Preface
Janice A. Gault, MD, FACS
In putting together this book, my hope is that the question-and-answer “Socratic method” format recreates how a large portion of clinical medical education takes place, on rounds, in clinic, and in testing formats. I hope it answers questions you find when seeing patients in your practices daily and you can refer to it easily and often.
I greatly appreciate the efforts of the many talented contributors who have shared their wisdom and experiences to create this book and update the subsequent editions I have received much positive feedback on the first four editions of this book, among residents and fellows here at Wills Eye Hospital and elsewhere. I hope that clinicians and students will continue to enjoy this new edition and find it valuable.
Contributors
Usiwoma Abugo, MD, Howard University Hospital, Washington, DC
Brandon D. Ayres, MD, Attending Physician, Cornea, Wills Eye Hospital, Philadelphia, Pennsylvania
Augusto Azuara-Blanco, MD, PhD
Professor of Ophthalmology, Centre for Public Health, Queen’s University Belfast, Belfast, Great Britain
Honorary Consultant Ophthalmologist, Ophthalmology, Belfast Health and Social Care Trust, Belfast, Great Britain
Robert S. Bailey, MD, Director and Attending Surgeon, CPEC, Wills Eye Hospital, Philadelphia, Pennsylvania
Upneet Kaur Bains, MD, Assistant Professor of Ophthalmology, Ophthalmology, Lewis Katz School of Medicine at Temple University, Philadelphia, Pennsylvania
Vincent F. Baldassano, MD, Doctor, Ophthalmology, Geisinger Eye Institute, Danville, Philadelphia, Pennsylvania
Caroline R. Baumal, MD, FRCSC, Professor, Vitreoretinal Surgery, New England Eye Center, Tufts University School of Medicine, Newton, Massachusetts
Edward H. Bedrossian, BA, MS, MD
Attending Surgeon, Ophthalmic Plastic & Reconstructive Surgery, Wills Eye Hospital, Philadelphia, Pennsylvania
Clinical Professor of Ophthalmology, Ophthalmology, Temple University School of Medicine, Philadelphia, Pennsylvania
Director, Ophthalmic Plastic and Reconstructive Surgery, Ophthalmology, Temple University School of Medicine, Philadelphia, Pennsylvania
Paramjit K. Bhullar, MD, Ophthalmology, Duke University School of Medicine, Durham, North Carolina
Jurij R. Bilyk, MD
Attending Surgeon, Skull Base Division, Neuro-Ophthalmology Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Professor of Ophthalmology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
Jeffrey P. Blice, MD, Clinical Professor, Ophthalmology, Storm Eye Institute Medical, Univeristy of South Carolina, Charleston, South Carolina
Michael J. Borne, MD, Founding Partner, Mississippi Retina Associates, Jackson, Mississippi
Steven E. Brooks, MD, Professor, Ophthalmology, Columbia University, New York, New York
David G. Buerger, MD, Clinical Instructor of Ophthalmology, Ophthalmology, University of Pittsburgh, Pittsburgh, Pennsylvania
Alan N. Carlson, MD Professor of Ophthalmology, Duke University School of Medicine, Durham, North Carolina
Corneal Specialist, Ophthalmologist, Duke University Eye Center, Durham, North Carolina
Marc S. Cohen, MD, Associate Professor of Ophthalmology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
Mary Jude Cox, MD Instructor, Glaucoma, Wills Eye Hospital, Philadelphia, Pennsylvania Ophthalmologist, Eye Physicians, Voorhess, New Jersey
Kristin M. DiDomenico, MD, FCPP, Comprehensive Ophthalmology, Cataract and Primary Eye Care, Wills Eye Hospital, Philadelphia, Pennsylvania
John Donald Dugan, MD, Attending Surgeon, Cataract and Primary Eye Care, Wills Eye Hospital, Philadelphia, Pennsylvania
Jacob Starr Duker, MD, Fellow Physician, Ophthalmology, Ophthalmic Consultants of Boston, Boston, Massachusetts
Ralph Conrad Eagle, MD, Director, Department of Pathology, Wills Eye Hospital, Philadelphia, Pennsylvania
Mitchell S. Fineman, MD
Attending Surgeon, Retina Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Associate Professor of Ophthalmology, Thomas Jefferson University, Philadelphia, Pennsylvania
Janice A. Gault, MD, FACS
Associate Surgeon, Cataract and Primary Eye Care, Wills Eye Hospital, Philadelphia, Pennsylvania
Assistant Professor, Ophthalmology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
Roberta E. Gausas, MD, Associate Clinical Professor, Ophthalmology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania
Kenneth B. Gum, MD, Traverse City Eye, Traverse City, Michigan
Shipra Gupta, MD, PGY3 Resident, Ophthalmology, University Hospitals-Case Medical Center, Cleveland, Ohio
Sadeer B. Hannush, MD
Attending Surgeon, Cornea Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Professor of Ophthalmology, Department of Ophthalmology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, Pennsylvania
Jeffrey D. Henderer, MD
Dr. Edward Hagop Bedrossian Chair, Department of Ophthalmology, Lewis Katz
School of Medicine at Temple University, Philadelphia, Pennsylvania
Professor, Department of Ophthalmology, Lewis Katz School of Medicine at Temple University, Philadelphia, Pennsylvania
Terry Kim, MD, Professor of Ophthalmology, Duke University School of Medicine, Duke University Eye Center, Durham, North Carolina
Kendra A. Klein, MD, Ophthalmology, Associated Retina Consultants, Phoenix, Arizona
Nicole A. Langelier, Scheie Eye Institute, University of Pennsylvania Health System, Philadelphia, Pennsylvania
Joseph I. Maguire, MD, Assistant Professor, Ophthalmology, Wills Eye Hospital, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
Marlene R. Moster, MD
Professor, Ophthalmology, Thomas Jefferson School of Medicine, Philadelphia, Pennsylvania
Glaucoma, Wills Eye Hospital, Philadelphia, Pennsylvania
OPP Vantage, Bala Cynwyd, Pennsylvania
Mark L. Moster, MD
Director, Neuro-Ophthalmology Fellowship, Neuro-Ophthalmology, Wills Eye Hospital, Philadelphia, Pennsylvania
Professor, Neurology and Ophthalmology, Sidney Kimmel Medical College of Thomas Jefferson University, Philadelphia, Pennsylvania
Leonard B. Nelson, MD, MBA, Co-Director, Pediatric ophthalmology and ocular genetics, Wills Eye Hospital, Philadelphia, Pennsylvania
Scott E. Olitsky, MD, MBA, Professor, Ophthalmology, UMKC, Kansas City, Missouri
Joshua Paul, MD, Resident, Ophthalmology, Temple University Hospital, Philadelphia, Pennsylvania
Robert B. Penne, MD
Clinical Professor, Ophthalmology, Sydney Kimmel Medical College Thomas Jefferson University, Philadelphia, Pennsylvania
Director, Ophthalmic Plastic Surgery Department, Wills Eye Hospital, Philadelphia, Pennsylvania
Attending Surgeon, Ophthalmology, Lankenau Hospital, Wynnewood, Pennsylvania
Julian D. Perry, MD, Physician, Ophthalmology, Cole Eye Institute, Cleveland, Ohio
Irving Raber, MD, F.R.C.S. (C), Attending Surgeon, Cornea Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Ehsan Rahimy, MD, Vitreoretinal Surgeon, Ophthalmology, Palo Alto Medical Foundation, Palo Alto, California
Christopher J. Rapuano, MD
Chief, Cornea Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Professor of Ophthalmology, Sidney Kimmel Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania
Carolyn S. Repke, MD
Assistant Surgeon, Cataract and Primary Eye Care, Wills Eye Hospital, Philadelphia, Pennsylvania
Physician partner, Vantage Eye Care, Philadelphia Eye Associates Division, Philadelphia, Pennsylvania
Douglas J. Rhee, MD
Professor and Chair, Ophthalmology & Visual Sciences, Case Western Reserve
University School of Medicine, Cleveland, Ohio
Director, University Hospitals Eye Institute, Cleveland, Ohio
Lorena Riveroll-Hannush, MD
Clinical Coordinator, Cataract and Cornea Associates, Langhorne, Pennsylvania
Ex-Adscrito, Servicio de Cornea, Hospital Para Evitar la Ceguera en Mexico, Mexico City, Mexico DF
Warren Robinson, BS, Pharmacist, Temple University Hospital, Philadelphia, Pennsylvania
Tal J. Rubinstein, MD, Assistant Professor, Ophthalmology, Ophthalmic Plastic Surgery, Albany Medical Center, Albany, New York
Brooke D. Saffren, BS
Bradway Scholar Research Fellow, Pediatric Ophthalmology and Ocular Genetics, Wills Eye Hospital, Philadelphia, Pennsylvania
OMS-IV, Philadelphia College of Osteopathic Medicine, Philadelphia, Pennsylvania
Jonathan H. Salvin, MD
Pediatric Ophthalmology, Division of Ophthalmology, Nemours/A.I. duPont Hospital for Children, Wilmington, Delaware
Clinical Associate Professor, Ophthalmology and Pediatrics, Sydney Kimmel College of Medicine, Philadelphia, Pennsylvania
Department of Pediatric Ophthalmology, Wills Eye Hospital, Philadelphia, Pennsylvania
Bruce M. Schnall, MD, Associate Surgeon, Pediatric Ophthalmology, Wills Eye Hospital, Philadelphia, Pennsylvania
Carol L. Shields, MD, Director, Ocular Oncology Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Jerry A. Shields, MD, Wills Eye Hospital, Oncology, Wills Eye Hospital, Philadelphia, Pennsylvania
Andrew P. Shyu, MD, Resident Physician, Ophthalmology, Temple University Hospital, Philadelphia, Pennsylvania
George L. Spaeth, BA, MD, Esposito Research Professor, Glaucoma, Wills Eye Hospital/T Jefferson University, Philadelphia, Pennsylvania
Archana Srinivasan, MD, Fellow, Neuro ophthalmology, Wills eye hospital, Philadelphia, Pennsylvania
Richard E. Sutton, MD, PhD, Professor, Section of Infectious Diseases, Department of Medicine, Yale School of Medicine, New Haven, Connecticut
Nancy G. Swartz, MS, MD, FACS
Director of Facial Rejuvenation, Myrna Brind Center of Integrative Medicine, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
Associate Surgeon, Neuro-Ophthalmology Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Clinical Associate, Ophthalmology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
Instructor, Ophthalmology, Thomas Jefferson University Medical College, Philadelphia, Pennsylvania
Janine G. Tabas, MD
Ophthalmologist, Kay, Tabas, Niknam & DiDomenico Ophthalmology Associates, Bala Cynwyd, Pennsylvania
Ophthalmologist, Cataract and Primary Eye Care, Wills Eye Hospital, Philadelphia, Pennsylvania
William S. Tasman, MD, Ophthalmologist, Retina Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Richard Tipperman, MD, Attending Surgeon, Cataract Surgery, Wills Eye Hospital, Philadelphia, Pennsylvania
Sydney Tyson, MD, M.P.H, Attending Surgeon, Cataract and Primary Eyecare Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Neil Vadhar, MD, Fellow, Cornea, Wills Eye Hospital, Philadelphia, Pennsylvania
Priya Sharma Vakharia, MD, Physician, Ophthalmology, Retina Group of Washington, Greenbelt, Maryland
James F. Vander, MD
Attending Surgeon, Retina Service, Wills Eye Hospital, Philadelphia, Pennsylvania
Clinical Professor, Ophthalmology, Thomas Jefferson University, Philadelphia, Pennsylvania
Nandini Venkatswaran, MD
Ophthalmologist; Cataract, Cornea, and Refractive Surgeon, Massachusetts Eye and Ear, Boston, Massachusetts
Clinical Instructor of Ophthalmology, Harvard Medical School, Boston, Massachusetts
Tamara R. Vrabec, MD
Ophthalmology, Geisinger Medical Center, Danville, Pennsylvania
Clinical Professor, Ophthalmolgoy, Temple University Hospital, Philadelphia, Pennsylvania
Lauren B. Yeager, MD, Assistant Professor of Ophthalmology, Ophthalmology, Columbia University Irving Medical Center, New York, New York
Top 100 secrets
Janice A. Gault, James F. Vander
These secrets summarize the concepts, principles, and most salient details of ophthalmology.
1. The goal of refractive correction is to place the circle of least confusion on the retina
2. To find the spherical equivalent of an astigmatic correction, add half the cylinder to the sphere.
3. Recheck if the axial length measures less than 22 mm or more than 25 mm or if there is more than a 0.3-mm difference between the two eyes. For each 1 mm in error, the intraocular lens (IOL) power calculation is off by 2 5 diopters (D) Recheck keratometry readings if the average K power is <40 D or >47 D or if there is a difference of more than 1 D between eyes. For every 0.25-D error, the IOL power calculation is in error by 0.25 D.
4. According to Kollner’s rule, retinal diseases cause acquired blue-yellow color vision defects, whereas optic nerve diseases affect red-green discrimination.
5 A junctional scotoma is a unilateral central scotoma associated with a contralateral superotemporal field defect and is caused by compression of the contralateral optic nerve near the chiasm.
6. False-negative errors cause a visual field to appear worse than it actually is. False-positive errors cause a visual field to look better than it actually is.
7 Lesions anterior to the optic chiasm cause unequal visual acuity, a relative afferent papillary defect, and color abnormalities. The optic disc may also have asymmetric cupping and pallor.
8. A drop of 2.5% neosynephrine is a simple test to distinguish between episcleritis (these vessels will blanch) and scleritis (these vessels do not) two entities with very different prognoses and evaluations. Because 50% of patients with scleritis have systemic disease, referral to an internist is necessary for further evaluation
9. Immediately irrigate any patient with a chemical ocular injury from an alkali or an acid, even before checking visual acuity. Normalize the pH before examining the patient to prevent further damage to the eye.
10. Rule out uncontrolled hypertension or blood dyscrasias in patients with recurrent subconjunctival hemorrhages
11. A corneal ulcer is infectious until proven otherwise. You are never wrong to
culture an ulcer; any ulcer not responding to therapy should be recultured.
12. Systemic treatment is necessary for gonococcal, chlamydial, and herpetic neonatal conjunctivitis because of the potential for serious disseminated disease. The mother and her sexual partners must be evaluated for other sexually transmitted diseases, including HIV.
13 Treatments that are effective for prophylaxis of gonococcal and chlamydial neonatal conjunctivitis include 1% silver nitrate, 0.5% erythromycin, and 1% tetracycline. Silver nitrate is rarely used, however, because of its potential for causing chemical conjunctivitis.
14. Topical steroids may promote herpetic keratitis if viral shedding is coincident with administration.
15 Steroid-induced increases in intraocular pressure occur in about 6% of patients on topical dexamethasone. This risk is higher in patients with known glaucoma or a family history of glaucoma.
16. Patients may be symptomatic with dry eye even with a normal slit lamp exam.
17. Ask about gastric bypass procedures in patients who have recent severe dry eye with no discernible cause Vitamin A deficiency may be the reason Similarly, patients after gastric bypass may present with Wernicke-Korsakoff syndrome (nystagmus, diplopia, ptosis, and mental confusion) due to vitamin B1 deficiency.
18 If a patient presents with symptoms consistent with recurrent corneal erosion syndrome but no findings on slit lamp exam of the same, look for an underlying dystrophy, specifically epithelial basement membrane dystrophy.
19. If a patient with a corneal dystrophy is undergoing corneal transplantation but also has a clinically significant cataract, consider staging the cataract extraction a few months after the corneal transplant, offering the patient the advantage of better IOL power calculation and postoperative refractive result Alternatively, Descemet’s stripping endothelial keratoplasty, which does not alter corneal contour, may be combined with cataract surgery with a more predictable refractive outcome.
20. Corneal opacification in a neonate has a differential diagnosis of STUMPED: sclerocornea, trauma, ulcers, metabolic disorder, Peter’s anomaly, endothelial dystrophy, and dermoid.
21. Most patients with keratoconus can be managed successfully with contact lens wear. Corneal transplantation is very successful in treating patients whose visual needs are not satisfied with glasses or contact lens correction, although corneal crosslinking may prevent keratoconus from progressing, thus preventing the need for a transplant
22. As many as 30% to 50% of individuals with glaucomatous optic nerve damage and visual field loss have an initial intraocular pressure measurement less than 22 mm Hg.
23. The treatment of both primary open-angle glaucoma and low-tension glaucoma aims to preserve vision and quality of life through the lowering of intraocular pressure.
24 When evaluating a patient with angle-closure glaucoma, it is important to look at the fellow eye. Except for cases of marked anisometropia, the fellow eye should have a similar anterior chamber depth and narrow angle. If it does not, consider other nonrelative papillary block mechanisms of angle closure.
25. Patients with sporadic inheritance of aniridia need to be evaluated for Wilms’ tumor, as it is found in 25% of cases
26. Allergy from topical medications can present months to years after starting the drop.
27. If a patient’s glaucoma continues to worsen, even with seemingly reduced intraocular pressure during office visits, think of noncompliance.
28. Before trabeculectomy surgery, identify high-risk patients in whom sudden hypotony should be avoided: those with angle-closure glaucoma, shallow anterior chambers, very high preoperative intraocular pressure, elevated episcleral venous pressure, or high myopia. Hemorrhagic choroidals and expulsive hemorrhages are more likely.
29. Patients with traumatic ocular injuries must be evaluated for systemic injuries as well
30. Posterior fractures most commonly occur in the posteromedial orbital floor.
31. Patients recovering from a traumatic hyphema are at increased risk for glaucoma and retinal detachments in the future. They need ongoing ophthalmic evaluation for the rest of their lives.
32. Always check the pressure in the contralateral eye in a patient with ocular trauma Asymmetrically low intraocular pressure may be an important clue to a possible ruptured globe.
33. Complete systemic evaluation by a pediatrician is mandatory for any infant with a congenital cataract.
34. Patients must have a documented functional interference in quality of life from a visual standpoint before cataract surgery is indicated
35. Glare testing can reveal significant functional visual problems even in patients with excellent visual acuity on Snellen testing.
36. Amblyopia is a diagnosis of exclusion. If amblyopia is associated with an afferent pupillary defect, a lesion of the retina or optic nerve should be suspected and ruled out.
37 The critical period of visual development is from birth through age 6 to 7 years Amblyopia is most successfully treated during this time. However, treatment can be successful at older ages with good compliance. Atropine penalization can be as effective as patching.
38. Early treatment for congenital esotropia gives the best chance for the development of binocular vision Be certain that a patient with a partial accommodative esotropia is wearing the maximum tolerated hyperopic prescription.
39. Check the light reflex test and cover test to determine if a true deviation exists. If the light reflex is in the appropriate place and there is no refixation on cover testing, the patient is orthophoric.
40 A young patient with asthenopia should be evaluated for exophoria at near (convergence insufficiency) as well as for their cycloplegic refraction for undercorrected hyperopia (accommodative insufficiency).
41. Any patient with chronic progressive external ophthalmoplegia needs an electrocardiogram to rule out heart block. These patients may need a pacemaker to prevent sudden death
42. A patient with acute onset of any combination of third, fourth, fifth, and sixth cranial nerve palsies; extreme headache; and decreased vision must be immediately placed on intravenous steroids and referred to neurosurgery for pituitary apoplexy.
43. The signs of endophthalmitis typically appear 1 to 4 days after strabismus surgery and include lethargy, asymmetric eye redness, eyelid swelling, and fever.
44. Before evaluating for strabismus, make sure patients with double vision have binocular diplopia. Strabismus does not cause monocular diplopia.
45. Always consider myasthenia gravis and thyroid eye disease in patients presenting with diplopia and normal pupils
46. When performing surgery on both oblique and rectus muscles, hook the obliques first.
47. In a recess–resect procedure, the recession should be done first.
48. If a patient has a significant deviation in primary gaze or an abnormal head posture, strabismus surgery is indicated in most incomitant strabismus cases.
49 Try for fusion of all patients with nystagmus Aim for exophoria with fusion
50. Smoking is a controllable risk factor for thyroid eye disease.
51. All patients with optic neuritis should experience some improvement in vision. However, 5% of patients who presented with visual acuity of less than 20/200 were still 20/200 or less at 6 months.
52 An abnormal magnetic resonance imaging (MRI) in a patient with optic neuritis is the strongest predictor of developing multiple sclerosis (MS). Fifty-six percent of patients with optic neuritis and a white matter lesion on MRI will develop MS within 10 years.
53. The closer a patient stands to a visual-field testing screen, the smaller the field should be. This is helpful in determining a malingering patient.
54 Any patient suspected of giant cell arteritis should immediately be started on high doses of steroids to prevent involvement of the other eye even if the temporal artery biopsy cannot be done beforehand.
55. Dacryocystitis must be treated emergently to prevent cellulitis or intracranial spread of the infection.
56 Computed tomography (CT) scanning is superior to MRI in most cases of orbital disease owing to better bone–tissue delineation.
57. The most common cause of unilateral or bilateral proptosis is thyroid eye disease (Graves ophthalmopathy). Most patients with thyroid-related ophthalmopathy (TRO) will not require surgery for their disease; it will burn out with time.
58 The most common cause of unilateral proptosis in children is orbital cellulitis
59. A child with rapidly progressive proptosis, inferior displacement of the globe, and upper eyelid edema should have immediate neuroimaging followed by an orbital biopsy to rule out rhabdomyosarcoma.
60. Suspect TRO in patients with nonspecific redness and inflammation of the eyes even if there is no history of a systemic thyroid imbalance
61. Myositis, a nonspecific inflammation of an extraocular muscle, can be distinguished from thyroid-associated ophthalmopathy (TAO) by the location of muscle inflammation. TAO demonstrates thickening of the muscle belly, but only myositis shows thickening of the tendon insertion as well.
62. Persistent proptosis and progression of orbital infection while on intravenous antibiotics for orbital cellulitis should prompt a repeat CT scan to rule out an orbital abscess.
63. The sinuses are the most common source of an orbital infection. The ethmoid sinus is the most frequent culprit as its lateral wall is the thinnest orbital wall, the lamina papyracea.
64 Surgical drainage should be undertaken in orbital cellulitis if sinuses are completely opacified, response to antibiotics is poor by 48 to 72 hours, vision decreases, or an afferent pupillary defect presents.
65. Mild ptosis associated with miosis and neck or facial pain should raise suspicion of a carotid artery dissection, prompting an urgent workup.
66. Acute ptosis and ocular misalignment mandate a careful evaluation of the pupil to rule out pupil-involving third-nerve palsy A dilated pupil requires neurologic evaluation for a compressive aneurysm.
67. Basal cell carcinoma is the most common malignant eyelid tumor. It has a 3% mortality rate because of invasion into the orbit and brain via the lacrimal drainage system, prior radiation therapy, or clinical neglect.
68 Squamous cell carcinoma may metastasize systemically
69. Keratoacanthomas often resolve spontaneously but should be removed surgically if near the lid margin to prevent permanent deformity.
70. Rule out sebaceous cell carcinomas in a patient with a recurrent chalazion in the same spot.
71. Young patients with xanthelasma should be evaluated for diabetes mellitus and hypercholesterolemia
72. All patients who have anterior uveitis must have a dilated examination to exclude associated posterior segment disease.
73. Consider masquerade syndromes in the very young, the elderly, and in patients who have uveitis that does not respond to treatment. Uveitis in patients with acquired immunodeficiency syndrome is almost invariably part of a disseminated systemic infection. Lymphoma may masquerade as retinitis.
74. Never aspirate subretinal exudates for diagnostic purposes in a patient with potential Coats disease, unless retinoblastoma has been absolutely ruled out. It may take as long as 1 to 2 years for exudation to clear after successful treatment of the abnormal peripheral retinal vessels.
