Using a nationally representative sample of older US adults, we found that over 8.1% of the individuals had a new diagnosis of neuropathy and met our inclusion criteria during the 10 years of our study. Many tests were ordered during the diagnostic period for peripheral neuropathy, but the evaluation was highly variable. MRIs of the brain and/or spine were frequently ordered, whereas the glucose tolerance test was rarely ordered. Significant increases in cost occurred during the diagnostic period compared to the baseline period. These findings suggest substantial opportunity to improve efficiency in the evaluation of peripheral neuropathy.
The large variation in testing indicates little consensus on an appropriate testing strategy in this population. With over 400 total patterns of tests and no pattern accounting for more than 4.8% of the total number, no standard approach to the evaluation of peripheral neuropathy currently exists. Similarly, the number of nerves tested on nerve conduction studies exhibited substantial variation and was close to the recommended number of nerves for patients entering a clinical trial as suggested by a 2009 AAN practice parameter. Substantial utilization of diagnostic tests was observed, exhibited by a median of 4 tests ordered out of the 15 tests evaluated. Interestingly, those with more nerves evaluated on NCS also had a higher chance of receiving a MRI, another expensive test. More research is needed to determine the optimal approach to this prevalent condition and to disseminate this information to the physicians that care for these patients.
When examining test utilization, two significant deviations from expected clinical practice and the tests supported by the best available evidence were discovered. The first was that a large proportion of these patients received MRIs of the brain and/or spine. In fact, each segment of the neuroaxis (brain, cervical, thoracic, and lumbar spine) was performed at a higher than expected frequency. When combining all MRI tests together, the utilization was even more dramatic with nearly one in four receiving at least one MRI. For a condition that affects the peripheral nervous system, this degree of utilization is substantial and suggests that many physicians have significant uncertainty when localizing neuropathy symptoms to the peripheral nervous system. The use of MRI may also result from the large proportion of patients with idiopathic neuropathy, the fact that electrodiagnostic studies can be non-diagnostic or normal, or from patient preferences. Another possibility is that neuropathy patients are at higher risk for other conditions or symptoms that warrant MRI.
The second deviation from expected practice is that GTTs are rarely ordered. In fact, only 1% of this neuropathy population received the GTT. The prevalence of impaired glucose tolerance in otherwise idiopathic neuropathy patients is higher compared to historical controls and the type of neuropathy in these patients is different (more sensory and painful neuropathies)9,10
. Therefore, emerging data supports impaired glucose tolerance as potentially one of the most common etiologies of neuropathy although controversy still exists13–15
. This condition is also one of the few potentially treatable causes of neuropathy with diet and exercise preventing a large percentage of patients from going on to develop diabetes and its inherent risk of neuropathy progression16
. One potential reason for the extremely low utilization of this test includes the fact that many physicians use hemoglobin A1C to identify those with pre-diabetes16
. Unfortunately, the cut point used to define pre-diabetes with this test has a low sensitivity and many of the patients in the Diabetes Prevention Program trial would not have been included using this criterion17
. These results indicate that two of the first steps in increasing the effectiveness and efficiency of the evaluation of peripheral neuropathy may be investigating why so many MRIs are ordered and determining the barriers to utilization of the glucose tolerance test.
The other three tests supported by the AAN systematic review (fasting glucose, B12, SPEP) were ordered less frequently than expected. In fact, only 49.8% of neuropathy patients received one or more of these three tests and only 17.3% received two or more. Even though some patients with peripheral neuropathy may not need these tests if they have a well-established cause, these numbers are still significantly lower than if the 25–40% of patients that end up with an idiopathic diagnosis were evaluated11
. Interestingly, B12 levels are ordered much more frequently than SPEPs, emphasizing the fact that many physicians do not recognize the evidence in support of ordering this test. While these data were collected from a time period prior to the release of the AAN review, they highlight that physicians were not ordering the tests with the highest levels of evidence to support their use. Understanding the obstacles to the utilization of these tests will be paramount to improving the efficiency of diagnostic testing in this population.
Medicare expenditures in this population rose substantially during the diagnostic period. The expenditures decreased during the 12-month follow up period, but did not return to baseline. This pattern is not surprising given our findings that patients with a new diagnosis of neuropathy undergo an extensive evaluation. These expenditures, however, may also reflect other broad expenditures related to their disabling condition including orthotics, walking-assist devices, office visits, and hospitalizations to name a few. These other expenditures likely explain the persistent increase in expenditures in this population, but the transient increase in the diagnostic period is at least partially explained by costs associated with diagnostic tests. Therefore, understanding the relative impact of these tests is important in allowing physicians to practice efficient care, especially in a patient population where the etiology frequently remains unclear and there are few disease modifying therapies. Future studies examining which diagnostic tests are driving the costs and whether they are effective and useful within this population will be essential.
This study has important limitations. ICD-9 diagnosis codes were used to identify patients with peripheral neuropathy, which may lead to misclassification bias. Peripheral neuropathy is a heterogeneous condition, and certain rare subtypes of neuropathy may require a different evaluation than those with DSP. However, very few patients in this study had an ICD-9 diagnosis indicating a rarer subtype of neuropathy, and a sensitivity analysis excluding patients with a diagnosis of mononeuritis multiplex, demyelinating neuropathy or hereditary neuropathy, did not significantly change our results. Many patients were included who have a known cause of neuropathy, and these patients may not require any work up for the cause of their neuropathy. On the other hand, 80% of the patients without diabetes were coded as idiopathic neuropathy, and the patterns observed in this group were similar to the entire cohort. Another limitation is that there are likely patients with neuropathy that did not receive an ICD-9 diagnosis and our population may be biased towards those with more severe neuropathy. Yet the high incidence of neuropathy in our cohort gives support to the likelihood that we are capturing a large proportion of the population with neuropathy. An additional limitation is that we were unable to investigate detailed information on why patients are receiving specific tests. For instance, some of the patients who had MRIs performed may have had another indication for this test, such as spinal arthritis. It is also possible that patients with a neuropathy diagnosis were more likely to see a specialist in neurology, which subsequently led to an increase in neurology-specific tests. However, the magnitude of the MRI utilization makes these factors unlikely to account for all of these tests. We also do not know whether the increase in Medicare expenditures is specifically related to the evaluation of neuropathy. We investigated total expenditures, which includes other non-diagnostic test related expenditures. On the other hand, the expenditures increased substantially around the time of diagnosis, and then decreased towards but not entirely back to that of the baseline period in the subsequent year. We also studied a Medicare population made up largely of patients over the age of 67. How these results apply to a younger population or one with private or no insurance is unclear.