Skip to main content

Advertisement

Log in

New vistas in the diagnosis of diabetic polyneuropathy

  • Review
  • Published:
Endocrine Aims and scope Submit manuscript

Abstract

New modalities are now available to improve the diagnosis of diabetic polyneuropathy (DPN). The present review discusses the progress achieved in this area. First, the minimal diagnostic criteria have been better clarified. Moreover, there are now new bedside tests available, such as the indicator test Neuropad, NeuroQuick, Ipswich Touch Test (IpTT), Vibratip, NC-stat®/DPNCheck™ for automated nerve conduction study (NCS), tactile circumferential discriminator, steel ball-bearing, and SUDOSCAN®, while more sophisticated modalities include skin biopsy and corneal confocal microscopy (CCM). Some tests can be used as screening tools, including primary care setting (Neuropad, IpTT, Vibratip, automated NCS), while others are more suitable for research, including evaluation of DPN in prospective studies (CCM, skin biopsy). Importantly, there is some evidence of earlier DPN diagnosis with the aid of some tests (Neuropad, skin biopsy, CCM). Further advantages provided by different tests are educational value and self-examination. Thus far, the potential of these tests has not been fully utilised. In particular, they have not been validated against standardised clinical examination scores in terms of predicting foot ulcers and amputations. Hence, it now remains to investigate the potential benefits from the widespread use of these tests for earlier and easier diagnosis of DPN in the everyday clinic.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Similar content being viewed by others

References

  1. A.J. Boulton, A.I. Vinik, J.C. Arezzo, V. Bril, E.L. Feldman, R. Freeman, R.A. Malik, R.E. Maser, J.M. Sosenko, D. Ziegler, American Diabetes Association. Diabetic neuropathies: a statement by the American Diabetes Association. Diabetes Care 28, 956–962 (2005)

    Article  PubMed  Google Scholar 

  2. S. Tesfaye, A.J. Boulton, P.J. Dyck, R. Freeman, M. Horowitz, P. Kempler, G. Lauria, R.A. Malik, V. Spallone, A. Vinik, L. Bernardi, P. Valensi, Toronto Diabetic Neuropathy Expert Group. Diabetic neuropathies: update on definitions, diagnostic criteria, estimation of severity, and treatments. Diabetes Care 33, 2285–2293 (2010)

    Article  PubMed Central  PubMed  Google Scholar 

  3. A.L. Carrington, J.E. Shaw, C.H. Van Schie, C.A. Abbott, L. Vileikyte, A.J. Boulton, Can motor nerve conduction velocity predict foot problems in diabetic subjects over a 6-year outcome period? Diabetes Care 25, 2010–2015 (2002)

    Article  PubMed  Google Scholar 

  4. N. Papanas, E. Maltezos, The diabetic foot: established and emerging treatments. Acta Clin. Belg. 62, 230–238 (2007)

    Article  CAS  PubMed  Google Scholar 

  5. A.J. Boulton, The diabetic foot: grand overview, epidemiology and pathogenesis. Diabetes Metab. Res. Rev. 24(Suppl 1), S3–S6 (2008)

    Article  PubMed  Google Scholar 

  6. N. Papanas, E. Maltezos, M. Edmonds, Salvation of the diabetic foot: still a quest for the Holy Grail? Vasa 40, 267–269 (2011)

    Article  CAS  PubMed  Google Scholar 

  7. A.J. Boulton, R.S. Kirsner, L. Vileikyte, Clinical practice. Neuropathic diabetic foot ulcers. N. Engl. J. Med. 351, 48–55 (2004)

    Article  CAS  PubMed  Google Scholar 

  8. J. Apelqvist, Diagnostics and treatment of the diabetic foot. Endocrine 41, 384–397 (2012)

    Article  CAS  PubMed  Google Scholar 

  9. N.K. Chammas, R.L. Hill, A.V. Foster, M.E. Edmonds, Is neuropathic ulceration the key to understanding increased mortality due to ischaemic heart disease in diabetic foot ulcer patients? A population approach using a proportionate model. J. Int. Med. Res. 30, 553–559 (2002)

    Article  CAS  PubMed  Google Scholar 

  10. J. van Baal, R. Hubbard, F. Game, W. Jeffcoate, Mortality associated with acute Charcot foot and neuropathic foot ulceration. Diabetes Care 33, 1086–1089 (2010)

    Article  PubMed Central  PubMed  Google Scholar 

  11. N. Papanas, A.I. Vinik, D. Ziegler, Neuropathy in prediabetes: does the clock start ticking early? Nat. Rev. Endocrinol. 7, 682–690 (2011)

    Article  CAS  PubMed  Google Scholar 

  12. N. Papanas, D. Ziegler, Polyneuropathy in impaired glucose tolerance: is postprandial hyperglycemia the main culprit? A mini-review. Gerontology 59, 193–198 (2013)

    Article  CAS  PubMed  Google Scholar 

  13. N. Papanas, D. Ziegler, New diagnostic tests for diabetic distal symmetric polyneuropathy. J. Diabetes Complic. 25, 44–51 (2011)

    Article  Google Scholar 

  14. N. Papanas, A.J. Boulton, R.A. Malik, C. Manes, O. Schnell, V. Spallone, N. Tentolouris, S. Tesfaye, P. Valensi, D. Ziegler, P. Kempler, A simple new non-invasive sweat indicator test for the diagnosis of diabetic neuropathy. Diabet. Med. 30, 525–534 (2013)

    Article  CAS  PubMed  Google Scholar 

  15. D. Ziegler, E. Siekierka-Kleiser, B. Meyer, M. Schweers, Validation of a novel screening device (NeuroQuick) for quantitative assessment of small nerve fiber dysfunction as an early feature of diabetic polyneuropathy. Diabetes Care 28, 1169–1174 (2005)

    Article  PubMed  Google Scholar 

  16. G. Rayman, P.R. Vas, N. Baker, C.G. Taylor Jr, C. Gooday, A.I. Alder, M. Donohoe, The Ipswich Touch Test: a simple and novel method to identify inpatients with diabetes at risk of foot ulceration. Diabetes Care 34, 1517–1518 (2011)

    Article  PubMed Central  PubMed  Google Scholar 

  17. F.L. Bowling, C.A. Abbott, W.E. Harris, S. Atanasov, R.A. Malik, A.J. Boulton, A pocket-sized disposable device for testing the integrity of sensation in the outpatient setting. Diabet. Med. 29, 1550–1552 (2012)

    Article  CAS  PubMed  Google Scholar 

  18. N. Bracewell, F. Game, W. Jeffcoate, B.E. Scammell, Clinical evaluation of a new device in the assessment of peripheral sensory neuropathy in diabetes. Diabet. Med. 29, 1553–1555 (2012)

    Article  CAS  PubMed  Google Scholar 

  19. A.I. Vinik, X. Kong, J.T. Megerian, S.N. Gozani, Diabetic nerve conduction abnormalities in the primary care setting. Diabetes Technol. Ther. 8, 654–662 (2006)

    Article  PubMed  Google Scholar 

  20. L. Vileikyte, G. Hutchings, S. Hollis, A.J. Boulton, The tactile circumferential discriminator. A new, simple screening device to identify diabetic patients at risk of foot ulceration. Diabetes Care 20, 623–626 (1997)

    Article  CAS  PubMed  Google Scholar 

  21. N. Papanas, A. Gries, E. Maltezos, R. Zick, The steel ball-bearing test: a new test for evaluating protective sensation in the diabetic foot. Diabetologia 49, 739–743 (2006)

    Article  CAS  PubMed  Google Scholar 

  22. J.H. Calvet, J. Dupin, H. Winiecki, P.E. Schwarz, Assessment of small fiber neuropathy through a quick, simple and non invasive method in a German diabetes outpatient clinic. Exp. Clin. Endocrinol. Diabetes 121, 80–83 (2013)

    CAS  PubMed  Google Scholar 

  23. G. Lauria, M. Bakkers, C. Schmitz, R. Lombardi, P. Penza, G. Devigili, A.G. Smith, S.T. Hsieh, S.I. Mellgren, T. Umapathi, D. Ziegler, C.G. Faber, I.S. Merkies, Intraepidermal nerve fiber density at the distal leg: a worldwide normative reference study. J. Peripher. Nerv. Syst. 15, 202–207 (2010)

    Article  PubMed  Google Scholar 

  24. N. Papanas, D. Ziegler, Corneal confocal microscopy: a new technique for early detection of diabetic neuropathy. Curr. Diab. Rep. 13, 488–499 (2013)

    Article  CAS  PubMed  Google Scholar 

  25. N. Papanas, K. Papatheodorou, D. Christakidis, D. Papazoglou, G. Giassakis, H. Piperidou, C. Monastiriotis, E. Maltezos, Evaluation of a new indicator test for sudomotor function (Neuropad®) in the diagnosis of peripheral neuropathy in type 2 diabetic patients. Exp. Clin. Endocrinol. Diabetes 113, 195–198 (2005)

    Article  CAS  PubMed  Google Scholar 

  26. N. Papanas, K. Papatheodorou, D. Papazoglou, D. Christakidis, C. Monastiriotis, E. Maltezos, The new indicator test (Neuropad): a valuable diagnostic tool for small-fiber impairment in patients with type 2 diabetes. Diabetes Educ. 33, 257–266 (2007)

    Article  PubMed  Google Scholar 

  27. V. Spallone, R. Morganti, M. Siampli, T. Fedele, C. D’Amato, L. Cacciotti, M.R. Maiello, Neuropad as a diagnostic tool for diabetic autonomic and sensorimotor neuropathy. Diabet. Med. 26, 686–692 (2009)

    Article  CAS  PubMed  Google Scholar 

  28. N. Papanas, P. Paschos, D. Papazoglou, K. Papatheodorou, K. Paletas, E. Maltezos, A. Tsapas, Accuracy of the Neuropad test for the diagnosis of distal symmetric polyneuropathy in type 2 diabetes. Diabetes Care 34, 1378–1382 (2011)

    Article  PubMed Central  PubMed  Google Scholar 

  29. N. Papanas, G. Giassakis, K. Papatheodorou, D. Papazoglou, C. Monastiriotis, D. Christakidis, H. Piperidou, E. Maltezos, Use of the new indicator test (Neuropad) for the assessment of the staged severity of neuropathy in type 2 diabetic patients. Exp. Clin. Endocrinol. Diabetes 115, 58–61 (2007)

    Article  CAS  PubMed  Google Scholar 

  30. D. Ziegler, N. Papanas, M. Roden, for the GDC Study Group, Neuropad: evaluation of three cut-off points of sudomotor dysfunction for early detection of polyneuropathy in recently diagnosed diabetes. Diabet. Med. 28, 1412–1415 (2011)

    Article  CAS  PubMed  Google Scholar 

  31. C. Manes, N. Papanas, T. Exiara, N. Katsiki, S. Papantoniou, E. Kirlaki, S. Tsotoulidis, N. Kefalogiannis, E. Maltezos, The indicator test Neuropad in the assessment of small and overall nerve fibre dysfunction in patients with type 2 diabetes: a large multicentre study. Exp. Clin. Endocrinol. Diabetes 122, 195–199 (2014)

    Article  CAS  PubMed  Google Scholar 

  32. A. Tsapas, A. Liakos, P. Paschos, T. Karagiannis, E. Bekiari, N. Tentolouris, P. Boura, A simple plaster for screening for diabetic neuropathy: a diagnostic test accuracy systematic review and meta-analysis. Metabolism 63, 584–592 (2014)

    Article  CAS  PubMed  Google Scholar 

  33. N. Tentolouris, V. Achtsidis, K. Marinou, N. Katsilambros, Evaluation of the self-administered indicator plaster Neuropad for the diagnosis of neuropathy in diabetes. Diabetes Care 31, 236–237 (2008)

    Article  PubMed  Google Scholar 

  34. N. Papanas, K. Papatheodorou, D. Papazoglou, S. Kotsiou, E. Maltezos, A prospective study on the use of the indicator test Neuropad for the early diagnosis of peripheral neuropathy in type 2 diabetes. Exp. Clin. Endocrinol. Diabetes 119, 122–125 (2011)

    Article  CAS  PubMed  Google Scholar 

  35. N. Papanas, K. Papatheodorou, D. Papazoglou, D. Christakidis, C. Monastiriotis, E. Maltezos, Reproducibility of the new indicator test for sudomotor function (Neuropad) in patients with type 2 diabetes mellitus: short communication. Exp. Clin. Endocrinol. Diabetes 113, 577–581 (2005)

    Article  CAS  PubMed  Google Scholar 

  36. D. Ziegler, N. Papanas, W. Rathmann, M. Heier, M. Scheer, C. Meisinger, KORA Study Group. Evaluation of the Neuropad sudomotor function test as a screening tool for polyneuropathy in the elderly population with diabetes and pre-diabetes: the KORA F4 survey. Diabetes Metab. Res. Rev. 28, 692–697 (2012)

    Article  CAS  PubMed  Google Scholar 

  37. M.J. Young, A.J. Boulton, A.F. MacLeod, D.R. Williams, P.H. Sonksen, A multicentre study of the prevalence of diabetic peripheral neuropathy in the United Kingdom hospital clinic population. Diabetologia 36, 150–154 (1993)

    Article  CAS  PubMed  Google Scholar 

  38. B.A. Perkins, J. Grewal, E. Ng, M. Ngo, V. Bril, Validation of a novel point-of-care nerve conduction device for the detection of diabetic sensorimotor polyneuropathy. Diabetes Care 29, 2023–2027 (2006)

    Article  PubMed  Google Scholar 

  39. A.I. Vinik, M.S. Emley, J.T. Megerian, S.N. Gozani, Median and ulnar nerve conduction measurements in patients with symptoms of diabetic peripheral neuropathy using the NC-stat system. Diabetes Technol. Ther. 6, 816–824 (2004)

    Article  PubMed  Google Scholar 

  40. X. Kong, S.N. Gozani, M.T. Hayes, D.H. Weinberg, NC-stat sensory nerve conduction studies in the median and ulnar nerves of symptomatic patients. Clin. Neurophysiol. 117, 405–413 (2006)

    Article  PubMed  Google Scholar 

  41. X. Kong, E.A. Lesser, J.T. Megerian, S.N. Gozani, Repeatability of nerve conduction measurements using automation. J. Clin. Monit. Comput. 20, 405–410 (2006)

    Article  PubMed Central  PubMed  Google Scholar 

  42. B. Boettcher, B. Tracey, X. Kong, M. Williams, S.N. Gozani, A rapid, low-cost, point-of-care test for diabetic peripheral neuropathy. Diabetes 60, 637 (2011)

    Google Scholar 

  43. X. Kong, B. Boettcher, K. Snow, S. Gozani, Reproducibility of a rapid point-of-care sural nerve conduction test. Diabetes 61, 595 (2012)

    Google Scholar 

  44. A.I. Vinik, S.N. Gozani, Loss of sural responses estimated by a rapid and easily administered technique identifies greater neuropathy severity and risk in a general diabetes population. Diabetes 60, 650 (2011)

    Google Scholar 

  45. J.A. Lee, E.M. Halpern, L.E. Lovblom, E. Yeung, V. Bril, B.A. Perkins, Reliability and validity of a point-of-care sural nerve conduction device for identification of diabetic neuropathy. PLoS ONE 9, e86515 (2014)

    Article  PubMed Central  PubMed  Google Scholar 

  46. R.E. Maser, C. Laudadio, M.J. Lenhard, G.S. DeCherney, A cross-sectional study comparing two quantitative sensory testing devices in individuals with diabetes. Diabetes Care 20, 179–181 (1997)

    Article  CAS  PubMed  Google Scholar 

  47. H. Gin, R. Baudoin, C.H. Raffaitin, V. Rigalleau, C. Gonzalez, Non-invasive and quantitative assessment of sudomotor function for peripheral diabetic neuropathy evaluation. Diabetes Metab. 37, 527–532 (2011)

    Article  CAS  PubMed  Google Scholar 

  48. C.M. Casellini, H.K. Parson, M.S. Richardson, M.L. Nevoret, A.I. Vinik, Sudoscan, a noninvasive tool for detecting diabetic small fiber neuropathy and autonomic dysfunction. Diabetes Technol. Ther. 15, 948–953 (2013)

    Article  PubMed Central  PubMed  Google Scholar 

  49. S. Løseth, E. Stålberg, R. Jorde, S.I. Mellgren, Early diabetic neuropathy: thermal thresholds and intraepidermal nerve fibre density in patients with normal nerve conduction studies. J. Neurol. 255, 1197–1202 (2008)

    Article  PubMed  Google Scholar 

  50. Joint Task Force of the EFNS and the PNS, European Federation of Neurological Societies/Peripheral Nerve Society Guideline on the use of skin biopsy in the diagnosis of small fiber neuropathy. Report of a joint task force of the European Federation of Neurological Societies and the Peripheral Nerve Society. J. Peripher. Nerv. Syst. 15, 79–92 (2010)

    Article  Google Scholar 

  51. M.I. Myers, A.C. Peltier, Uses of skin biopsy for sensory and autonomic nerve assessment. Curr. Neurol. Neurosci. Rep. 13, 323 (2013)

    Article  PubMed Central  PubMed  Google Scholar 

  52. C. Quattrini, M. Jeziorska, A.J. Boulton, R.A. Malik, Reduced vascular endothelial growth factor expression and intra-epidermal nerve fiber loss in human diabetic neuropathy. Diabetes Care 31, 140–145 (2008)

    Article  PubMed  Google Scholar 

  53. C. Quattrini, M. Tavakoli, M. Jeziorska, P. Kallinikos, S. Tesfaye, J. Finnigan, A. Marshall, A.J. Boulton, N. Efron, R.A. Malik, Surrogate markers of small fiber damage in human diabetic neuropathy. Diabetes 56, 2148–2154 (2007)

    Article  CAS  PubMed  Google Scholar 

  54. H. Narayanaswamy, P. Facer, V.P. Misra, M. Timmers, G. Byttebier, T. Meert, P. Anand, A longitudinal study of sensory biomarkers of progression in patients with diabetic peripheral neuropathy using skin biopsies. J. Clin. Neurosci. 19, 1490–1496 (2012)

    Article  CAS  PubMed  Google Scholar 

  55. M. Polydefkis, P. Hauer, S. Sheth, M. Sirdofsky, J.W. Griffin, J.C. McArthur, The time course of epidermal nerve fibre regeneration: studies in normal controls and in people with diabetes, with and without neuropathy. Brain 127, 1606–1615 (2004)

    Article  PubMed  Google Scholar 

  56. D. Ziegler, N. Papanas, A. Zhivov, S. Allgeier, K. Winter, I. Ziegler, J. Brüggemann, A. Strom, S. Peschel, B. Köhler, O. Stachs, R. F. Guthoff, M. Roden, for the GDS Group, Early detection of nerve fiber loss by corneal confocal microscopy and skin biopsy in recently diagnosed type 2 diabetes. Diabetes. doi:10.2337/db13-1819

  57. A. Strom, J. Brüggemann, I. Ziegler, K. Jeruschke, J. Weiss, H. Al-Hasani, M. Roden, D. Ziegler, for the GDS Group, Pronounced reduction of cutaneous Langerhans cell density in recently diagnosed type 2 diabetes. Diabetes 63, 1148–1153 (2014)

    Article  CAS  PubMed  Google Scholar 

  58. B. Köhler, S. Allgeier, F. Eberle, R. Guthoff, S. Maier, O. Stachs, A. Zhivov, D. Ziegler, G. Bretthauer, Image reconstruction of the corneal subbasal nerve plexus with extended field of view from focus image stacks of a confocal laser scanning microscope. Klin. Monbl. Augenheilkd. 228, 1060–1066 (2011)

    Article  PubMed  Google Scholar 

  59. E.A. Sellers, I. Clark, M. Tavakoli, H.J. Dean, J. McGavock, R.A. Malik, The acceptability and feasibility of corneal confocal microscopy to detect early diabetic neuropathy in children: a pilot study. Diabet. Med. 30, 630–631 (2013)

    Article  CAS  PubMed  Google Scholar 

  60. C. Quattrini, M. Tavakoli, P. Kallinikos et al., Comparing skin biopsy with corneal confocal microscopy: diagnostic yield of nerve fiber density. Diabetologia 53(Suppl 1), A1114 (2010)

    Google Scholar 

  61. M. Tavakoli, C. Quattrini, P. Begum et al., Neuropad and corneal confocal microscopy: new indications for human diabetic neuropathy. Diabetologia 53(Suppl 1), A1112 (2010)

    Google Scholar 

  62. F. Ishibashi, R. Kojima, A. Kawasaki, E. Yamanaka, A. Kosaka, H. Uetake, Correlation between sudomotor function, sweat gland duct size and corneal nerve fiber pathology in patients with type 2 diabetes mellitus. J. Diabetes Invest. (2013). doi:10.1111/jdi.12171

    Google Scholar 

  63. M. Tavakoli, P. Kallinikos, A. Iqbal, A. Herbert, H. Fadavi, N. Efron, A.J. Boulton, R.A. Malik, Corneal confocal microscopy detects improvement in corneal nerve morphology with an improvement in risk factors for diabetic neuropathy. Diabet. Med. 28, 1261–1267 (2011)

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  64. M. Tavakoli, M. Mitu-Pretorian, I.N. Petropoulos, H. Fadavi, O. Asghar, U. Alam, G. Ponirakis, M. Jeziorska, A. Marshall, N. Efron, A.J. Boulton, T. Augustine, R.A. Malik, Corneal confocal microscopy detects early nerve regeneration in diabetic neuropathy after simultaneous pancreas and kidney transplantation. Diabetes 62, 254–260 (2013)

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  65. N. Efron, K. Edwards, N. Roper, N. Pritchard, G.P. Sampson, A.M. Shahidi, D. Vagenas, A. Russell, J. Graham, M.A. Dabbah, R.A. Malik, Repeatability of measuring corneal subbasal nerve fiber length in individuals with type 2 diabetes. Eye Contact Lens 36, 245–248 (2010)

    Article  PubMed  Google Scholar 

  66. P. Hertz, V. Bril, A. Orszag, A. Ahmed, E. Ng, P. Nwe, M. Ngo, P.A. Perkins, Reproducibility of in vivo corneal confocal microscopy as a novel screening test for early diabetic sensorimotor polyneuropathy. Diabet. Med. 28, 1253–1260 (2011)

    Article  CAS  PubMed  Google Scholar 

  67. I.N. Petropoulos, T. Manzoor, P. Morgan, H. Fadavi, O. Asghar, U. Alam, G. Ponirakis, M.A. Dabbah, X. Chen, J. Graham, M. Tavakoli, R.A. Malik, Repeatability of in vivo corneal confocal microscopy to quantify corneal nerve morphology. Cornea 32, e83–e89 (2013)

    Article  PubMed  Google Scholar 

  68. W.H. Herman, L. Kennedy, Underdiagnosis of peripheral neuropathy in type 2 diabetes. Diabetes Care 28, 1480–1841 (2005)

    Article  PubMed  Google Scholar 

  69. C. Gazzaruso, A. Coppola, T. Montalcini, E. Baffero, A. Garzaniti, G. Pelissero, S. Collaviti, A. Grugnetti, P. Gallotti, A. Pujia, S.B. Solerte, A. Giustina, Lipoprotein(a) and homocysteine as genetic risk factors for vascular and neuropathic diabetic foot in type 2 diabetes mellitus. Endocrine 41, 89–95 (2012)

    Article  CAS  PubMed  Google Scholar 

  70. American Diabetes Association, Standards of medical care in diabetes. Diabetes Care 36(Suppl 1), S11–S66 (2013)

    Article  PubMed Central  Google Scholar 

  71. R. Sayın, M. Aslan, M. E. Kucukoglu, A. Luleci, M. Atmaca, R. Esen, H. Demir, Serum prolidase enzyme activity and oxidative stress levels in patients with diabetic neuropathy. Endocrine. doi:10.1007/s12020-013-0136-3

Download references

Conflicts of interest

NP and DZ have served as members in the scientific advisory board of TrigoCare International, distributor of Neuropad.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to Dan Ziegler.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Papanas, N., Ziegler, D. New vistas in the diagnosis of diabetic polyneuropathy. Endocrine 47, 690–698 (2014). https://doi.org/10.1007/s12020-014-0285-z

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s12020-014-0285-z

Keywords

Navigation