QST: The Only Non-invasive Clinical Test
Quantitative Sensory Testing (QST) is essential in the evaluation of small-caliber A-delta & C-fibers, the primary transmitters of thermal & pain sensation. Small-fiber sensory nerves comprise over 70% of the body’s peripheral nervous system. Sensory Testing of the thermal senses is the only non-invasive clinical test, which quantitatively assesses the functional state of somatic small fibers from peripheral receptors through their central nervous system connections. As a complimentary test to traditional EMG/NCV, QST can demonstrate neurosensory abnormality when it is selective to the small fibers despite negative EMG/NCV findings, which evaluate large-caliber fibers only.
Disorders of ‘sensation’ are very common in clinical medicine. In the last decade, after the availability of new tools for investigating unmyelinated C and thinly myelinated A-delta fibers, Small Fiber Neuropathy (SFN) has been recognized as a distinct nosologic entity. Although small fibers encompass thermal and nociceptive sensation as well as autonomic functions, SFN commonly refers to somatic neuropathy alone and the overlap with the term ‘painful neuropathy’ is accepted.
Early signs of Small-fiber loss
Small fibers are invisible to routine nerve conduction studies and their damage most frequently causes a neuropathic pain syndrome, making the diagnosis of SFN often particularly difficult. In fact, spontaneous and stimulus-evoked positive sensory symptoms frequently dominate the clinical picture and can hide the signs of small fiber loss, namely thermal and pinprick hypoesthesia.
It is well documented that small-caliber C-fibers are responsible for pain transmission, which can mainly be evaluated with Quantitative Sensory Testing. QST, when combined with other clinical and sub-clinical tests, can be extremely helpful in diagnosing the pain-mediating, neurosensory nerve pathway and nerve root involved in a patient’s pain condition. Additionally, it can contribute in the differentiation between pain of a peripheral nature vs. pain involving central mechanisms, the latter being more difficult to treat.
Better Diagnosis of Small Fiber Loss
Through this testing technique, more optimal treatment courses can be identified / selected and unnecessary interventions can be avoided. Various chronic pain conditions can be diagnosed and more cost-effectively managed with Quantitative Sensory Testing: Chronic Regional Pain Syndrome (CRPS), Central Pain Syndromes (i.e., Fibromyalgia), Radiculopathies & Small-fiber Polyneuropathies.