The root causes of neuropathic pain put it in a different category to more
familiar types of pain, which may in part account for the relatively poor
performance of clinicians in diagnosing and treating the condition. It stems
from an abnormal activation of the body’s pain response, a disturbance of
function or pathological change in a nerve.
This can be the result of dysfunction or injury to the nervous system
– to a specific nerve, the spinal cord or the brain itself. Evidence to
date suggests that this often manifests itself as ectopic impulse formation,
hyper-excitability and loss of inhibitory control.
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Common causes of neuropathy include accidents, surgical procedures or damage
to the nerve structure from infections such as shingles, or through long-term
conditions such as diabetes. Damage to the peripheral nerve system can also be
caused by pressure on the nerve from a malignant tumour or, more commonly, from
spinal abnormalities.
In many cases, damaged nerves can heal without resulting in ongoing pain,
but in some cases, despite complete healing to the surrounding tissue, nerves
can become excitable, firing off pain impulses from the site of the nerve
damage. Essentially, the brain is being tricked into thinking that pain
impulses are coming from pain receptors in the skin or organs.
In patients suffering neuropathic pain, three types of observable changes
can occur: painful symptoms, visible skin changes and loss of sensation.
Essentially, these patients’ pain systems bombard the spinal cord and
brain with unusual patterns of impulses, often producing bizarre sensations
– both painful and not – which can be vital clues for clinicians in
diagnosing neuropathic pain.
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Spontaneous experience of pain, which seemingly has no cause, or “evoked”
pain, where an ordinary physical stimulus produces an unusual or exaggerated
sensation of pain, are further indications that neuropathy may be the cause.
Some spontaneous pain is continuous. In other cases, it manifests as paroxysmal
pain, characterised by intermittent sensations that occur without stimulus.
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By GlobalDataEvoked pain can come from contact or circumstance that would normally go
unnoticed, such as brushing clothes against the skin or a change in
temperature. It can also include the sensation of pain in a different part of
the body to that which is receiving the stimulus, or the radiation of pain from
a specific area to a wider region.
FOUNDATIONS FOR DIAGNOSIS
Given the obvious factors that might indicate neuropathy, it may be
surprising that up to two-thirds of GPs have difficulty recognising neuropathic
pain. Many specialists, including endocrinologists treating diabetes and even
pain specialists, also report problems in diagnosing the condition.
A clinical diagnosis of neuropathic pain should only be made when the
distribution of pain and the associated sensory abnormalities, jointly and in a
clinical context, point to a neurological condition. Put simply, neuropathic
pain is pain that often occurs in an area of abnormal or absent sensation.
Patients with neuropathic pain usually complain of spontaneous pain, such as
the continuous sensation largely felt in cutaneous or deep tissues and, on
occasion, from viscera, or of paroxysmal and evoked pain. However, the
demonstration of nerve dysfunction is crucial corroborating evidence in the
diagnosis of neuropathic pain.
Clinicians diagnose neuropathic pain on the basis of a combination of
symptoms and examination evidence that suggests nerve dysfunction. Given that
pain is a subjective phenomenon and that traditional tests, such as vibrametry
and nerve conduction studies, have only measured function in large myelinated
fibres, basing a diagnosis on examination evidence can be quite a
challenge.
In some cases, special nerve testing is performed to detect nerve damage, as
is common in response to reports of pain from changes in temperature. These
tests may well highlight nerve damage, but cannot in themselves prove or
disprove that a patient is suffering with pain caused by that nerve damage.
It is fortunate that the diagnosis of neuropathic pain is now receiving much
more attention from researchers. This has resulted in the development of pain
scales that can help doctors detect whether neuropathic pain might be the cause
of their patient’s pain. Recently, neurophysiological tests have been
developed. These enable clinical assessment of the peripheral and central
systems for sensing pain.
Alternative approaches have focused on the development of standardised
assessment tools based on the symptoms reported by patients. Lists of symptoms
that can be described verbally are often used to identify specific neuropathic
pain descriptors. One problem, however, is that presently these lists have not
been successfully tested for their ability to allow clinicians to discriminate
between neuropathic and other kinds of pain.
Nevertheless, these verbal descriptors can be useful, as they allow
clinicians to rate a patient on the neuropathic pain scale (NPS). This consists
of ten verbal descriptions, each rated on an 11-point numerical rating scale.
The process was developed from clinical experience and used single descriptors
to discriminate between four diagnostic categories of neuropathic pain.
Only post-herpetic neuralgia could be distinguished from the other
diagnostic groups, such as reflex sympathetic dystrophy, diabetic neuropathy
and peripheral nerve injury. The NPS was not used to discriminate between
neuropathic pain and pain resulting from a measurable physiological event.
THE LANSS PAIN SCALE
Clearly, additional assessment techniques were required. The Leeds
Assessment of Neuropathic Symptoms and Signs (LANSS) Pain Scale is comprised of
seven elements, consisting of five symptom items and two examination items.
These are delivered by a clinician in an interview format.
The purpose of this test is to more accurately assess the probability that a
patient’s pain is dominated by neuropathic mechanisms. It is not intended
to help clinicians assess the severity of the pain or its causes. So far, it is
the only published tool with validity when it comes to discriminating between
neuropathic and nociceptive pain irrespective of disease-based diagnostic
categories.
The LANSS scale, which is generally perceived as easy to use, is now more
widely available to clinicians and patients. Thanks in part to the Neuropathy
Trust, the LANSS pain scale is available online for patients and professionals
alike to view, test and evaluate. This includes the S-LANSS, or self-report
LANSS test, which can be used by patients without the presence of a
clinician.
Both the LANSS and S-LANSS tests result in the patient receiving a score out
of 24. If a patient registers 12 or more on this scale then there is a strong
suggestion that neuropathic pain is present to some degree. The current track
record of LANSS tests has shown that four out of five patients with chronic
pain were correctly classified as suffering from nociceptive or neuropathic
pain.
It must be stressed, however, that there is still a small chance that a
patient who scores below 12 on the scale may indeed have some neuropathic pain,
and also that a patient with a score of more than 12 may not. For this reason
it is advisable that a patient undertaking the S-LANSS test should consult a
doctor to get a more precise diagnosis before treatment. With LANSS and S-LANSS
easy to complete online through the Neuropathy Trust website, it is hoped that
this will provide a platform on which to build more detailed histories.
Although a consensus on the classification and assessment of neuropathic
pain has yet to be reached, the development of detailed records will make it
easier to assess the cause of pain and its response to treatment, allowing
patients and their doctors to deal more confidently with neuropathic pain.
