Ischemic Pain Testing Demonstrates Hypoxia Dependent Early Pain and Hypoxia Independent Persistent Pain Responses in Adults with Sickle Cell Anemia
Authors: Vaughan Kathleen , Kim Joseph , Lichy Daniel , Fishman Nathan S , Khandhadia Amit , Kim You Keun , Yoon Stephen , Sheth Niral , Ahad James , Ackerman Hans , Belfer Inna , Gorbach Alexander M. , Taylor James G. 
Published: November 2019

Abstract
The hallmark manifestation of homozygous sickle cell disease (SCD) is acute pain. The underlying etiology is unknown. It is believed that hypoxia induces changes in erythrocyte shape and produces microvasculature occlusion by sickled red cells. Vascular occlusion is believed to represent the event inducing tissue hypoxia, reperfusion injury and pain.
Studies have only recently characterized pain responses to experimental stimuli in rodents and humans with SCD using quantitative sensory testing (QST). SCD is characterized by thermal and mechanical sensitivity, and there is evidence of central sensitization (heightened sensitivity to pain) from QST, temporal summation and functional MRI studies. However, the effect of hypoxia upon nociception has not been studied despite the need for hypoxia to induce sickle hemoglobin polymerization. The ischemic tourniquet pain test is a QST used to investigate nociception. It was the first QST to demonstrate opioid analgesia. While it is assumed that the resulting pain is due to hypoxia, no studies have demonstrated the degree of hypoxia needed to produce pain.
Methods: Thirty adults with SCD and 30 age and sex matched 30 controls underwent the ischemic pain test. A pneumatic cuff was inflated around the upper arm to induce forearm hypoxia, and the time from cuff inflation to first reported pain (pain threshold) and until pain became intolerable (pain tolerance) was measured. Pain was quantified on a 20 point scale. Time to pain threshold and tolerance were the primary end points. Testing was repeated after a washout period in a subset of the initial subjects: 18 SCD patients and 22 controls. Repeat testing proceeded with a 15 minute observation period followed by the ischemic pain test with the time and pain score recorded at threshold and tolerance. Subjects were monitored after cuff deflation during recovery with pain scores obtained at 30 seconds, then every minute for 5 minutes and finally every 3 minutes over a total recovery period of 20 minutes. Skeletal muscle tissue oxygenation at the thenar eminence was measured continuously with a near infrared spectroscopy (NIRS) monitor (OxiplexTS; ISS, Champaign, IL).
Results: SCD subjects reached the pain threshold at a mean of 6.44 versus 11.57 minutes in controls (p=0.07). Time to pain tolerance was significantly different (SS 11.29 versus controls 19.59 minutes; p=0.004). Both groups reported identical pain scores. By stepwise linear regression, SCD (p=0.002) and gender (p=0.0008) were associated with time to pain tolerance (r2=0.29), while recent opioid use and pain crisis frequency were not. Because these results demonstrated that SCD adults reach tolerance more rapidly than controls, we sought to determine if this difference was due to tissue hypoxia. Pain scores with repeat testing were not significantly different at threshold or tolerance. However, pain score curves over testing and recovery were significantly different (p=0.003, Fig 1A). SCA had pain above baseline after 11 minutes of recovery (p=0.02) suggesting a persistent pain response compared to controls. Oxyhemoglobin and deoxyhemoglobin from NIRS were not different between groups at the primary endpoints (Fig 1B and 1C). StO2 (% tissue oxygenation, Fig 1D) at both threshold and tolerance was not different between SCD and controls, suggesting that the onset and maximal level of pain during this QST is defined an StO2 thresholds 40% common to sickle cell patients and controls. However, oxyhemoglobin, deoxyhemoglobin and StO2 had already recovered to baseline by the time the pain was evident (Fig 1B, 1C and 1D).
Conclusions: Adults with SCD reach hypoxia induced, experimental pain earlier than NVs. Time to maximal pain is associated with SCD and sex. NIRS showed that pain tolerance occurred at a uniform tissue oxygenation threshold indicating hypoxia is the primary determinant for sensing acute pain. However, persistent SCD pain was independent of tissue oxygenation, and could be explained by the presence of peripheral or central sensitization. These data suggest that SCD pain treatment may require improved oxygen delivery to affected tissues and analgesics.
Figure 1. Nociceptive and NIRS responses to ischemic pain testing in adults with sickle cell anemia. Figure 1. Nociceptive and NIRS responses to ischemic pain testing in adults with sickle cell anemia.

Disclosures
No relevant conflicts of interest to declare.