Table 1

Characteristics of included studies

Study	Groups	N	Age (years)	Sex (M/F)	Diabetes type (1/2)	Duration of diabetes (years)	HbA_1c (%)	Exclusion	Type of CCM	Assessment with CCM
Study	Groups	N	Age (years)	Sex (M/F)	Diabetes type (1/2)	Duration of diabetes (years)	HbA_1c (%)	criteria*	Type of CCM	NFD	NBD	NFL	TC
Ahmed et al5	Controls	64	38.9±17.6	30/34	–	–	5.5±0.4	√	LSCM	√	√	√	√
	No-DPN	56	34.9±14.8	27/29	56/0	17.6±14.0	7.4±1.3	√		√	√	√	√
	DPN	33	50.0±14.3	16/17	33/0	31.4±13.5	8.7±2.1	√		√	√	√	√
Edwards et al6	Controls	61	52±14	27/34	–	–	5.4±0.3	√	LSCM		√	√	√
	No-DPN	143	48±16	66/77	105/38	14±12	7.8±1.2	√			√	√	√
	DPN	88	58±9	57/31	51/37	23±14	8.2±1.7	√			√	√	√
Hertz et al7	Controls	20	41.4±17.3	5/15	–	–	5.5±0.4	N/A	LSCM	√	√	√	√
	No-DPN	12	42.8±16.9	18/8	26/0	22.7±16.4	8.0±1.9	N/A		√	√	√	√
	DPN	14	42.8±16.9	18/8	26/0	22.7±16.4	8.0±1.9	N/A		√	√	√	√
Kallinikos et al8	Controls	18	57.8±11.5	N/A	–	–	<6.5	N/A	SSCM				√
	DPN	18	57.8±12.8	N/A	7/11	22.9±6.3	8.1±1.2	√					√
Malik et al9	Controls	18	57.8±11.5	N/A	–	–	<6.5	N/A	SSCM	√	√	√
	DPN	18	57.8±12.8	N/A	7/11	22.9±6.3	8.1±1.2	√		√	√	√
Mehra et al10	Controls	15	46±3	N/A	–	–	N/A	N/A	SSCM	√	√	√	√
	DPN	20	41±1	N/A	20/0	27±2	8.9±1.4	N/A		√	√	√	√
Petropoulos et al11	Controls	55	51.7±11.4	28/27	–	–	5.5±0.3	√	LSCM	√	√	√
	No-DPN	86	50.4±14.1	108/78	N/A	24.2±21.2	7.7±1.6	√		√	√	√
	DPN	100	50.4±14.1	108/78	N/A	34.4±17.3	7.9±1.6	√		√	√	√
Pritchard et al12	Controls	154	46±15	70/84	–	–	5.5±0.3	√	LSCM		√	√
	No-DPN	168	43±16	85/83	168/0	20±15	8.0±1.2	√			√	√
	DPN	74	57±11	41/33	74/0	34±16	8.6±1.8	√			√	√
Quattrini et al13	Controls	15	55.0±18.5	6/9	–	–	–	√	SSCM	√	√	√
	No-DPN	10	53.5±10.2	6/4	3/7	16.7±14.0	7.2±1.3	√		√	√	√
	DPN	44	59.1±11.0	36/8	13/31	21.2±12.1	8.0±1.8	√		√	√	√
Sivaskandarajah et al14	Controls	64	38.3±16.4	34/30	–	–	5.6±0.4	√	LSCM	√	√	√	√
Sivaskandarajah et al14	No-DPN	63	32.7±13.6	29/34	63/0	17.3±12.2	7.5±1.2	√		√	√	√	√
	DPN	33	48.5±13.7	14/19	33/0	32.3±13.1	8.4±1.6	√		√	√	√	√
Tavakoli et al15	Controls	17	55±19.8	8/9	–	–	<6.5	√	SSCM	√	√	√
	No-DPN	34	55±11.1	19/15	2/32	10.7±10.6	8.1±1.2	√		√	√	√
	DPN	67	58.9±11.3	54/13	15/52	17.0±12.3	8.1±1.4	√		√	√	√
Tavakoli et al16	Controls	18	57.0±12.7	10/8	–	–	5.7±0.4	√	SSCM	√	√	√	√
	DPN	25	52.0±10.0	20/5	15/10	26.5±12.5	8.1±1.5	√		√	√	√	√
Xue et al17	Controls	10	48.6±7.8	5/5	–	–	–	N/A	LSCM		√	√	√
	No-DPN	20	56.3±8.9	12/8	0/20	8.4±5.1	7.1±1.3	N/A			√	√	√
	DPN	25	52.0±10.6	15/10	0/25	9.2±6.7	7.9±1.6	N/A			√	√	√

Data are presented as mean±SD for age, duration of diabetes and HbA_1c.
The patients with DPN met the international diagnostic criteria for DPN.1 Cases of peripheral neuropathy were defined according to established consensus criteria using nerve conduction studies and clinical examination.
*The exclusion criteria was described in the article, for example, patients with any other known cause of neuropathy or previous corneal abnormality were excluded.
CCM, corneal confocal microscopy; DPN, diabetic peripheral neuropathy; HbA_1c, glycated haemoglobin; LSCM, laser-scanning confocal microscope; N/A, not applicable for this group; NBD, corneal nerve branch density; NFD, corneal nerve fibre density; NFL, corneal nerve fibre length; No-DPN, diabetes without diabetic peripheral neuropathy; SSCM, slit-scanning confocal microscope; TC, tortuosity coefficient.