To the best of our knowledge, this is the first report of an epidemiological investigation into iNPH and LSS. The typical triad of iNPH comprises gait disturbance, dementia, and urinary dysfunction. However, gait disturbance and urinary dysfunction can also be caused by LSS. Furthermore, the number of patients with dementia and LSS is increasing in the current aging society. We investigated the relationship between iNPH and LSS because the symptoms of iNPH and LSS are similar. We found that 32.3% of the patients with iNPH patients had LSS. Previous studies reported a prevalence of LSS of 6% in 850 lumbar myelograms26 or 13.1% in 17,744 patients27. Furthermore, the risks of LSS and iNPH were reported to be significantly higher in older adults compared with younger individuals28. The reason for the increased prevalence of LSS in our study compared with previous studies may be that our participants were patients with iNPH and had a relatively old mean age.
Obesity affects pain related to spinal stenosis and is associated with mechanical forces in addition to the presence of chronic circulating inflammatory chemicals from active adipose tissue29. Our study showed that a higher BMI was characteristic of patients with iNPH with LSS. In addition, we found that a smaller DSCSA correlated with older age and a higher BMI. Therefore, our findings suggest that older age and a higher BMI may be useful predictors of LSS in patients with iNPH.
In the present study, we used the TUG test as an index of gait ability and used the MMSE as an index of dementia. The TUG test and MMSE had a relatively strong negative correlation. General cognitive function was reported to be associated with improvements in the physical performance of patients with mild cognitive impairment30. The TUG test is a sensitive and specific measure of the risk of falls31. Previous studies reported that patients with LSS have a mean raw TUG test time of 10.2 seconds before surgery32 and that patients with iNPH have a median TUG test time of 15.2 (11.7–21) seconds19. In our study, the median TUG test time was 23 (17–38.5) seconds, and these were patients with relatively severe symptoms.
Although VPS is the main surgical procedure for iNPH, therapeutic intervention with LPS was also reported to be effective33. The efficacy and safety of adjustable LPS are comparable with VPS for the treatment of patients with iNPH. LPS may be the best treatment because it is less invasive and avoids damage to the brain, despite the relatively high rate of shunt failure33,34. In this study, we mainly performed LPS in patients without lumbar spinal canal stenosis.
After surgery, the rate of improvement in gait disturbance was significantly lower in the iNPH-LSS group compared with the iNPH group. This suggests that some of the patients who did not improve after surgery for iNPH might have been affected by LSS symptoms. One patient in the iNPH-LSS group underwent lumbar decompression surgery after shunt surgery. Four patients in the iNPH group underwent shunt surgery after lumbar decompression surgery.
There were several limitations in our study. First, this was a retrospective single-center study; thus, selection bias may have occurred. We should perform a multicenter study to confirm our findings. Second, patients were diagnosed with LSS alone, by evaluating medical records, on the basis of the most narrowed part of the DSCSA being narrower than 75 mm2 on MRI. We could not evaluate more detailed comprehensive physical examination findings of LSS, such as the presence of numbness and pain of the lower extremities. Considering the possibility of LSS in patients with iNPH, we should keep in mind that elderly patients who present with gait disturbance may not have a single disease, but may also have a comorbidity.