obtained from patients with FS.
It is beyond the scope of this review to detail all of those findings,
so the aim will be to focus on a small number of key new concepts which
provide a basis for a better understanding of the biochemical defects
in this disorder.
CEREBROSPINAL FLUID
Our group had predicted that the pathogenesis of fibromyalgia must involve
one or more abnormalities in the processes by which the central nervous
system interprets incoming pain signals. It seemed logical, therefore,
to examine the metabolic status of cerebrospinal fluid (CSF) samples as
a biological fluid which enjoys intimate contact with brain tissue and
reflects the biochemical activities of the brain. CSF Substance P A prior
study conducted by Vaeroy et al., in Norway found substance P (SP) to
be 3-fold elevated in FS CSF compared with NC CSF, but did not document
its relationship to symptoms. We conducted a similar study in San Antonio,
collecting and analyzing CSF from 32 FS and 30 healthy NC. The findings
were very similar to those reported by the Norweigan group. We found an
average SP concentration of 42.79 plus or minus 14.94 fmole/ml in FS CSF
and only 16.30 plus or minus 6.04 in CSF from NC. Those differences were
statistically highly significant (Wilcoxon P<0.001). It was interesting
to note that ethnicity had some influence on the Sp levels among FS patinets.
SP in 12 Causasians FS (49.9 plus or minus 3.3) was higher (P = 0.02)
than in 19 Hispanics FS (37.9 plus or minus 3.4). SP levels were not different
on the basis of gender despite the fact that FS is so predominantly a
female disorder (9 females affected for each male affected). There was
clearly no relationship of SF SP concentration to age. As dramatic as
the differences in CSF SP were between FS and NC, it was surprising that
the correlations with clinical measures of pain were not stronger than
were found. A borderline significant correlation was observed with the
average tender point threshold (TPA) (R = 0.32, P = 0.07) and the total
tender point severity index (TPI) (R = -0.30, P = -0.10), but there was
no relationship between SP and the FS patients' subjective self-assessment
of pain severity on a visual analog scale. It seems likely that the elevated
SP levels in the CSF of FS patients do indicate something about the pathogenesis
of FS but the findings also suggest that it is not acting alone. Rather,
the data mandates a "second hit" biochemical abnormality to
explain the constellation of symptoms so characteristic of FS patients.
It is still possible, as we earlier predicted, that the other abnormality
is related to serotonin, a metabolite of tryptophan.
CSF Tryptophan
The same CSF samples collected from 32 well-characterized FS patients
and 30 NC were studied for tryptophan concentrations. The mean CSF tryptophan
concentration in the FS patients was found to be numerically lower than
the mean for NC CSF, but the differences were not statistically significant.
Tryptophan can be metabolized to serotonin or can take an alternate metabolic
route by metabolism through the kynurenine pathway. The question then,
was whether the serotonin or the kynurenine pathways of tryptophan metabolism
would be found to evidence abnormalities in the CSF of FS patients. CSF
Serotinin is a potent, often inhibitory, neuromodulator which is effective
locally at the interneural sites of its release. As a result, its levels
in CSF are so low that no laboratory has yet devised methodology to accurately
measure it. Serotonin's metabolic product, 5-hydroxyindole acetic acid
(5HIAA), was measured because it is stable and is thought to accurately
reflect the rate of serotonin turnover in brain tissue. Two different
studies were conducted to answer this question and both gave essentially
the same answer. The first study was performed on CSF from 17 FS patients
collected in Norway and transported to San Antonio for examination along
with CSF from 8 NC and 7 CSF from patients with RA. The second study involved
CSF collected in San Antonio from FS and NC volunteers. In both experiments,
the mean concentration of 5HIAA in CSF from FS patients was numerically
lower than normal but in neither case was it significantly so. Whether
that should steer investigation away from serotonin or simply indicate
that the localized processes may be poorly documented by the CSF metabolite
concentration will require more study. The question of serotonin involvement
cannot be dismissed however, because it is clearly abnormal in the serum
of FS patients and that abnormality has been shown to correlate with the
painful symptoms of FS.
CSF Kynurenine
The kynurenine pathway was also examined in the CSF. For that purpose,
we used the same 32 FS and 30 healthy NC CSF samples which had previously
been examined for the concentrations of substance P. The results indicated
that the metabolic conversion of tryptophan to kynurenine, catalyzed by
the enzyme indole-2,3-dioxygenase (IDO) was abnormal in FS. Despite the
lower mean concentration of tryptophan (the substrate for IDO) in FS patients,
the CSF concentration of kynurenine (the reaction's product) was significantly
higher in the CSF of FS patients than in the CSF of NC. The next enzyme
in the sequence is kynurenine-3-monooxygenase (KMO) which catalyzes the
conversion of kynurenine to kynurenine-3-hydroxylase. Surprisingly, the
concentration of kynurenine-3-oxygenase was significantly lower than normal
in FS CSF. The explanation of these findings is not yet fully known. One
possibility is that a single chemical messenger is responsible for both
changes. The enzyme IDO, is known to be inductible by interfron-gamma
(IFN-gamma). The activity of the enzyme KMO is dependent upon the concentration
of its cofactor nicotinamide adenine dinucleotide phosphate (NADP). Interferon
is also capable of inducing an intracellular depletion of NADP by inducing
nucleotide transferase activity.
RBC NUCLEOTIDES
A series of experiments with FS and NC red blood cells (RBC) were initiated
to examine intracellular biochemistry. The specific purpose was to determine
whether the abnormal kynurenine metabolism, particularly the "bottleneck"
at the KMO which requires NADP as a cofactor, might be explained by an
intracellular deficiency of NADP. We examined the concentrations of nucleotides
in the RBC from the venous blood of 27 demographically-matched FS and
NC subjects, RBC nucleotides were extracted by the method of Stocchi et
al., fractionated by high-performance liquid chromatography using an ion-paired,
reverse-phase, gradient elution technique and detected by optical density
at 260 mu. The nucleotides identified and quantified included AMP, ADP,
ATP, NAD, NADH, NADP, and NADPH. The concentrations of several nucleotides
were numerically low in FS RBC, but NADP (P < 0.05), NADH (P < 0.02),
and ATP (P < 0.005) were significantly lower than normal. Within the
FS patient group, the NADP concentrations correlated highly with the ATP
concentration (R = 0.73, P < 0.001), and ATP correlated with the subject's
age (P < 0.006), but none of the nucleotides concentrations related
with the patients' perception of their pain, with their perceived severity
of fatigue or with the semi-objective measures of pain severity such as
the TPI or the TPA. There were no important differences by gender and
the only ethnic difference of note was that NADPH in the Caucasian FS
patients, who exhibited a greater severity of pain and morning stiffness,
was only half that of the Hispanic FS patients (P = 0.001).
IFN-GAMMA?
The abnormalities in tryptophan metabolism implied by these data are consistent
with the literature reports of alterations inducible by elevated levels
of IFN-gamma. In fact, several recognized biochemical abnormalities in
FS seem to fit that concept. A diversion of tryptophan to kynurenine would
decrease the level of tryptophan for serotonin synthesis so the level
of serotonin would fall, making pain perception and deep sleep more problematic.
Tryptophan is an essential amino acid whose availability is also required
for protein synthesis, some functions which depend on replacement proteins,
such as skeletal muscle repair, might suffer. The problem is to determine
whether IFN-gamma really is elevated in FS and, if so, to conceive why
it might be elevated in the CNS of these patients. In the blood, IFN-gamma
is thought to be released in response to viral infections or other inducers
of inflammation. Since FS is not considered to be an inflammatory disorder,
it is not clear why IFN-gamma should be present in excess. In addition,
the white blood cells known to produce IFN-gamma in response to cytokine
signals are not likely to be present in the CNS.
IFN AND SP
A search of the medical literature for a CNS connection between substance
SP and IFN was without much success. There is published evidence to indicate
that substance P can enhance production of IL-2 by T lymphocytes. Increased
IFN production by those cells parallels that increase in IL-2 production.
Production of much IFN in the CNS, however, would require larger numbers
of leukocytes than are normally present in the brain tissue. One could
speculate that there is a CNS stromal cell capable of producing IFN in
response to substance P. For example, there is evidence that IL-1 can
be produced by astrocytes, and that substance P stimulates that process
by a calcium ion-dependent mechanism. Conversely, substance P production
in neural tissue is responsive to IL-1 and IL-1 down-regulates the numbers
of receptors for substance P on astrocytes. We have not yet identified
any published accounts of IFN production in the CNS except in the presence
of a bacterial or viral infection. Another potential alternative is that
IFN is produced elsewhere, in response to a signal other than substance
P, and then effects elsewhere and the products (e.g., kynurenine) might
cross the blood brain barrier where we measured them in the CSF. That
could explain the biochemical changes we have observed in other compartments
in FS patients, i.e., the low tryptophan in serum and CSF, low serotonin
in serum, high kynurenine in CSF, low 3-hydroxykynurenine in CSF, and
low NADP in peripheral red cells. The abnormality in tryptophan transport
described by Yunus et al. could result from competition by kynurenine
for transport with the energy-requiring large amino acid receptor at the
level of the blood brain transport.
5HT AND SP
Data from two laboratories, involving subjects from different ethnic backgrounds,
have provided convincing evidence that substance P is increased in the
CSF of FS patients. The cause and mechanism are still unknown, but considerations
must include a hereditary or congenital defect, toxin exposure, tissue
trauma or defective regulation. Substance P is normally released in the
spinal cord by afferent neurons in response to painful peripheral stimuli.
It is then involved in the early stages of nociceptive signaling, so an
excess of substance P could amplify the perception of pain. We suspect
that some of the other biochemical abnormalities we have recently identified
in FS patients (low tryptophan, low serum serotonin, increased CSF kynurenine,
lower than normal adenine nucleotides in RBC may be epiphenomena. On the
other hand, they probaly are pointing to critical abnormalities, as yet
undetected, which are responsible for the array of symptoms experienced
by FS patients. For that reason, a direct link between substance P and
a tryptophan metabolite such as serotonin would be of interest. There
is evidence from a rat model that the nociceptive effects of SP are counteracted
by 5HT agonists. The relationship between SP and 5HT is more complex than
that, however. In the brain 5HT seems to augment the synthesis of SP while
blockade of 5HT receptors in the brain decreases SP production. Conversely,
a decrease in brain 5HT which causes a decrease in brain SP results in
an increase in the spinal cord SP content. These data indicate that brain
SP can be lower than normal while cord SP is relatively increased. Our
own finding of increased lumbar CSF SP in FS would be consistent with
that model and would further predict lower than normal 5HT effect in the
brain. The apparent dichotomy can be explained as follows: Brain SP seems
to serve as a central down-regulator of nociception in the spinal cord,
working through cord-level 5HT and opiates. SP release in the spinal cord
induces nociception while in the brain it initiates retrograde anti-nociception
down in the cord. Brain 5HT levels seem to be positively related to brain
SP levels and inversely with cord SP levels. Therefore, a hypothetical
model of relative central 5HT deficiency would predict lower normal central
SP, but an excess of SP at the cord level resulting in excessive nociception
which is inadequately down-regulated by deficient cord 5HT. Substance
P may also influence the function of the hypothalamic-pititutary-adrenal
axis but dysregulation by 5HT seems even more likely. Perhaps one or both
are responsible for some of the abnormalities FS patients exhibit in corticotrophin
releasing hormone (CRH) and adrenal corticotrophic hormone (ACTH) production
and growth hormone (via IGF1) levels in FS.
WORKING MODEL
Our current working model is deceptively simple but it has the potential
to explain many of the observed biochemical abnormalities and the exaggeration
of pain perception in FS. It is perceived as a step on the way to a more
complete understanding of FS pathogenesis partly because it predicts a
large number of very testable subhypotheses. It is as follows:
Abnormal levels of or a disproportion of serotonin and substance P in
the brain and the spinal cord result in a variety of neuroendocrine, biothythm,
and nociceptive abornmalities which explain the signs and symptoms of
fibromyalgia syndrome. In the spinal cord they cause an exaggerated nociception
from fairly normal stimuli (allodynia). In the brain, a relative deficiency
of serotonin could be responsible for the sleep disturbances and the neuroendocrine
abnormalities. In the abdomen, they could cause the dysfunctional bowel
synptoms experienced by FS patients. Finally, a relative deficiency of
tryptophan for protein synthesis, in an environment deficient of IFG1
and DHEAS could be responsible for the intolerance to and delayed recovery
from physical exercise among FS patients.