optics Flashcards
(163 cards)
1
Q
What is the distance measured using a A scan US
A
Axial length is traditionally measured using A-scan ultrasound
Measures distance from anterior cornea to internal limiting membrane
2
Q
Types of A scan ?
A
- Applanation biometry
- Immersion A scan
3
Q
How does he optical biometry measure axial length
A
Uses delay and intensity of infrared light reflected back from a diode to determine axial length
Measures distance from anterior cornea to retinal pigment epithelium
4
Q
Advantage of optical biometry compared to US
A
Measures to fovea versus ultrasound measuring longest axis to posterior pole
In highly myopic/ staphyloma eyes, ultrasound overestimates axial length, underestimation of IOL power
More accurate in pseudophakic and silicone oil-filled eye
5
Q
Disadvantages of optical biometry
A
Need clear path for infrared laser to travel to the fovea
Opacities can interfere with measurement
Cornea pathology
Dense/ hypermature, posterior subcapsular cataracts
Vitreous opacity
Maculopathy and retinal detachments
6
Q
What biometry to use for axial length <22
A
Haigis, Hoffer Q
7
Q
What biometry to use for Axial length between 22-26
A
SRK/T or Barret Universal II
8
Q
What biometry to use for previous refractive surgery ?
A
Haigis L
8
Q
What biometry to use for Axial length >26mm
A
Haigis, SRK/T
9
Q
when should contact lens be removed prior to biometry
A
Soft lenses 1 week
Rigid gas permeable contact lens 2-4 weeks
10
Q
Perimetric errors 4
A
Miotic pupil-pupil size is documented on HVF printout
Cloverleaf defect-pattern of visual field does not correlate to any anatomical defect and is a sign of poor visual attention/ malingering
Lens rim artifact-thick rim cause blockage of peripheral stimuli
Refractive errors
11
Q
Pachymetry false positive:
A
The patient signals when no target is displayed
12
Q
Pachymetry false negative
A
The patient fails to signal when a target brighter than the previously detected stimulus is displayed in the same spot
13
Q
What does SITA stand for and it benefits
A
SITA: Swedish Interactive Threshold Algorithm
Shortens the time needed to complete the test
50% less time
SITA standard typically takes 7 minutes per eye.
14
Q
Advantages over kinetic perimetry such as Goldman Visual Fields include
A
Standardized testing conditions
Less user dependent
Numerical data as output for statistical analysis
15
Q
The HVF 24-2 evaluates the – with how many points?
A
The HVF 24-2 evaluates the central 24 degrees with 54 points.
16
Q
The HVF 10-2 evaluates the – with how many points?
A
The central 10 degrees with 68 points
17
Q
The HVF 30-2 evaluates the – with how many points?
A
evaluates the central 30 degrees with 76 points
18
Q
What is the measure of raw light intensity used in automated perimetry?
A
Apostib
19
Q
Fluroscein is excited and emited - at what colors and wavelengths?
A
Fluorescein is a water-soluble dye which helps visualize the choroidal and retinal vasculature. It is stimulated by blue light (490nm), and subsequently emits green light ( 530nm
20
Q
what are the 6 phases of fluroscein ?
A
- Arm to retina
- Choroidal
- Arterial
- Arteriovenous
- Venous
- Tissues
21
Q
How many after injection does the tissue phase occur?
A
5-10 after injection
22
Q
How long des the arm to retina phase take ?
A
Fluorescein reaches retinal vessels from the arm to retina in 10-12 seconds.
23
Q
when is a bscan useful?
A
Particularly useful to visualize the retina if there is an opacity anterior to the retina: Corneal scarring, cataracts, vitreous haemorrhage. Can be visualized through the eyelid.
24
Quadrants and nomenacluture of the eye when using US?
Four quadrants of the eye are typically denominated with the following nomenclature based on clock hours
T12 ( superior quadrant)
T9 ( lateral or nasal quadrant)
T3 ( nasal or lateral quadrant)
T6 ( inferior quadrant)
25
US and density of tissue ?
Dense - hyperechoic
Less Dense - hypoechoic
26
Relationship between gain and detecting signals ?
Higher gain allow weak signals - vitreous opacity, small foreign body, PVD
Lower gain allow high strong signal - masses and tumours
27
Common A scan error with misalignment
Probe not perpendicular to lens or macular, or aligned to optic nerve
Falsely short reading
Cornea compression as discussed above
Falsely long reading
Fluid meniscus between probe and cornea, posterior staphyloma
Incorrect velocity
Important to consider if the eye is phakic, aphakic, pseudophakic, or if there is silicone oil as this can result in changes the sound velocity. A correction factor should be applied.
28
A scan error when gain is too high
High gain increases sensitivity, but reduces resolution of spikes, causing retina and scleral spikes to merge together
29
A scan error for falsely short reading
Cornea compression as discussed above
Falsely long reading
Fluid meniscus between probe and cornea, posterior staphyloma
Incorrect velocity
30
A scan falsely long reading ?
Fluid meniscus between probe and cornea, posterior staphyloma
31
A scan incorrect velocity
Important to consider if the eye is phakic, aphakic, pseudophakic, or if there is silicone oil as this can result in changes the sound velocity. A correction factor should be applied.
32
Readout spikes of the A scan biometry
Initial spike ( probe tip and cornea)
Anterior lens capsule
Posterior lens capsule
Retina
Sclera
Orbital fat
33
Features of keratoconus progression
Kmax change =≥1D
Topographical astigmatism change =≥1D
Corneal thickness change >30um
Remember keratoconus can present in nearly all pattern
34
Features of early keratoconus
1) Kmax => 47D
2) Kmax difference between both eyes is > 2D
35
Red flags for ectasia :
Superior and inferior thickness difference greater than 30 um.
A difference in thinnest value between both eyes of more than 30um.
- A difference between the apex thickness and thinnest location of more than 10um.
Posterior elevation map > 15um, or Anterior elevation > 12um
36
Corneal thickness maps important for 3 things:
1) Important for ectasias i.e. keratoconus and PMD.
2) Useful for Fuch's endothelial dystrophy
3) Helps determine the need for cross-linking
37
What is the k max ?
This is the maximal K reading of the anterior corneal surface. Should be measured using the tangential map. Its location tells you the apex of the cone.
38
What is the supposed K max?
47.2 D
39
A k max value that is a risk of keratocnus ?
>50D
40
What shape of the corneal surface is at increased risk of keratoconus?
Hyper-prolate <-1
41
Vertical bow tie
Astigmatism with the rule - vertical meridian is steeper
42
Horizontal bow tie
Astigmatism against the rule - horizontal meridian is steeper
43
What is the progression index ?
Progression index: assess CHANGE in corneal thickness of the whole cornea.
Useful for assessing ectatic disease
> 1.1 suggests ectatic disease
44
What is the q val?
Q-val: Tells you about corneal shape (normally based on 6mm zone)
Normal value between -1 and 0
Positive > 0 = Oblate cornea
Negative < -1 = Prolate (keratoconus)
45
Lipofuscin is primarily derived from what structure?
Outer photoreceptor segment
46
Confocal microscopy and function
Principle of confocal-single point of tissue illuminated by a point source of light; while simultaneously imaged by a camera in the same plane.
Uses:
Identify organisms causing infectious keratitis such as Acanthamoeba, fungus, microspores, herpetic eye disease
Evaluate cornea nerve morphology
Evaluate cornea endothelial layer
Differentiate corneal dystrophy
47
Each small square on a Hess Chart subtends to a degree of?
5 degrees at 50cm working distance
48
IOP is derived from the amount of force require to flatten an area of how many diameter on the cornea?
3.06mm diameter
49
Instrument used to preform ERG
Ganzfeld bowl which illuminates the whole retina with a full-field luminance stimulus, based on the ISCEV standard.
50
what inferometry
A light beam is split into two beams. One beam is sent and reflected off the ocular structures. The other is reflected off a reference mirror
51
Which of the following method is best used to evaluate paracentral defects?
Humphrey 10-2
52
Mechanism of OCTA
Multiple OCT B-scans are taken at the same point in the retina. Based on en-face OCT technique, reconstruct scans performed multiple times in a vertical plane into a single image
differences between the B scans generate movement related contrast
particularly contrast related to erythrocyte/ red blood cell movement
53
Loss of which cells from which layer will occur from retinal laser treatment?
Retinal pigment epithelium
54
Loss of which cells from which layer will occur from retinal laser treatment?
Green
55
56
Where is the inner retinal capillary plexus typically found?
NFL and ganglion cell layer
57
Where is the inner retinal capillary plexus typically found?
Pattern standard deviation
58
What is a normal field of vision i degrees ?
A normal visual field is an island of vision measuring 90 degrees temporally to central Fixation, 50 degrees superiorly and nasally, and 60 degrees inferiorly
59
The IS/OS junction represents the junction of the inner and outer segments of the photoreceptors. Why is it more pronounced near the fovea compared to the more periphereal macula?
The cones have a longer outer segment
60
The 30-2 visual field static perimery how many degrees are the central points away from fixation?
3
61
the 30-2 visual field static perimery how many degrees are the central points away from fixation?
focal cone problem
62
What is a significant axial length difference between both eyes, where you should consider repeating biometry?
If axial length difference > 0.3mm between eyes
63
What is the compression of the cornea in biometry
Compression of the cornea s between t0.2 4mm to 0.27mm
64
How long do cotton wool spots last ?
4-12 weeks
65
OMSC tumour examples
Meningioma - arachnoid cells, circumferentially tram track pattern, compress ON located centerally
Glioma- expand nerve circumferentially, nerve kinking, skip lesion
66
MRI that is best suited for ON inflammation ?
Increased T2 signal
contrast enhancement best seen with fat compression in coronal plane
Enhancement seen in 90% cases of MRI performed within 3 weeks of symptoms onset
67
MRI protocol progression
Pre-contrast Axial T1,
Axial T2 after gadolinium injection,
Coronal T2 with fat suppression
Post contrast fat suppression/ STIR/ ONSC
68
Precontrast axial T1 is best for
Best for assesisin marrow, bony orbit walls, EOM. intracranial masses
69
In FFA the macula is darker because
because of xanthophyllic
70
which uptake is quicker FFA or ICG?
FFA - 10-12 seconds to reach retina vessels
71
what are the two methods of viewing the fundus ?
1. Fundus camera
2. Scanning laser ophthalmoscopy
72
what color light does the fundus camera use ? and how many flashes of light ? it reduces light interference from what structure ?
Green light. Single flash of light. Reduces interference from the lens
73
what excitation filter des the scanning laser ophthalmoscopy use?
488nm excitation filter
74
OCTA is based on what principle?
Diffractive particle movement detection
75
Mechanism behind OCTA?
Multiple B scans are taken at the same point in the retina - B scan generate movement related to contrast, particularly contrast related to RBCs
76
Relationship between decibel and apostilbs ?
Decibel is directly proportional to the retinal sensitivity.
Decibel is inversely proportional to the aspotibls.
Therfore the apostilbs is inversely porportional to the retina sensitivity.
O decibels is 10,000 apostilbs - brightest, lowest retinal sensitivity
40 decibels is related to 1 apostib - dimmest, high retinal sensitivity
77
Roman numerals on the Goldman Print out
Roman numerals( I-V) indicate the size of the target. With every drop in number the diameter halves. V is the largest.
78
Goldmann print out number
Number (1-4) indicates the brightness of the stimulus. The larger the number, the brighter the target. Each consecutive drop represents a 5db change
79
Goldmann print out letters mean
8) Letter ( a-e)
Allows finer calibration of brightness.
Each consecutive drop in letter represents a 1db change
80
Fixation loss with Goldmann VF
-patient identifies stimulus in physiologic blind spot
81
why is a 10-2 goldmann preferable?
Better resolution - more points in the centre
82
what tis the staircaising/bracketing strategy
Bracketing: Seen/unseen
Staircase: The stimulus intensity/ luminance is varied in steps.
First stimulus is presented at the testing point. If it is percevied, it is dimmed in 4 dbs till the stimulus is no longer seen.
The intensity is then increase in steps of 2 decibels and the first point where it is percevied becomes the threshold for that testing point.
83
Limitation of SITA?
Do not measure Sort Term Fluctuation and corrected pattern standard deviatoin.
Not available for macular and nasal step programmes
84
Pupil diameter and the recording of visual field?
3-4mm in size. Less than 2mm casues diffuse field depression or edge scotomas
85
what is the foveal threshold and what must it correspond with ?
Normal >30bD in 6/12 and above.
if VA is good but threshold low --> early damage or improper fixation
VA is poor but fveal threshold good --> refractive correction
86
what percentage of the stimuli are presented to the bindspot?
5%
87
what does the RAW DATA correspond to ?
It is the exact retainal sensitivity in dB units of the selected points
88
what is the total deviation numerical plot
The deviation between measured retinal sensitivity and normal retinal sensitivity of the same age.
Zero = expected threshold for that age
+ve = more sensitive
-ve = depressed threshold
89
The total deviation probabilities plot ?
Each deviation value of the total deviation numerical plot is given a symbol based on it P value and represented symbolically
90
Pattern deviation numberical plot?
Modified TDNP to bring out deep scotomas
91
Pattern deviation probability pot
IS he symbolic representaion of values of each numerical threshold value of the PDNP is called the PDPP.
IT brings out the localised field defects and localised scotomas (PATTERN) masked by generalised depression
92
What is the Mean deviation?
Mean deviation is the average of all the numbers in the total deviation numerical plot. Expressed in dB units along with a P value.
+ve: better overall sensitivity than the normal and a-ve MD indicates a worse tan average retinal sensivity
93
what is the PSD ? Pattern Standard deviation
is the standard deviation across the mean deviation index.
Indicates the degree to which the numbers in the total deviation numerical plot are not similar to each other
Zero PSD mean no difference among the point and hence a smoot contour
94
Two types of definitive diagnosis for tuberculosis uveitis ?
Acid fast smear of mycobacterial culture
PCR based assays of ocular fluids
95
Challenges with definitive diagnosis for tuberculosis uveitis ?
low sensitivity
96
Four types of definitive diagnosis for tuberculosis uveitis ?
1. +VE Tb SKIN TEST
2.Positive INFgamma release assay
3.Lesions of imaging of the chest
4.Resolution/non recurrence of uveitis following TB
97
Limitations of TST?
1. Lack of standardisation for test, administeration and reading
2. High flase postivie rates in patients immunised with BCG/ exposed to TB in the past
3. False negative in severe illness/ immunodeficency
98
what is the mechanism of IGRA ?
Measure IFgamma response from sensitized T calls produce against Tuberculosis; Measure of ESAT 6, CFP10, TB7.7. The proteins above are absent in BCG
99
Pitfall of TST and IGRA ?
Cant differentiate between active and latent TB
100
2 Types of IGRA
Quantiferon TB Gold (QFT)
Quantify amount of IFN-gamma release in response to ESAT 6, CFP 10, TB7.7 antigen
ELISpot assay ( T spot)
Counts TB specific interferon-gamma secreting T cells
Limitation: time and temperature sensitive
More sensitive in patients with reduced T-cell counts, eg: immunocompromised
101
What is Quantifereon TB gold (QFT)
Quantiferon TB Gold (QFT)
Quantify amount of IFN-gamma release in response to ESAT 6, CFP 10, TB7.7 antigen
ELISpot assay ( T spot)
102
Which is FALSE regarding biometry with the Zeiss IOLMaster?
it utilises two fully coherent, coaxial laser beams
it is a non-contact biometry method
it can measure anterior chamber depth
it is less operator dependent than A-scan biometry
The Zeiss IOLMaster is a non-contact method that uses two coaxial laser beams that are partially coherent. The beams produce an interference pattern that is used to interpret measurements, thus the technology is referred to as partial coherence interferometry. It is highly reproducible and less operator-dependent than ultrasound biometry. The machine can measure keratometry, axial length, anterior chamber depth and corneal white-to-white diameter
103
Clinical application of of LLVA:
Clinical application of of LLVA:
Marker of foveal and parafoveal cone function in low light
Risk factor for disease progression in geographic atrophy
Currently used in research to invetigate ther ophthalmic disease
104
Photopic
Mesopic
Scotopic
Photopic: measures the cone function
Mesopic: measures the rod and cone function
Scotopic: measures the rod function
105
What are the lighting levels for mesopic coniditons
Mesopic conditions involves lower lighting levels: 0.01 to 10cd/m2
Equivalent to moonlight/ standard indoor lighting
106
What is LLVA?
LLVA is essentially VA tested under mesopic conditions
107
Methods to achieve LLVA
Different methods to achieve LLVA measurements:
Vary light levels
Neutral density filters ( ND filtrers) to reduce luminance level entering the ey
108
When reviewing a patient's Humphrey Visual Field (HVF) test, which of the following should always be done first?
Correlate the HVF with fundus imaging
109
The IS/OS junction represents the junction of the inner and outer segments of the photoreceptors. Why is it more pronounced near the fovea compared to the more peripheral macula?
The cones have a longer outer segment in the fovea
110
The non organic visual loss VF on Goldmann versus Humphrey?
Spiraling, Crossing, Stacking of Isoptres on Goldmann Visual Field
Clover leaf visual field defect on HVF 24-2
111
Each small square on a hess chart subtends to a degree of ?
5 degrees at 50cm working distance
112
What area of the central retina does the mfERG cover in degrees?
30
113
which is most associated with RD
Choroidal mel, mets or capillary haemangioma?
Choroidal mets, followed by melanoma, followd by capillary haemangioma
114
Reversible hypofluorescent areas in all phases of ICGA?
First pattern of hypofluorescent areas noticeable in all phases of angiography
Attributed to choriocapillaris nonperfusion
This hypofluorescence can be either permanent, as seen in atrophic lesions, or reversible, as observed in conditions temporarily affecting the choriocapillaris, like multiple evanescent white dot syndrome (MEWDS), acute posterior multifocal placoid pigment epitheliopathy (APMPPE)
Or the hypofluorescent satellite dark dots around a toxoplasmic retinochoroiditis focus.
115
How many milliseconds is a stimulus typically shone for during a HVF?
200ms
116
The sensitivity of ischara chats ?
95.5%
117
DA 0.01 reduced with DA10/30 reduced?
Photoreceptor problem
118
DA 0.01 reduced with DA10/30 normal?
Inner retina problem
119
Astigmatism common in
younger patient
older patient
Younger; with the rule
Older: against the ule
120
What is the dioptre of loupe
+32DS
121
Suprathreshold intensity?
Intesnity that you can see 95% of the time
121
What do you call the principle meridian of weaker power of the toric surface?
Base curve
122
The loss of retinal photopigment has what effect on autofluorescence?
increase or decrease
The loss of retinal photopigment has what effect on autofluorescence?
Increase
123
What is a significant axial length difference between both eyes, where you should consider repeating biometry?
>0.3
124
Antibodies in MG;
ACH
Musk
Striational
Antibody testing
Acetylcholine receptor (AChR) antibodies
Present in 90% of systemic MG
50-70% of ocular MG
MuSK protein antibodies positive in 50% of those with -ve AChR antibodies
If positive, less likely to have ocular features, thymoma
Striational antibodies
Found more often in thymoma
Marker for more severe MG
125
Methods used to detect suppression
The presence of suppression can be detected using the following:
Bagolini Glasses
Worth's light test
Synotophore
4 dioptre prism test
TNO stereopsis with the suppression test plate
126
What is the first principal focus?
Point of origin of rays which becomes parallel to principle axis after refraction by lens
127
Upward deflection ?
Gaze error
128
Does corneal oedmea cause under or overestimation of IOP?
Corneal Oedema
129
If a lens of +20 dioptre power lens is moved 1cm away from the eye, what is the dioptric power of the lens in the new position so that the image still forms on the retina ?
16.6D
130
best used to evaluate paracentral defects?
Humphrey 10-2
131
What does the Amsler grid evaluate?
20 degrees of VF centered on fixation
132
SO palsy features:
Superior oblique palsy features:
Hyperdeviation when fixing with unaffected eye
Deviation greater for near
Head posture
Chin depression
Hess chart
Greatest negative displacement on depression
Extorsion is common
Bielschowsky head tilt test
Usually positive
133
SR palsy features
Superior rectus palsy features:
Hypodeviation when fixing with unaffected eye
Deviation greater for distance
Head posture
Chin elevation
Hess chart
Greatest negative displacement on elevation
Extrosion rare
Bielschowsky head tilt test
Usually negative
134
Hess chart facts - re torsion, scotoma, type of strabismus
Hess charts can reveal presence of torsion. However, torsion is better detected with methods such as Maddox Rods, Lancester Screens. Hess Chart requires normal retinal correspondence. It can occasionally reveal scotomata/ hemianopic field defect if patient demonstrates difficulty in locating target
135
Which plot measures the numerical deviation of the patient from the normal age-matched values after accounting for generalized depression?
Numerical Raw Data
Greyscale Raw Data
Total Deviation Numerical Plot
Pattern Deviation Numerical Plot
Pattern Deviation Numerical Plot
136
VR silicone oil changes
Silicone oil
Higher refractive index than the crystalline lens
The posterior surface of the lens becomes a diverging interface
This results in a hypermetropic shift of +5.00 to +7.00
If left aphakic, the curve anterior surface and higher refraction index of silicone oil compared with the crystalline lens is more strongly converging
This produces a myopic shift
The hypermetropia which normally results from aphakia will therefore be of order only +4.00D to +6.00D
137
Encircling buckle effect on the eye
Encircling scleral buckle: Causes an increase in the axial length and induces myopia
Can also cause astigmatism if compress eye asymmetrically.
138
Gas in the eye can cause what ?
Gas in the eye
Causes myopic shift
May allow indirect ophthalmoscopy without condensing lens
139
Normal VF extends
For each eye a normal visual field extends approximately 50⁰ superiorly, 60⁰ nasally, 70⁰ inferiorly and 90⁰ temporally from central fixation
140
Type of strabismus in Duanes ?
Mechanical Paralytic stabismus
The Brown classification is probably easier to remember:
Type A: Limited abduction and adduction ; adduction less limited than abduction
Type B: Limited abduction
Type C: Limited abduction and adduction-adduction more limited than abduction
141
Cause of Duane SYndrome
nnervational:
Aberrant co-innervation of lateral rectus and medial rectus by third nerve; this has possible association with IV nerve hypoplasia
Mechanical:
Abnormal attachment of medial rectus to orbital apex, medial rectus is tight and unable to relax or contract
142
When is fat suppression performed ?
After contrast injection
143
The sign of convention of lenses are designated by which of the following?
First focal length
First principal focus
Reciprocal of first focal length
Second focal length
Second principle focus
Second focal legnth
144
Patients are unlikely able to tolerate post operative anisometropia more than ?
2.5D
145
A delay in the 30Hz flicker is an indication of what pathology?
General Cone problem
146
What is the size of the normal blind spot?
147
blind spot
20 diameter and 10-20 from point of fixation
148
Which of the following is most corelated with visual acuity on an OCT scan?
central subfield
149
30hx flicker delay and no delay?
Delay - focal cone problem - if they are delayed the focus is on them..
No delay - general cone problem
150
Abnormal retinal correspondence usually occurs at how many prism dioptres of manifest deviation?
<20
151
An object is placed 20cm to the left of a -3.00 D lens. Where would you expect to roughly find the image?
An object is placed 20cm to the left of a -3.00 D lens. Where would you expect to roughly find the image?
50 cm to the right of the lens
Vergence of light entering the lens
U=100/20cm= -5D .
Negative sign indicates divergent light rays
Using the lens equation:
U +D= V
-5 +(-3)=-8
100/V=-8
V=-12.5cm
This means that the resulting image is 12.5 cm to the left of the lens
152
What is pleomorphism and what does high pleomorphism indicate?
Pleomorphism indicates variability in cell shape.
High pleo >50 - may not be suitable for ocular surgery
153
The autofluorescence (AF) fundus camera is good at reducing interference from which structure?
Lens
154
Astigmatic lens is
Astigmatic lens
Distortion of cross when not moving the lens unless its axes coincide with cross lines
Rotation of lens causes scissors movement
155
The prescription of a lens can be identified when viewing image formed through the lens, with two lines crossed at how many degrees?
90 deg
156
Gain is measured in ?
Gain
Measure in dB ( decibles)
Affects the amplitude of displayed echoes
157
What is the prismatic power of decentring a lens of 10 dioptres by 20mm?
20
10 dioptres X 2cm
158
False negative perimetry? patten and data
Greyscale
Cloverleaf pattern
159
uncorrected VA for both distance and near vision
-1.00 is usually considered a reasonable trade off between uncorrected visual acuity for both distance and near vision.
160
indirect accommodation required and binocular effect?
Binocular indirect has +2.0D in prismatic eye piece-viewer does not need to accommodate
161
Aphakic lens problems
Altered depth perception
Pin cushion distortion
Ring scotoma generated by prismatic effect ( jack in the box phenomenon)
Cosmetic problems-patient's eye appears magnified
Sensitivity to any minor misalignment of vertex distanc
