Table 2. Results of the main choroidal parameters of the studies from the review.

CA: choroidal area, CC: choriocapillaris, CCT: central choroidal thickness, CSME: clinically significant macular edema, CT: choroidal thickness, CVI: choroidal vascular index, DME: diabetic macular edema, DT: systemic diabetic treatment, LA: luminal area, L/C ratio: luminal-to-choroidal ratio, ME: macular edema, mmNPDR: mild-to-moderate non-proliferative diabetic retinopathy, msNPDR: moderate-to-severe non-proliferative diabetic retinopathy, NPDR: non-proliferative diabetic retinopathy, OCTA: optical coherence tomography angiography, PDR: proliferative diabetic retinopathy, PRP: pan-retinal photocoagulation, SA: stromal area, SFCT: subfoveal choroidal thickness, SMD: serous macular detachment, TCA: total choroidal area.

Study	Groups	Choroidal Part	Thickness Measurement (μm)	CVI	Conclusion
Endo et al. [15]	Healthy control	Total CCT	254±83		The total and outer CCT layers of diabetic eyes were significantly thickened in the DME+DT− as compared with the DME−DT+ group. The total CCT layer and the outer choroidal layer thickness were significantly thicker in the DME+ than in the DME− group in all DME cases examined.
	DME+		283±88
	DME−		251±70
	DME+DT+		274±88
	DME−DT+		247±66
	DME+DT−		290±84
	DME−DT−		258±75
	Healthy control	Outer CCT	195±75
	DME+		222±83
	DME−		193±63
	DME+DT+		214±83
	DME−DT+		189±58
	DME+DT−		228±77
	DME−DT−		201±70
Wang and Tao [16]	DM	SFCT	213.21±19.02	0.63±0.04	Eyes of patients with DM showed that the L/C ratio (equals to CVI) decreased compared with normal controls. The SFCT increased, but the L/C ratio significantly decreased with worsening of DR compared with DM with no DR, and normal eyes.
	Healthy control		212.63±11.99	0.68±0.06
	DM without DR		194.18±5.68	0.65±0.03
	PRP-untreated NPDR		217.29±14.07	0.63±0.05
	PRP-untreated PDR		229.25±13.89	0.61±0.04
Gupta et al. [17]	Healthy control	SFCT	284.53±56.45	67.51±2.86	SFCT was significantly increased in eyes with DME as compared to the controls and showed an ascending trend with worsening of DR, though this difference was not statistically significant. CVI was significantly decreased in DME with DR eyes as compared to controls. CVI was also significantly decreased with worsening DR.
	Mild NPDR		304.33±40.39	66.38±0.3
	Moderate NPDR		327.81±47.39	65.28±0.37
	Severe NPDR		357.72±62.65	63.50±0.47
	PDR		334.59±47.4	61.27±0.9
	DME		334.47±51.81	63.89±1.89
Rewbury et al. [18]	Mild NPDR	Mean SFCT	217.7±62		SFCT increased with the severity of DR, especially in the PDR group. DME was associated with a non-statistically significant increase in CT compared with eyes without DME.
	msNPDR		221.7±62
	PDR		242.1±48
	DME−		209.3±61
	DME+		225.4±60
Ohara et al. [19]	Before PRP	SFCT	268.4±102.9		CT significantly decreased after PRP, which continued for at least six months after treatment. CT of severe NPDR and PDR was significantly thicker than that of mild-to-moderate NPDR.
	One month after PRP		253.4±103.1
	Three months after PRP		253.8±107.1
	Six months after PRP		252.9±110.5
	Healthy control		243±71.4
	DM without DR		251.3±61.9
	mmNPDR		227.1±71.3
	Severe NPDR		323.1±66.0
	PDR		301.7±80.8
	before PRP	Central Field CT	268.6±104.5
	One month after PRP		254.5±105.3
	Three months after PRP		254.2±108.2
	Six months after PRP		248.1±101.8
	Healthy control		248.3±70.7
	DM without DR		250.2±55.4
	mmNPDR		230.0±70.3
	Severe NPDR		323.2±61.3
	PDR		307.3±84.1
Tan et al. [20]	Healthy control	Average Choroidal (subfoveal, nasal, and temporal)	180.4±70.50	67.20±0.16	Eyes of patients with DM showed significantly decreased CVI with no corresponding change in CT compared to controls. However, there was a significant decrease in CVI and an increase in TCA, LA, SA, and average CT in DR patients compared with DM without DR.
	DM group		168.37±52.07	65.10±0.20
	DM without DR		157.24±48.29	65.30±0.21
	DR group		193.68±53.65	64.20±0.16
Hua et al. [21]	DME+ SMD−	SFCT	276		In group two, both the SFCT and CA of eyes with DME and SMD were significantly greater than those in the other eyes. The CA in PRP treated cases was also greater than that in non-PRP treated cases.
	DME+ SMD+		364
	Non-PRP treated		288
	PRP-treated		365
Kim et al. [14]	Healthy controls	SFCT	320±77.92	69.08±2.29	The eyes of patients with DM, even without DR, exhibited a significantly lower CVI than those of healthy controls. Notably, the PDR group exhibited a significantly lower mean CVI relative to the other DR stages. Eyes of diabetic patients exhibited a lower SFCT than the eyes of healthy controls. Among the eyes of diabetic patients, the lowest CT values were observed in the PDR group.
	DM without DR		258.13±89.02	67.07±3.71
	mmNPDR		310.22±72.41	66.28±2.70
	Severe NPDR		304.53±69.26	66.2±2.56
	PDR		258.75±73.29	63.48±2.89
	PRP-treated DR		276.29±79.51	65.38±3.15
	CSME		312.58±89.59	66.28±2.85
Sudhalkar et al. [22]	Healthy controls	Mean SFCT	281.7±47.7		Control eyes had greater SFCT compared to subjects with DM, with and without retinopathy. The thinning progressed with increasing severity of DR. SFCT in eyes with ME was not significantly different from eyes without ME.
	DM without DR		261.71±51.8
	DM with any form of DR		252.8±55.6
	NPDR		248.0±56.3
	PDR		243.9±56.2
	PRP-treated DR		258.4±48.3
	Non-PRP-treated DR		251.78±56.9
	DME−		246.805±55.61
	DME+		256.629±55.24
Totan et al. [23]	Healthy controls	SFCT	321.4±36.5		Both pulsatile choroidal blood flow and CT were decreased in patients with DME.
	DME		273.5±30.2
Gerendas et al. [24]	Healthy controls	Total CT in the 6-mm region on the foveal grid	190±23		Total CT in the 6-mm region on the foveal grid is significantly reduced in DME and non-edematous fellow eyes of patients compared with healthy control eyes. There was no statistically significant difference in overall CT between patients’ study eyes with DME and their fellow eyes without DME.
	Non-edematous fellow eyes		177±20
	DME		175±23
Lee et al. [25]	Healthy controls	SFCT	228.5±38.9		The CT of subfoveal regions was significantly decreased in DR patients compared with controls. The proliferative changes or presence of ME did not result in additional choroidal thinning.
	No diabetic change		219.1±47.8
	mmNPDR		158.9±56.3
	Severe NPDR		161.2±38.5
	PDR		157.4±45.7
	DME+		164.1±63.0
	DME−		157.2±71.1
Adhi et al. [26]	Healthy controls	SFCT	276.4±13.4		Choroidal morphological features are altered in patients with moderate to severe DR. The SFCT and the subfoveal medium choroidal vessel layer and CC layer thicknesses are significantly reduced in patients with DR, PDR, and DME compared to controls.
	NPDR		252.9±20.27
	PDR		209.6±12.42
	DME		211.6±17.05
Regatieri et al. [27]	Normal	Mean SFCT	232.3±15.2		There is a significant decrease in the CT in patients with DME or treated PDR compared to normal subjects. No significant difference between normal subjects and the NPDR group was observed. Between DME and treated PDR groups, there was no significant difference.
	NPDR		222±21.6
	DME		169.5±14.7
	PDR		162.7±7.0
Gołębiewska et al. [28]	Diabetic	SFCT	355.65		CT remains unchanged in children with Type one DM. There was no significant difference between subjects and controls in the CT in the fovea, nasal, temporal, superior, and inferior quadrants of the macula. However, regardless of the prevalence of Type one DM in the studied children, CT was significantly thicker in girls than in boys, except for the superior quadrant.
				Non-diabetic		327.98
	Kim et al. [29]		Healthy controls						69.21±2.24	CVI correlated negatively with worsening DR severity, P-value= 0.009.
			DM without DR				67.06±3.98
Mild NPDR				66.60±3.03
Moderate NPDR				66.18±3.04
Severe NPDR				66.15±2.63
PDR				63.10±3.45
Dodo et al. [30]					On the ~10-μm and ~29-μm-thick CC slab images, the areas of flow void increased gradually according to the DR severity, and eyes with severe NPDR and PDR had significantly larger areas of flow void and larger non-flow areas than those with no apparent retinopathy. In 12 eyes with ischemic maculopathy, the CC layer beneath the disrupted ellipsoid zone of the photoreceptor (EZ) had greater areas of flow void than did the area beneath an intact EZ.
Nesper et al. [31]					Retinal and CC vascular nonperfusion in OCTA increased significantly with disease severity in eyes with DR.