Test 191  (Ocular physiology)

1. With regard to nystagmus:

a. caloric nystagmus occurs when iced water is poured 
    into the ear

b. vestibular nystagmus occurs as a consequence 
    of head rotation

c. optokinetic nystagmus occurs as a consequence 
    of the relative motion of the visual field

d. the direction of a jerk nystagmus is characterized by 
    the direction of the slow phase 

e. pendular nystagmus has equal velocity in each direction

 
Answera.T   b.T  c.T   d.F  e.T
The direction of a jerk nystagmus is characterized by the direction of the fast phase.

2. Visual-evoked response:

a. is used primarily to detect visual loss due to retinal 
    disease

b. produces biphasic wavefronts

c. may give falsely delayed latency if the patient is 
    not concentrating

d. produces responses in normal subjects with a latency 
    of 100msec

e. is obtained from the lateral canthi

 

Answera.F  b.F  c.T  d.T  e.F
Visual-evoked response is used primarily to detect visual loss due to optic nerve. It produces multiphasic 
wavefronts which have a latency of 100msec or so called P100. The result may not be accurate if the patient's is not concentrating. It is obtained from the scalp.

 
 

3. An increase in intra-ocular pressure occurs with:
 

a. normal blinking

b. coughing

c. hypercarbia

d. pupillary dilatation

e. head down position

 
Answer
a.T   b.T  c.T  d.T  e.T

The main physiological determinant of the intraocular 

pressure is the balance between production and removal of aqueous humour. Aqueous humour is secreted via a 
Na/K dependant ATPase pump from the ciliary processes
of the posterior chamber (2/3), and  passive ultrafiltration through blood vessels on the anterior surface of the iris (1/3) at an overall rate of 2ul/min. It flows between the lens and iris, through the pupil, into the anterior 
chamber. It is eliminated via passage through the 
trabeculae of the episcleral venous system (spaces of 
Fontana) and eventually through the canal of Schlemm, which lies at the iridocorneal angle.
Changes in choroidal blood volume (CBV) also affect 
IOP significantly. The choroid is a vascular meshwork 
of arterial anastomoses located in the posterior chamber. Choroidal blood flow is usually autoregulated over the normal range of perfusion pressures to keep IOP 
stable. Sudden increases in systemic blood pressure 
may cause the perfusion pressure to rise above its 
autoregulatory threshold as with cerebral blood flow, 
resulting in transient swelling of CBV and elevation of 
IOP. Similarly, hypotension may cause decreases in 
CBV and IOP. Choroidal blood volume will also increase in response to acidosis, hypercarbia, and hypoxaemia. 
The force of the eyelid during a normal blink may cause an increase of 10 mmHg; a forceful squeeze can increase IOP to over 50 mmHg. 

 

. 4. With regard to dark adaptation:

a. is a quicker process than light adaptation

b. the sensitivity of the cones increases more rapidly 
    than the rods

c. it is monophasic in rod monochromatism

d. the first limb of the curve represents rod recovery

e. the peak sensitivity of the retina to light with longer 
    wavelengths increases with dark adaptation

Answer
a.F  b.T  c.T  d.F  e.T
Dark adaptation is slower than light adaptation. It is 
biphasic in normal people, the first limb of the curve represents cone recovery and the second limb represents rod recovery. The response of the retina to light with longer wave-length increases with dark adaptation and this change is called Purkinje shift.

5. True statements about EOG (electro-oculogram) include: 

a. the EOG light-peak to dark-trough ratio is reduced 
    in central retinal vein occlusion

b. the light peak of EOG is abnormal in Best's disease

c. the EOG light rise is produced by depolarization of the 
    basal membrane of the retinal pigment epithelium

d. intact ganglion cells are necessary to generate the 
    light-rise portion of the EOG

e. the amount of melanin within the retinal pigment 
    epithelial cells determines the amplitude of the EOG 
    light rise

 
Answer
a.T   b.T  c.T  d.F  e.F
Intact photoreceptors and retinal pigment epithelium are important for the generation of light-rise portion of the EOG. The amount of melanin within the retinal pigment epithelial cells does not determine the amplitude of the EOG light rise.

 

More MCQs