C h a p t e r 3 5
Somatosensory Function, Pain, and Headache
869
Alterations in Pain Sensitivity
and Special Types of Pain
Pain sensitivity and types of pain vary according to the
body structures, the initiating event, and the duration of
the pain. It also varies among persons and in the same
person under different conditions.
Alterations in Pain Sensitivity
Sensitivity to and perception of pain varies among
persons and in the same person under different condi-
tions and in different parts of the body. Irritation, mild
hypoxia, and mild compression of a peripheral nerve
often result in hyperexcitability of the sensory nerve
fibers or cell bodies. This is experienced as unpleas-
ant hypersensitivity (i.e.,
hyperesthesia
) or increased
painfulness (i.e.,
hyperalgesia
). Primary hyperalgesia
describes pain sensitivity that occurs directly in damaged
tissues. Secondary hyperalgesia occurs in the surround-
ing uninjured tissue. An example of primary hyperalge-
sia is the extreme sensitivity of sunburned skin, which
results from sensitization of the skin pain endings by
local products from the burn.
More severe pathologic processes can result in reduced
or lost tactile (e.g.,
hypoesthesia, anesthesia
), tempera-
ture (e.g.,
hypothermia, athermia
), and pain sensation
(i.e.,
hypoalgesia
).
Analgesia
, in addition to meaning the
relief of pain without loss of consciousness, is a pathol-
ogy characterized by the absence of pain on noxious
stimulation. The inability to sense pain may result in
trauma, infection, and even loss of a body part or parts.
It is thought to result from a peripheral nerve defect or
a cortical defect, either of which disrupts the perception
of pain. Whatever the cause, persons who lack the abil-
ity to perceive pain are at constant risk of tissue damage
because pain cannot serve its protective function.
Allodynia
(Greek
allo,
“other,” and
odynia,
“painful”)
is the puzzling phenomenon of pain that follows a non-
noxious stimulus to apparently normal skin.
2
Examples of
such non-noxious stimuli are wind, touching sheets, and
showering. The condition may arise when otherwise nor-
mal tissues are abnormally innervated or are referral sites.
It can also result from increased responsiveness within the
spinal cord (central sensitization) or a reduction in the
threshold for nociceptor activation (peripheral sensitiza-
tion). One type of allodynia involves
trigger points,
which
are highly localized points on the skin or mucous mem-
brane that can produce immediate intense pain at that
site or elsewhere with light tactile stimulation.
Special Types of Pain
Neuropathic Pain
“Neuropathic pain refers to pain that originates from
pathology of the nervous system.”
31
When peripheral
nerves are affected by injury or disease, it can lead to
unusual and sometimes intractable sensory disturbances.
The notable features that point to neuropathic processes
as a cause of pain include widespread pain that is not
otherwise explainable, evidence of sensory deficit (e.g.,
numbness, paresthesias), burning pain, pain that occurs
with light stroking of the skin, and attacks of pain that
occur without seeming provocation.
31
Depending on the
cause, few or many axons could be damaged and the
condition could be unilateral or bilateral.
Causes of neuropathic pain can be categorized
according to the extent of peripheral nerve involve-
ment. Conditions that can lead to pain by causing dam-
age to peripheral nerves in a single area include nerve
entrapment, nerve compression from a tumor mass,
and various neuralgias (e.g., trigeminal, postherpetic,
and post-traumatic). Conditions that can lead to pain
by causing damage to peripheral nerves in a wide area
include diabetes mellitus, long-term alcohol use, hypo-
thyroidism, renal insufficiency, and drug treatment with
neurotoxic agents.
32
Diabetes often causes a length-
dependent neuropathy (meaning that the longest axons
in a peripheral nerve are most vulnerable). Injury to a
nerve also can lead to a multisymptom, multisystem syn-
drome, called
complex regional pain syndrome.
Nerve
damage associated with amputation is believed to be a
cause of phantom limb pain.
Neuropathic pain can vary with the extent and loca-
tion of disease or injury. There may be allodynia or
pain that is stabbing, jabbing, burning, or shooting.
■■
Modulation of the pain experience occurs by
way of the endogenous analgesic center in the
midbrain, the pontine noradrenergic neurons,
and the nucleus raphe magnus in the medulla,
which sends inhibitory signals to dorsal horn
neurons in the spinal cord or trigeminal nerve.
■■
Pain can be classified according to location,
referral, and duration as well as associated
medical diagnoses. Pain can arise from
stimulation of cutaneous, deep somatic, or
visceral pain receptors. Referred pain is pain
perceived at a site different from its origin. Acute
pain is self-limiting pain that ends when the
injured tissue heals. Chronic pain is pain that
lasts much longer than the anticipated healing
time for the underlying cause of the pain.
■■
Treatment modalities for pain include the use of
nonpharmacologic and pharmacologic agents
either singly or in combination. Management
of acute pain includes therapy directed at
providing pain relief by interrupting the
nociceptive stimulus. Chronic pain management
is much more complex and is based on multiple
considerations, including life expectancy.
