Pain Oriented Sensory Testing

This article is a stub.

The ANZCA have published guidelines on pain oriented sensory testing (POST) which can be found here

Neurophysiology

See also: Nociception

Peripheral transmission follows transduction. Figure 1 shows a response being recorded from a sciatic nerve after an applied stimulus. The recording electrode experiences large changes in electrical potential. There are three major waves corresponding to three groups of nerves - A wave, B wave, and a smaller later C wave. Velocities can be calculated, and we find that the diameter of the nerve fibres are proportional to their conduction velocities.

The Aα and Aγ nerves are largely made up of motor neurons. The Aβ waves are produced by large diameter sensory nerve fibres that information related to proprioception, touch, and pressure. All fibres capable of transmitting nociception are Aδ fibres or C fibres.

There is no such thing as a "pain fibre" or specific nociceptive fibre. There are a variety of nerve fibres that are capable of transmitting nociception, but that may not be their only function. In various situations there may be more than one fibre involved in transmitting nociceptive information.

Characteristics of sensory nerve fibres

Nerve fibre	Myelinated axons	Diameter (µm)	Conduction velocity (m/s)	Sensory information	Usefulness of electroneuromyography	Usefulness of QST
Aα	Yes	13-20	80-120	Proprioception, muscle spindle primary endings (Ia), golgi tendon organs (Ib) (and alpha motor neurons)	Yes (H reflex)	No
Aβ	Yes	6-12	33-75	Discriminative mechanoreception (touch, vibration), proprioception, pain modulation (block nociceptive information, allodynia in sensitisation)	Yes (sensory nerve conduction)	Yes
Aγ	Yes	4-8	15-40	Touch, pressure (and gamma motor neurons)		No
Aδ	Thin	1-5	3-30	"rapid" pain, crude touch, pressure, temperature. AMH type I for rapid mechanical pain (high heat threshold >53C), AMH type II for rapid heat pain (lower heat threshold 43-47C).	No	Yes
C	No	0.3-1.5	0.5-2.0	"slow" pain, touch, pressure temperature (and postganglionic autonomic). Polymodal.	No	Yes

Examination

Below is a table comparing clinical vs QST methods that I have modified from several sources.

Summary of choice methods of assessing nerve function per sensation.

Fibres	Sensation	Finding Descriptor	Clinical Relevance	Testing Equipment and Instructions
				Clinical	QST	Laboratory
Aβ	Touch	Dynamic mechanical allodynia	Common to most NP. Central sensitisation.	cotton wool or camel hairbrush - 2cm stroke over 1 second and repeat	Von Frey filaments	NCS, SEPs
Aβ	Vibration	Vibration detection threshold	Infrequent but strongly suggestive of NP	Tuning fork (128 Hz)	Vibrameter†	NCS, SEPs
Aδ	Pinprick, sharp pain	Punctate mechanical allodynia and hyperalgesia	Common to most NP. Central sensitisation.	Punctate mechanical allodynia - use a toothpick apply 2 stimuli per second (2 Hz) and repeat. Punctate mechanical hyperalgesia - optionally tested with a neurotips needle.	Weighted needles	LEPs, IENF
Aδ	Touch	Static or Pressure-evoked mechanical allodynia	Common to most NP, also observed in inflammatory pain. Central sensitisation.	Finger tip - apply pressure until blanching of nail bed for 1 second and repeat.	Von Frey filaments
Aδ	Cold	Cold allodynia	Infrequent but strongly suggestive of NP. Central sensitisation.	Stainless steel 128 Hz tuning fork prongs applied to the skin for 1 second and repeat	Thermode‡	None
Aδ	Touch, Pain	Temporal summation indicating hyperpathia	Central sensitisation, test for "wind-up"	Toothpick applied 2 stimuli per second (2Hz) for 30 seconds. Assess change in pre- and post- pain scores, and aftersensation.
C	Warmth	Warm allodynia	Infrequent but strongly suggestive of NP. Peripheral sensitisation.	Thermoroller or warmed C size battery at 45° applied for 1 second and repeat	Thermode‡	LEPs, IENF
† or other device providing graded vibratory stimuli ‡ or other device providing graded thermal stimuli NCS: nerve conduction studies, SEP: sensory evoked potentials, LEP: laser evoked potentials, IENF: intra-epidermal nerve fibre density. NP: neuropathic pain Modified from various sources

The function of the various sensory peripheral nerves are reproduced below.See Also

• Neuropathic Pain
• Central Sensitisation
• Allodynia, Hyperalgesia, and Hyperpathia
• Nerve Conduction Studies and Electromyography