DOCSLIB.ORG

The Night Vision Threshold Test (NVTT): a Simple Instrument for Testing Dark Adaptation in Young Children

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Night Vision Threshold Test (NVTT): a Simple Instrument for Testing Dark Adaptation in Young Children The Night Vision Threshold Test (NVTT): A Simple Instrument for Testing Dark Adaptation in Young Children by Burris Duncana, Louise Canfieldb, Brent Barbera, John Greivenkampc, Francis O. Oriokotd, and Florence Naluyindad aDepartment of Pediatrics, College of Medicine and Children’s Research Center, College of Medicine, University of Arizona, USA bDepartment of Biochemistry, College of Medicine, and cThe College of Optical Sciences, University of Arizona, 1501 N Campbell Avenue, Tucson, AZ 85724, USA dDepartment of Pediatrics and Mwanamugimu Nutrition Unit, Makerere Medical School, Kampala, Uganda Summary It is estimated that 41 per cent of the population aged under 5 in the developing world has an inadequate vitamin A dietary intake resulting in increased morbidity and mortality. Half a million children go blind each year as a result of vitamin A deficiency. Thirteen and a half million have night blindness, the first sign of vitamin A deficiency. Unfortunately, there is no simple, sensitive and inexpensive means to identify the child who has marginal levels of vitamin A and thus institute means to prevent their development of severe deficiency. A low cost, simple, easy-to-use instrument designed to detect a young child’s ability to adapt to darkness was tested in children admitted to the Mwanamugimu Nutrition Unit at Makerere Medical School in Kampala, Uganda. Despite the severe degree of malnutrition found in these children, Night Vision Threshold Test results and serum retinol levels were related (r ¼ 0:41, p < 0:05). Further efficacy trials for this instrument are planned at community sites in Nepal. Introduction relatively inexpensive, their distribution is expensive and UNICEF estimates that 231 million children under the a means to target those communities where there is a age of 5 years or 41 per cent of the entire population documented significant prevalence of vitamin A defi- under 5 years old in the developing world have an ciency would be economically sound. However, all inadequate dietary intake of vitamin A. Thirteen and a currently available methods for the assessment of a half million children have night blindness, the first population for vitamin A deficiency have serious limitations: some require serum and expensive analysis clinical sign of vitamin A deficiency. Three million 3 children have xerophthalmia and 500 000 have severe of retinol levels; some lack sensitivity and specificity, 1 and others require considerable cooperation and are eye damage resulting in blindness. Vitamin A is the 4–10 most important cause of preventable childhood blindness inappropriate for young children. and represents one-third of all cases of childhood A history of night blindness remains the most blindness world-wide. More than one-half of these frequently used and the most reliable indicator for the earliest and mildest expression of clinical xerophthal- children die within a few months of becoming blind. In 11 addition, children with inadequate vitamin A status have mia. The history of night blindness, however, is based a 23 per cent greater risk of dying from common on the person’s perception; it does not determine the childhood illnesses, such as gastroenteritis, respiratory degree or severity of dark adaptation and its accuracy is diseases and measles.12 The efficacy of vitamin A highly questionable in the infant or very young child. supplementation for the young child with vitamin A A reliable, inexpensive, non-invasive, rapid test for deficiency in decreasing morbidity and mortality from assessing the prevalence of vitamin A deficiency in measles and measles-associated diarrhoeal, and respira- infants and young children is needed. Such a test would tory illness has been documented2 prompting vitamin A provide the means for the selection of at-risk individuals distribution programmes as a current strategy of the or communities to receive vitamin A supplementation WHO and UNICEF. Although vitamin A capsules are and nutrition educational programmes. It could be used for monitoring the effectiveness of vitamin A interven- tion programmes designed to restore vitamin A status in individuals and in their communities. In addition, most Correspondence: B. Duncan, Department of Pediatrics, Room 3335, University Medical Centre, 1501 N Campbell Avenue, previous techniques have targetted severely vitamin A Tucson, AZ 85724, USA. E-mail <[email protected]. deficient children, i.e., those who have developed or are edu>. developing clinical symptoms of vitamin A deficiency. 30 q Oxford University Press 2000 Journal of Tropical Pediatrics Vol. 46 February 2000 B. DUNCAN ET AL. This group represents about 2 per cent of the population 30 cm. A green LED is used to match the spectral of most developing countries. Unfortunately, the effects response of the eye. Since the light output of an LED is a of severe vitamin A deficiency are not reversible and function of the electrical current through the LED, the children at this stage often die despite supplementation. illuminance of the projected spot can be easily varied by There are, however, many more children with marginal connecting the LED to a battery through a series of vitamin A stores (0.35–0.7 mmol/l). In this group, resistors. A 12-position rotary switch was used allowing vitamin A deficiency is completely reversible. More the projected illuminance to vary from about 400 mlux importantly, vitamin A supplementation significantly down to about 0.1 mlux. Each switch position reduced the protects this group of children against respiratory and illuminance by a factor of about a half. A digital ammeter gastrointestinal infections which together are responsible with numeric display on the outside of the case is provided for a very large number of deaths of children world- to measure the electrical current through the LED and to wide.1 Thus, this study was designed to evaluate a monitor the light output while the system is in use. technique for identifying children with marginal vitamin A deficiency so that interventions can be Testing compliance implemented prior to the development of severe vitamin A game was designed and adapted after the method used A deficiency. Specifically, this study was designed to by audiologists for testing the hearing of young children. test the efficacy of an inexpensive, non-invasive, easy- In the case of hearing, a sound stimulus is activated in to-use instrument, the Night Vision Threshold Tester one corner of a sound-proof room and the hearing-abled (NVTT), to evaluate infants and young children for their child will turn to the place where the sound originates. In ability to adapt to darkness or for the presence of night the game we designed to test for dark adaptation; after blindness; the first clinical sign of vitamin A deficiency. sitting in a dark room for 10 min, a light is directed to one corner of the room and scanned from one side to the other. If the child can see the light, he/she should follow Materials and Methods it and light perception can be determined by observing Instrument design the child’s head turn in response to the movement of the The design of the instrument is shown in Fig. 1. Its use is light. based on the premise that the function of the rods in the Twenty-four children between the ages of 7 and 45 retina, the eye’s ability to adapt to darkness, is a reflection months attending the Pediatric Clinic at the University of of the body’s retinol stores. The instrument is contained in Arizona were evaluated to determine if young children a plastic box approximately 12 × 18 × 5 cm. A light- would play this ‘game’. Four (17 per cent) could not be emitting diode (LED) is imaged and magnified by a tested because of inattention or being afraid of the dark. simple lens to project a round uniform circle of light on The remaining children (83 per cent), 13 (65 per cent) of the wall of a darkened room. The system was designed for whom were less than 24 months of age, co-operated with operation at 3 m that produces an illuminated spot of about the test procedure. Fig. 1. The Night Vision Threshold Tester (NVTT). Journal of Tropical Pediatrics Vol. 46 February 2000 31 B. DUNCAN ET AL. Fig. 2. Light perception of subjects with retinitis pigmentosa. Results of SSTDA as compared with light intensity from the NVTT instrument. Standardization of the NVTT Dietary information Nine subjects between the ages of 8 and 61 months with Both a 24-h and a 7-day food frequency dietary history, retinitis pigmentosa (RP) were tested for night blindness including an estimate of serving size, were obtained with the NVTT and the results compared with those from the child’s mother by the field research team. A obtained using the more sophisticated SST Dark qualitative analysis of the dietary data was performed to Adaptometer (SSTDA). RP is a condition that affects describe the dietary pattern for the study population. the rods of the retina and in its early phases causes night blindness. Each eye of nine subjects was tested Determination of the night vision threshold individually. Twelve of the 18 eyes tested were not The child sits on the mother’s lap in a completely dark able to detect light from the highest intensity emitted room facing a wide strip of white paper attached to the from the SSTDA but could detect the light beam when wall 3 m in front of the subject. The observer stands shown through the brightest light of our instrument, as it behind the mother and child and, while waiting for dark was far brighter than the brightest light emitted from adaptation to occur, discusses the importance of vitamin the SSTDA. The varying levels of light intensity detected A and how to enrich the child’s diet with it.
Load more

Recommended publications
  • Shedding New Light on the Generation of the Visual Chromophore PERSPECTIVE Krzysztof Palczewskia,B,C,1 and Philip D
    PERSPECTIVE Shedding new light on the generation of the visual chromophore PERSPECTIVE Krzysztof Palczewskia,b,c,1 and Philip D. Kiserb,d Edited by Jeremy Nathans, Johns Hopkins University School of Medicine, Baltimore, MD, and approved July 9, 2020 (received for review May 16, 2020) The visual phototransduction cascade begins with a cis–trans photoisomerization of a retinylidene chro- mophore associated with the visual pigments of rod and cone photoreceptors. Visual opsins release their all-trans-retinal chromophore following photoactivation, which necessitates the existence of pathways that produce 11-cis-retinal for continued formation of visual pigments and sustained vision. Proteins in the retinal pigment epithelium (RPE), a cell layer adjacent to the photoreceptor outer segments, form the well- established “dark” regeneration pathway known as the classical visual cycle. This pathway is sufficient to maintain continuous rod function and support cone photoreceptors as well although its throughput has to be augmented by additional mechanism(s) to maintain pigment levels in the face of high rates of photon capture. Recent studies indicate that the classical visual cycle works together with light-dependent pro- cesses in both the RPE and neural retina to ensure adequate 11-cis-retinal production under natural illu- minances that can span ten orders of magnitude. Further elucidation of the interplay between these complementary systems is fundamental to understanding how cone-mediated vision is sustained in vivo. Here, we describe recent
    [Show full text]
  • JMSCR Vol||08||Issue||02||Page 208-210||February 2020
    JMSCR Vol||08||Issue||02||Page 208-210||February 2020 http://jmscr.igmpublication.org/home/ ISSN (e)-2347-176x ISSN (p) 2455-0450 DOI: https://dx.doi.org/10.18535/jmscr/v8i2.40 Keratomalacia - A Case Report Authors Deepthi Pullepu1*, Deepthi Janga2, V Murali Krishna3 *Corresponding Author Deepthi Pullepu Department of Ophthalmology, Rangaraya Medical College, Kakinada, Andhra Pradesh, India Abstract Xerophthalmia refers to an ocular condition of destructive dryness of conjunctiva and cornea caused due to severe vitamin A deficiency. Usually affects infants, children and women of reproductive age group. It is estimated to affect about one-third of children under the age of five around the world. Keratomalacia means dry and cloudy cornea which melts and perforates, caused due to severe vitamin A deficiency. Here we describe a case of 21 year old female with neurofibromatosis type 1 presenting with keratomalacia. Keywords: Vitamin A deficiency, Xerophthalmia, Keratomalacia. Introduction blindness is a significant contributor Vitamin A is essential for the functioning of the In India, it is estimated that there are immune system, healthy growth and development approximately 6.8 million people who have vision of body and is usually acquired through diet. less than 6/60 in at least one eye due to corneal Globally, 190 million children under five years of diseases; of these, about a million have bilateral age are affected by vitamin A deficiency involvement. They suffer from an increased risk of visual Corneal blindness resulting due to this disease can impairment, illness and death from childhood be completely prevented by institution of effective infections such as measles and those causing preventive or prophylactic measures at the diarrhoea community level.
    [Show full text]
  • Abstract: a 19 Year Old Male Was Diagnosed with Vitamin a Deficiency
    Robert Adam Young Abstract: A 19 year old male was diagnosed with vitamin A deficiency (VAD). Clinical examination shows conjunctival changes with central and marginal corneal ulcers. Patient history and lab testing were used to confirm the diagnosis. I. Case History 19 year old Hispanic Male Presents with chief complaint of progressive blur at distance and near in both eyes, foreign body sensation, ocular pain, photophobia, and epiphora; signs/symptoms are worse in the morning upon wakening. Started 6 months to 1 year ago, and has progressively gotten worse. Patient reports that the right eye is worse than the left eye. Ocular history of Ocular Rosacea Medical history of Hypoaldosteronism, Pernicious Anemia, and Type 2 Polyglandular Autoimmune Syndrome Last eye examination was two weeks ago at a medical center Presenting topical/systemic medications - Tobramycin TID OU, Prednisolone Acetate QD OU (has used for two weeks); Fludrocortisone (used long-term per PCP) Other pertinent info includes reports that patient cannot gain weight, although he has a regular appetite. Patient presents looking very slim, malnourished, and undersized for his age. II. Pertinent findings Entering unaided acuities are 20/400 OD with no improvement with pinhole; 20/50 OS that improves to 20/25 with pinhole. Pupil testing shows (-)APD; Pupils are 6mm in dim light, and constrict to 4mm in bright light. Ocular motilities are full and smooth with no reports of diplopia or pain. Tonometry was performed with tonopen and revealed intraocular pressures of 12 mmHg OD and 11 mmHG OS. Slit lamp examination shows 2+ conjunctival injection with trace-mild bitot spots both nasal and temporal, OU.
    [Show full text]
  • Current Health Issues in the Caribbean BLINDNESS in THE
    Current Health Issues in the Caribbean BLINDNESS IN THE CARIBBEAN Alfred L. Anduze, M.D. St. Croix Vision Center St. Croix Hospital St. Croix U.S. Virgin Islands Caribbean Studies Association Merida, Mexico May 26, 1994 Abstract: Blindness in the Caribbean Background: The prevailing of blindness in the Caribbean region are reviewed in the context of world blindness statistics to identify differences and similarities that might exist. Method: A review of the status of blindness in the U.S. Virgin Islands, Barbados, Jamaica, Puerto Rico, Trinidad, and Mexico; individually with regard to causal etiology, epidemiology, treatment and possible future research. Results: Blindness in the Caribbean is the result of genetics, tropical environment and cultural habits of the inhabitants and consist of Age-related macular disease, Infectious diseases, Diabetes mellitus, Glaucoma, Congenital defects, Xerophthalmia, Trachoma, Trauma and Cataracts. Conclusion: There are almost 50 million people who are legally blind worldwide (i.e. with a vision of 20/200 or less) 2-3 million in the Caribbean region. The social and economic consequences are serious additional deterrents in developing countries. Outline: Causes of Blindness in the Caribbean I. Age-related macular disease a. Vascular insufficiency b. Senile macular degeneration II. Cataracts III. Glaucoma IV. Diabetes mellitus V. Infectious diseases a. Trachoma b. Onchocerciasis c. Leprosy d. Toxoplasmosis e. Toxocariasis f. AIDS VI. Trauma a. industrial/work-related b. sports c. home accidents VII. Nutritional a. Xerophthalmia/keratomalacia b. Iron-deficiency anemia c. Tobacco/Alcohol Retinopathy VIII. Congenital defects a. genetic syndromes b. strabismus Legal blindness is acceptably defined as vision 20/200 (6/60) or less.
    [Show full text]
  • 2002 Samel 10 Most Common Systemic Health Conditions with A
    Avanti Samel / March 15, 2002 10 Most Common Systemic Health Conditions with a Listing of Frequently Prescribed Medications and their Ocular Side Effects HYPERTENSION ACE Inhibitors: Captopril (Capoten}- blurred vision Enalapril (Vasotec) - Blurred vision, conjunctivitis, dry eyes, tearing Quinipril (Accupril)- amblyopia Benazepril (Lotensin)- N/A Lisinopril (Zestril) - Visual loss, diplopia, blurred vision, photophobia Beta-Blockers: Propranolol (Inderal)- visual disturbances, dry eyes Atenolol (Tenormin)- blurred vision, dry eyes, visual disturbances Metoprolol (Lopressor) - blurred vision, dry eyes calcium Channel Blockers: Diltiazem (Cardizem)- Amblyopia, eye irritation Amlodipine (Norvasc)- abnormal vision, conjunctivitis, diplopia, eye pain Verapamil (Calan)- Blurred vision Nifedipine (Procardia) - blurred vision, Transient blindness at the peak of plasma level · Diuretics: Thiazides: Chlorothiazide (Diuril) - Transient blurred vision, xanthopsia Loop: Furosemide (Lasix) - Blurred vision, Xanthopsia Potassium Sparing: Amiloride (Midamor) - Visual disturbances, Increased lOP Triamterene (Dyrenium)- N/A HYPERLIPIDEMIA Statins: Lovastatin (Mevacor) - Blurred vision, Eye irritation Simvastatin (Zocor)- Cataracts Atorvastatin (Lipitor)- Amblyopia, dry eyes, refraction disorder, eye hemorrhage, glaucoma Resins: Cholestyramine (Questran)- Uveitis Fibrates: Gemfibrozil (Lopid)- Blurred vision Niacin: Niacin (Niacor) - Toxic amblyopia, cystoid macular edema ASTHMA ( ) Beta 2 Agonists: Albuterol (Proventil) - N/A ( Salmeterol (Serevent)-
    [Show full text]
  • © Alan Macfarlane and Gerry Martin, 2002 1 MYOPIA: ITS
    © Alan Macfarlane and Gerry Martin, 2002 MYOPIA: ITS COMPARATIVE INCIDENCE AND PROBABLE CAUSES What Rasmussen encountered in his many years of working with Chinese eye problem astonished him. He summarized his impressions thus. 'Three facts stand out clearly in both ancient and modern records of Chinese eyesight: one that they are a highly myopic nation; two that keratomalacia is the major cause of ocular disease; and three that presbyopia is earlier than in the West by about five years. They stand out against an historical background of neglect and abuse, malnutrition, and centuries of State indifference.' (p.48) Moving on to the statistics of myopia, he summarized the research of a quarter of a century by saying that 'only 20 per cent. of all lenses supplied by the ancients, and similar amount by the moderns (excluding high astigmatic) were for old-sight lenses. It is the other 80 per cent. that matters; the 65 per cent. for myopia and the 15 per cent. for glare and therapeutics.' (p.20) Elsewhere he gives slightly different statistics, which applied to those aged under forty years, showed that approximately 75 per cent. of the lenses were supplied for myopia. In a detailed table he presents the figures; in the 'Ancient Chinese Sight Records', 65% were for myopia; in the 'Modern Chinese Sight Records' myopia (simple) or with astigmatism combined accounted for 70% of the lenses.(p.48) The second figure of 70% was based on 120,000 case histories collected by Rasmussen and his colleagues over a period of 25 years. (p.49) The significance of the degree of myopia only comes out if we compare the Chinese figures with those for other nationalities.
    [Show full text]
  • The Main Causes of Blindness and Low Vision in School for Blind
    26ORIGINAL ARTICLE DOI 10.5935/0034-7280.20160006 The main causes of blindness and low vision in school for blind As principais causas de cegueira e baixa visão em escola para deficientes visuais Abelardo Couto Junior1, Lucas Azeredo Gonçalves de Oliveira2 ABSTRACT Objective: Identify and analyze the main causes of blindness and low vision in school for blind. Methods: One hundred sixty-five medical records of visually impaired students were reviewed in an institution specialized in teaching the blind, treated between august 2013 and may 2014. The variables analyzed were age, sex, visual acuity, primary and secondary diagnoses, treatment, optical prescription features and prognosis. Results: 165 students were evaluated, 91 students (55%) are legally blind and only 74 (45%) of the students are classified as low vision. The main causes of blindness were: retinopathy of prematurity (21%), optic nerve atrophy (18%), congenital glaucoma (16%), retinal dystrophy (11%) and cancer (8%). The causes of low vision were: congenital cataract (18%), congenital glaucoma (15%) and retinochoroidal scarring (12%). Conclusion: The main causes of blindness and low vision in the Benjamin Constant Institute are from preventable diseases. Keywords: Blindness; Low vision; Visual acuity; Child; Health school RESUMO Objetivo: Identificar e analisar as principais causas de cegueira e baixa visão em escola para deficientes visuais. Métodos: Foram revisados 165 prontuários de alunos portadores de deficiência visual em instituição especializada no ensino de cegos, atendidos no período de agosto de 2013 a maio de 2014. As variáveis analisadas foram: idade, gênero, acuidade visual, diagnóstico principal e secundário, tratamento, recursos ópticos prescritos e prognóstico. Resultados: Dos 165 alunos avaliados, 91 alunos (55%) são legalmente cegos e apenas 74 (45%) dos alunos são enquadrados como baixa visão.
    [Show full text]
  • Ophthaproblem
    Pratique clinique Clinical Pract ice Ophthaproblem Shaun Segal Sanjay Sharma, MD, MSC, MBA, FRCSC : Ophthalmic Photography : Ophthalmic Photography Photo credit Photo Dieu Hotel University, Laboratory of Queen’s Kingston, Ont. Hospital, 30-year-old woman had been admitted to hospital in infancy with diarrhea, steatorrhea, and failure to thrive. During late childhood, she developed atypi- Acal retinitis pigmentosa involving the retina and a progressive ataxic neuropathy. Investigations showed her serum lacked beta-lipoprotein and her red corpuscles had a spiky shape. Recent ophthalmic examination revealed substantial choroidal atrophy around the disk and a speckled peripheral fundus. Which vitamins should this patient be given as therapy for her disorder? 1. Vitamin A 2. Vitamin E 3. Vitamin C 4. Vitamin B Answer on page 1085 Mr Segal is a medical student at Queen’s University in Kingston, Ont. Dr Sharma is an Associate Professor of Ophthalmology and an Assistant Professor of Epidemiology at Queen’s University VOL 5: AUGUST • AOÛT 2005 d Canadian Family Physician • Le Médecin de famille canadien 1079 Pratique clinique Clinical Pract ice Answer to Ophthaproblem continued from page 1079 1. Vitamin A and 2. vitamin E Th is patient has abetalipoproteinemia (ABL), some- times called Bassen-Kornzweig syndrome,1 a dis- ease characterized by malabsorption of fat. The condition is treated with supplements of the two fat-soluble vitamins A and E. Abetalipoproteinemia manifests itself through signs of lipid malabsorp- tion, dense contracted red blood
    [Show full text]
  • Prevalence and Causes of Low Vision and Blindness Worldwide
    S Afr Optom 2005 64 (2) 44 − 54 Prevalence and causes of low vision and blindness worldwide AO Oduntan* Department of Optometry, University of Limpopo, Private Bag X1106, Sovenga, 0727 South Africa <[email protected]> Abstract Introduction A recent review of the causes and prevalence There are many low vision and blind people of low vision and blindness world wide is lack- worldwide, and there is a considerable amount of ing. Such review is important for highlighting data available on the prevalence of low vision and the causes and prevalence of visual impairment blindness in many parts of the world. The data, in the different parts of the world. Also, it is however, vary significantly from one continent to important in providing information on the types another. In 1995, the World Health Organization and magnitude of eye care programs needed in (WHO) Task Force on data on blindness estimated different parts of the world. In this article, the that there were 37.1 million blind people world- causes and prevalence of low vision and blind- wide, indicating a global prevalence of 0.7 percent1. ness in different parts of the world are reviewed According to that report, the prevalence values and the socio-economic and psychological range from 0.3% in the developed countries to implications are briefly discussed. The review 1.4% in Sub-Saharan African countries. The preva- is based on an extensive review of the litera- lence of visual impairment is expected to be higher ture using computer data bases combined with in the developing countries due to the low level of review of available national, regional and inter- health care services in many of the countries.
    [Show full text]
  • Therapy-Resistant Dry Itchy Eyes Rima Wardeh* , Volker Besgen and Walter Sekundo
    Wardeh et al. Journal of Ophthalmic Inflammation and Infection (2019) 9:13 Journal of Ophthalmic https://doi.org/10.1186/s12348-019-0178-7 Inflammation and Infection BRIEFREPORT Open Access Therapy-resistant dry itchy eyes Rima Wardeh* , Volker Besgen and Walter Sekundo Abstract An 8 years old male presented to our clinic with dry eye symptomes. Different therapiy attemps were made in the last few months and did not lead to any improvement. Examining this patient revealed multiple signs of vitamin A deficiency, which could confirmed by laboratory examination. The initial substitution of vitamin A led to a fast rehabilitation and a following nutrition consulting kept the patient symptom-free over 6 month follow up. Vitamin A deficiency -although rare in the developed countries- is an importent differential diagnosis of the dry eye especially in children. Vitamin A deficiency not only causes ocular manifistaion, but also general symptoms. Dietary change and initial subtitution is the key element for a fast and sustaining improvement. Medical history follow-up examination showed no improvement in the An 8-year-old male child was referred to our pediatric visual acuity nor in the corneal surface. In addition, the ophthalmology department because of burning sensation conjunctiva had developed triangular-shaped superficial and itching in both eyes during the last 4 months. His spots with keratinization in the bulbar conjunctiva nas- mother reported that the child was always pinching his ally, inferiorly, and temporally near the limbus of both eyes while reading or focusing. Topical therapy with eyes. With these findings, the diagnosis of conjunctival dexamethasone eye drops, antihistamine eye drops and corneal xerosis due to vitamin A deficiency was sus- (ketotifen), antibiotic eye drops (ofloxacine) and im- pected.
    [Show full text]
  • Control of Rhodopsin Activity in Vision
    Control of rhodopsin DENIS A. BAYLOR, MARIE E. BURNS activity in vision Abstract high concentration in the cytoplasm in darkness and that binds to cationic channels in the Although rhodopsin's role in activating the surface membrane, holding them open. phototransduction cascade is well known, the Hydrolysis of cGMP allows the channels to processes that deactivate rhodopsin, and thus close, interrupting an inward current of sodium, the rest of the cascade, are less well calcium and magnesium ions and producing a understood. At least three proteins appear to hyperpolarisation of the membrane. The play a role: rhodopsin kinase, arrestin and hyperpolarisation reduces the rate at which recoverin. Here we review recent physiological neurotransmitter is released from the synaptic studies of the molecular mechanisms of terminal of the rod. rhodopsin deactivation. The approach was to The purpose of this paper is to review recent monitor the light responses of individual work on the important but still poorly mouse rods in which rhodopsin was altered or understood mechanisms that terminate the arrestin was deleted by transgenic techniques. light-evoked catalytic activity of rhodopsin. Removal of rhodopsin's carboxy-terminal These mechanisms, which fix the intensity and residues which contain phosphorylation sites duration of the activation of the transduction implicated in deactivation, prolonged the flash cascade, need to satisfy strong functional response 20-fold and caused it to become constraints. Rhodopsin activity must be highly variable. In rods that did not express arrestin the flash response recovered partially, terminated rapidly so that an absorbed photon but final recovery was slowed over lOO-fold.
    [Show full text]
  • Molecular Basis of Dark Adaptation in Rod Photoreceptors
    1 Molecular basis of dark c.s. LEIBROCK , T. REUTER, T.D. LAMB adaptation in rod photoreceptors Abstract visual threshold (logarithmically) against time, following 'bleaching' exposures of different Following exposure of the eye to an intense strengths. After an almost total bleach light that 'bleaches' a significant fraction of (uppermost trace) the visual threshold recovers the rhodopsin, one's visual threshold is along the classical bi-phasic curve: the initial initially greatly elevated, and takes tens of rapid recovery is due to cones, and the second minutes to recover to normal. The elevation of slower component occurs when the rod visual threshold arises from events occurring threshold drops below the cone threshold. within the rod photoreceptors, and the (Note that in this old work, the term 'photon' underlying molecular basis of these events was used for the unit now defined as the and of the rod's recovery is now becoming troland; x trolands is the illuminance at the clearer. Results obtained by exposing isolated retina when a light of 1 cd/m2 enters a pupil toad rods to hydroxylamine solution indicate with cross-sectional area x mm2.) that, following small bleaches, the primary intermediate causing elevation of visual threshold is metarhodopsin II, in its Questions and observations phosphorylated and arrestin-bound form. This The basic question in dark adaptation, which product activates transduction with an efficacy has not been answered convincingly in the six about 100 times greater than that of opsin. decades since the results of Fig. 1 were obtained, Key words Bleaching, Dark adaptation, is: Why is one not able to see very well during Metarhodopsin, Noise, Photoreceptors, the period following a bleaching exposure? Or, Sensitivity more explicitly: What is the molecular basis for the slow recovery of visual performance during dark adaptation? In considering the answers to these questions, there are three long-standing observations that need to be borne in mind.
    [Show full text]