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The most common cause of PMD is a mutation of the Proteolipid protein (PLP) gene producing a structural protein involved in the construction of central nervous system myelin 3-6. Familial Case of Pelizaeus-Merzbacher Disorder Detected by Oligoarray Comparative Genomic Hybridization: Genotype-to-Phenotype Diagnosis.

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Vatinee Bunya, MD, Maria A. Woodward, MD, Sarah Brown Weissbart, MD, Harikrishnan Vannadil, MD, MS (Ophthal), Yannis M. Paulus, M.D.

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by Sarah Brown Weissbart, MD on March 12, 2021.

Pellucid Marginal Degeneration (PMD) is a noninflammatory, nonhereditary cause of corneal ectasia with bilateral, clear, inferior (typically 4 o’clock to 8 o’clock), peripheral corneal thinning.

PMD is uncommon and is characterized by a crescent-shaped band of inferior corneal thinning approaching 20% of normal thickness that is 1 to 2 mm in height, 6 to 8 mm in horizontal extent, and 1 to 2 mm from the limbus. There is no associated inflammation and the central cornea is of normal thickness.

Disease

PMD is disease of peripheral corneal thinning that is slowly progressive over many years. It is important to recognize as it can cause severe deterioration in visual function. Also, patients who undergo refractive surgery with PMD can have deleterious results, so it is important to consider prior to refractive surgery.

Etiology

The etiology of PMD remains unknown. Music zapadcock games by: austin adcock.

Risk Factors

PMD, while rare, is the second most common noninflammatory corneal thinning disorder after keratoconus. Some people postulate, however, that this prevalence might be underestimated as the condition is often misdiagnosed as keratoconus. Keratoconus, keratoglobus, and PMD are postulated to be related because these conditions coexist in families. Ten percent of PMD cases are associated with keratoconus and 13% are associated with keratoglobus. In series performed in Japan, 17 of 27 cases of unilateral PMD revealed keratoconus or suspected keratoconus in the fellow eye. It has not been elucidated whether keratoconus, keratoglobus, and PMD are different diseases or phenotypic variations of the same disease.

PMD typically presents in the second to fifth decade of life. PMD has no sex or racial predilection, and does not appear to be hereditary, but moderate to high astigmatism has been noted in families with affected patients. A recent case showed that PMD was coexistent with corneal plana in an individual with a KERA mutation, thus, potentially implicating KERA in the development of PMD.

General Pathology

Histopathologic abnormalities in PMD resemble keratoconus. Classically, PMD is histologically thought to show an area of stromal thinning, normal epithelium, endothelium, and Descemet's membrane, and absent or broken (focal disruption) Bowman's membrane. Lipid deposits are typically absent and stromal ground substance rich in mucopolysaccharides is present. Electron microscopy of the thin regions reveals unusual electron-dense areas of fibrous long-spacing (FLS) collagen with a periodicity of 100 to 110 nm, whereas normal collagen has a periodicity of 60 to 64 nm. FLS collagen has also been observed in advanced KC. In cases of acute hydrops, breaks in Descemet's membrane with swelling of the stroma and inflammatory infiltrate is seen.

Pathophysiology

The exact pathophysiology of PMD has not been ascertained, but it is thought to be secondary to collagen abnormalities, similar to keratoconus. The thin, weakened cornea is hypothesized to protrude as a result of intraocular pressure.

Primary prevention

No preventative strategies exist for PMD. Given the corneal thinning and the potential association of eye rubbing with keratoconus and its relationship to PMD, it could be considered to advise patients not to rub their eyes.

Pellucid gets its name from the meaning 'transparent' as the cornea typically appears transparent in this degeneration. The diagnosis is made clinically as patient's are usually asymptomatic except for progressive visual deterioration.

History

Patients with PMD typically present asymptomatically except with progressive visual deterioration despite spectacle correction due to the irregular astigmatism. They someimes present with acute corneal hydrops and pain or acute decrease in vision, but this is relatively rare with only a few case reports.

Physical examination

Slit lamp examination is typically characterized by a peripheral band of corneal thinning in the inferior cornea from 4 to 8 o'clock. The thinning can reach 20% of normal corneal and is 1 to 2 mm from the limbus. The steepest corneal protrusion in PMD occurs above (central to) the area of stromal thinning, appearing like a 'beer belly' in cross section. This results in high and irregular 'against-the-rule' astigmatism of up to 20 diopters and a flattening of the vertical meridian. The astigmatism can instead be 'with the rule' if the inferior curvature is steeper than the horizontal curvature. The area of thinning in PMD is always epithelialized, clear, avascular, and without lipid deposition (distinguishing it from Terrien marginal degeneration). Prominent lymphatics are also associated with PMD at the inferior limbus parallel to the area of thinning.

Vertical striations at the level of the Descemet membrane (similar to Vogt striae), hydrops, vascularization, and scarring are relatively rare, as is spontaneous corneal perforation. The ectasia and thinning can also occur superiorly. PMD does not present with an iron ring, cone, apical scarring, or abnormal corneal sensation. Descemet’s folds can develop concentric to the inferior limbus and posterior stromal scarring can be noted in the thin areas.

Signs

Optical section of a patient of advanced Pellucid Marginal Degeneration showing significant corneal thinning inferiorly.

Signs of PMD include:

Inferior corneal thinning
Severely reduced uncorrected visual acuity that typically cannot be improved with spherocylinder lens
Practically normal pinhole visual acuity
Refraction and keratometry showing against-the-rule astigmatism

Symptoms

Patients typically report a gradual decrease in vision uncorrected by spectacles.

Clinical diagnosis

PMD is a clinical diagnosis based primarily on the slit lamp examination and history and supported by corneal topography/tomography.

Diagnostic procedures

Corneal topography of a patient with advanced Pellucid Marginal Degeneration showing the 'Butterfly' pattern.

Several tests exist to assist with the detection and early diagnosis of PMD:

Corneal topography/tomography
Computerized videokeratography: Shows a classic 'butterfly' appearance with PMD, showing low power along the central vertical axis, increasing power in the inferior cornea and high power along the infero-oblique meridians
Pachymetry: Used to measure for inferior corneal thinning, which is a reversal of the typical pattern in which the cornea thickens from center to periphery
Orbscan: Shows a classic 'kissing birds' appearance with PMD

Laboratory test

No laboratory tests exist at this time for PMD.

Differential diagnosis

Keratoconus
Keratoglobus
Terrien marginal degeneration
Furrow degeneration
Peripheral corneal melting disorders (for example, Mooren ulcer)

Management with PMD is difficult given the abnormal corneal topography and degree of corneal protrusion. Management is typically with either spectacles, contact lenses, or surgery.

General treatment

Because of extremely abnormal corneal topography, the treatment of PMD is difficult. Therapeutic options are limited by the degree of corneal protrusion. A recent study at a tertiary care center found that 88% of PMD cases were managed nonsurgically with spectacles (36%) or contacts (52%), whereas 12% underwent penetrating keratoplasty.

Medical therapy

The standard medical therapy is contact lens correction. Spectacle correction typically fails early in the course of PMD with the increasing astigmatism.

Contact lens (CTL) correction can be attempted when corneal ectasia is mild, but inferior decentration often makes proper contact lens fit difficult, more difficult than with keratoconus. Hybrid (gas-permeable lenses with a soft lens skirt) or scleral lens can also be considered.

The specific types of contact lens for mild PMD and their pros/cons are listed below:

Toric hydrophilic CTL: spherical hydrophilic CTL are not useful given the associated astigmatism
Hybrid CTL: they are easier to fit on patients, but their low oxygen permeability can result in corneal neovascularization
Rigid gas permeable CTL: provide good oxygenation of the cornea but are hard to fit. Can sometimes place in the upper eyelid with large-diameter lens, but this can cause irritation and can move excessively

All of these CTL treatments are symptomatic treatments to improve the patient's visual acuity and can result in flare and decreased contrast sensitivity. There is no evidence that these lens prevent progression of the disease.

Medical follow up

PMD is slowly progressive and be followed at regular, extended intervals.

Surgery

If medical therapy is unsatisfactory, surgical treatment may be required to restore vision or due to patient intolerance of contact lens. Clasically, this surgical treatment has been penetrating keratoplasty. However, for PMD, this requires large (9.0 mm or larger), inferiorly decentered penetrating keratoplasty grafts for highly ectatic cases. The transplant's large graft size and its location near the limbus increases the risk of vascularization and corneal graft rejection and is more surgically challenging. Regular-sized grafts deliberately decentered inferiorly are also suboptimal due to the large degree of astigmatism and high incidence of rejection.

Other surgical procedures have been advocated include:

Diathermy
Lamellar keratoplasty
C-shaped lamellar keratoplasty
Thermokeratoplasty
Lamellar crescentic resection of the affected area and re-apposition of normal thickness cornea from each side of the affected area
Wedge resection
Full-thickness crescentic resection
Replacement by a freehand corneoscleral graft
Tectonic lamellar grafting followed by a central penetrating keratoplasty
Large epikeratophakia graft
Intrasomal ring segment implantation using a femtosecond laser to make lamellar dissections
Intracorneal ring segments and bitoric or intralimbal rigid gas-permeable lenses may also improve vision.

Recently, the most favored treatment has been a peripheral lamellar crescentic keratoplasty followed by a central penetrating keratoplasty. Some have even advocated for performing both procedures simultaneously, but this is technically challenging.

Surgical follow up

Surgical follow-up depends on the surgical procedure performed. Corneal grafts require ongoing monitoring for failure, rejection, glaucoma, and other complications.

Complications

Complications can occur with PMD, but are more rare than keratoconus. Complications that occur include vertical stress lines and acute hydrops, but these are relatively rare. Spontaneous corneal perforation has also been reported. Vascularization and scarring of the cornea can occur but typically the cornea is clear without neovascularization or lipid deposition. Descemet’s folds can develop as can posterior stromal scarring.

Prognosis

PMD is slowly progressive over many years and results often in severe visual deterioration of working age people. No large longitudinal studies have been reported, with the largest study following 31 patients over 8 years and noting that 5 required corneal transplantation in one eye and 1 requiring transplantation in both eyes.

A helpful Web site for patients regarding PMD is: http://www.lensdesign.ca/pmd.htm

There are several other useful keratoconus Web sites, and given that many PMD patients are misdiagnosed, there groups often discuss and have information on PMD: www.nkcf.org/

1. Jain A, Paulus YM, Cockerham GC, Kenyon KR. Keratoconus and Other Non- inflammatory Thinning Conditions. Duane's Foundations of Clinical Ophthalmology Vol 4, Chapter 16C. Ed. William Tasman, MD and Edward A. Jaeger, MD. Philadelphia: Lippincott Williams and Wilkens, 2008.

2. Karabatsas CH, Cook SD: Topographic analysis in pellucid marginal corneal degeneration and keratoglobus. Eye 10:451–455, 1996

3. Krachmer J: Pellucid marginal corneal degeneration. Arch Ophthalmol 96:1217–1221, 1978

4. Maguire L, Klyce SD, McDonald MB, Kaufman HE: Corneal topography of pellucid marginal degeneration. Ophthalmol 94:519–524, 1987

5. Sridhar M, Mahesh S, Bansal AK et al: Pellucid marginal corneal degeneration. Ophthalmol 111:1102–1107, 2004

6. Kayazawa F, Nishimura K, Kodama Y et al: Keratoconus with pellucid marginal corneal degeneration. Arch Ophthalmol 102:895–896, 1984

7. Santo RM, Bechara SJ, Kara-Jose N: Corneal topography in asymptomatic family members of a patient with pellucid marginal degeneration. Am J Ophthalmol 127:205–7, 1999

8. Nagy M, Vigvary L: Etiology of the pellucid marginal degeneration of the cornea. Klin Monatsbl Augenheilkd 161:604–611, 1972

9. Khan A, Aldahmesh M, Al-Saif A, Meyer B: Pellucid marginal degeneration coexistent with cornea plana in one member of a family exhibiting a novel KERA mutation. Br J Ophthalmol 89:1538–1540, 2005

10. Rodrigues M, Newsome DA, Krachmer JH, Eiferman RA: Pellucid marginal corneal degeneration: A clinicopathologic study of two cases. Exp Eye Res 33:277–288,1981

11. Gruenauer-Kloevekorn C, Fischer U, Kloevekorn-Norgall K, Duncker GI: Pellucid marginal corneal degeneration: evaluation of the corneal surface and contact lens fitting. Br J Ophthalmol 90:318–323, 2006

Get big gameshome. 12. Tzelikis P, Cohen EJ, Rapuano CJ et al: Management of pellucid marginal corneal degeneration. Cornea 24:555–560, 2005

13. Rasheed K, Rabinowitz YS: Surgical treatment of advanced pellucid marginal degeneration. Ophthalmol 107:1836–1840, 2000 Youtubeclout games.

14. Speaker M, Arentsen JJ, Laibson PR: Long-term survival of large diameter penetrating keratoplasties for keratoconus and pellucid marginal degeneration. Acta Ophthalmol Suppl 192:17–19, 1989

15. Cheng C, Theng JT, Tan DT: Compressive C-shaped lamellar keratoplasty: A surgical alternative for the management of severe astigmatism from peripheral corneal degeneration. Ophthalmol 112:425–430, 2005

16. Cameron J: Results of lamellar crescentic resection for pellucid marginal corneal degeneration. Am J Ophthalmol 113:296–302, 1992

17. Fronterre A, Portesani GP: Epikeratoplasty for pellucid marginal corneal degeneration. Cornea 10:450–453, 1991

Pmd App On Computer

18. Ertan A, Bahadir M: Intrastromal ring segment insertion using a femtosecond laser to correct pellucid marginal corneal degeneration. J Cataract Refract Surg 32:1710–1716, 2006

19. Rodriguez-Prats J, Galal A, Garcia-Lledo M et al: Intracorneal rings for the correction of pellucid marginal degeneration. J Cataract Refract Surg 29:1421–1424, 2003

20. Barbara A, Shehadeh-Masha'our R, Zvi F, Garzozi HJ: Management of pellucid marginal degeneration with intracorneal ring segments. J Refract Surg 21:296–298, 2005

21. Mularoni A, Torreggiani A, di Biase A et al: Conservative treatment of early and moderate pellucid marginal degeneration: a new refractive approach with intracorneal rings. Ophthalmol 112:660–666, 2005

22. Akaishi L, Tzelikis PF, Raber IM: Ferrara intracorneal ring implantation and cataract surgery for the correction of pellucid marginal corneal degeneration. J Cataract Refract Surg 30:2427–2430, 2004

23. Dominguez C, Shah A, Weissman BA: Bitoric gas-permeable contact lens application in pellucid marginal corneal degeneration. Eye Contact Lens 31:241–243, 2005

24. Ozbek Z, Cohen EJ: Use of intralimbal rigid gas-permeable lenses for pellucid marginal degeneration, keratoconus, and after penetrating keratoplasty. Eye Contact Lens 32:33–36, 2006

25. Shimazaki J, Maeda N, Hieda O, et al: National Survey of pellucid marginal corneal degeneration in Japan. Jpn J Ophthalmol. 2016:60(5):341-8.

Retrieved from 'https://eyewiki.org/w/index.php?title=Pellucid_Marginal_Corneal_Degeneration&oldid=66138'

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PMD

microphthalmia, syndromic, type 9

A clinically heterogeneous disorder (OMIM:601186) of eye formation, ranging from unilateral microphthalmia to bilateral anophthalmia, which may be associated with pulmonary hypoplasia or aplasia.
Molecular pathology
Caused by defects of STRA6, which encodes a membrane protein involved in retinol metabolism.

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(di-vis') [Fr. devis, contrivance]

An apparatus, tool, or machine made for a specific function.

abduction device

A trapezoidal pillow, wedge, or splint placed between the arm and torso to prevent adduction. It is commonly used postoperatively for patients having total joint replacement or open reduction or internal fixation of the hip or shoulder.

See: illustration

adaptive device

Assistive technology.

adaptive seating device

Abbreviation: ASD

A device that provides a proper sitting position for those with limited motor control. Such devices include seating inserts, wheelchairs, and postural support systems designed to prevent deformities and enhance function.

Synonym: seating system

assistive technology device

Assistive technology.

augmentative device

A device that helps people with limited or no speech to communicate. Examples include communication boards, pictographs, or ideographs (symbols representing ideas, not sounds).

bag mask device

A manually operated resuscitator used to ventilate a nonbreathing patient or assist the ventilation of a patient who is not breathing at an effective rate or tidal volume. The device consists of a bag, an oxygen reservoir system, a one-way flow valve, and a clear face mask. It is designed to be attached to an oxygen source by tubing to deliver concentrations approaching 100%.

See: illustration

belay device

A device using friction to brake or slow the movement of a rope, or to protect a patient, basket, climber, or other rescuer.

biventricular assist device

Abbreviation: BiVAD

A device that helps both ventricles of the heart contract more effectively. It is used to treat heart failure by propelling blood out of the chambers of the heart.

cardiac rhythm management device

Abbreviation: CRMD

An umbrella term for pacemakers and implantable cardioverter/defibrillators.

cervical immobilization device

Abbreviation: CID

A stiff neck brace or collar to prevent movement of the cervical spine in order to maintain spinal alignment and prevent injury or paralysis.

charge-coupled device

Abbreviation: CCD

A device used in video and digital imaging (such as in CT scanning) that creates electronic images from light.

clitoral vacuum device

A mechanical device used to engorge and stimulate the clitoris. It is used as a U.S. FDA–approved treatment for female sexual dysfunction.

electronic infusion device

Abbreviation: EID

A device for monitoring intravenous infusions. The device may have an alarm in case the flow is restricted because of an occlusion of the line. In that case, the alarm will sound when a preset pressure limit is sensed. The device can also signal that an infusion is close to completion. The pressure is regulated by the height at which the container is positioned above the level of the heart when the patient is lying flat. A height of 36 in (91 cm) provides a pressure of 1.3 lb/sq in (70 mm Hg). Most EIDs are equipped to stop the flow of the infused liquid if accidental free flow occurs.

See: infusion pump

esophageal intubation detector device

A syringe attached to the endotracheal tube immediately after an intubation attempt.

Patient care

If aspiration is difficult or stomach contents are withdrawn, or both, the endotracheal (ET) tube may have been placed in the esophagus and needs to be removed and reinserted. If aspiration is easy and free of stomach contents, it is probable that the ET tube is located in the trachea; the rescuer should then confirm tube placement by other techniques, e.g., a combination of auscultation, x-ray, and pulse oximetry.

femoral compression device

A device used to apply pressure to the large artery or vein in the thigh after it has been cannulated in order to reduce bleeding from the punctured vessel. Femoral compression devices are used, e.g., after angiography.

flow-restricted oxygen-powered ventilation device

Abbreviation: FROPVD

A ventilation device that provides a peak flow rate of 100% oxygen at up to 40 L/min. See: oxygen-powered ventilation device

Flutter device

See: Flutter device

head immobilization device

A device that attaches to a long back board and holds the patient's head in neutral alignment.

See: long back board

humanitarian use device

Humanitarian device exemption.

improvised explosive device

Abbreviation: IED

Military jargon for a homemade bomb or land mine used in unconventional warfare.

input device

In assistive technology, the device that activates an electronic device. This can be a manual switch, a remote control, or a joystick.

See: switch

inspiratory impedance threshold device

Inspiratory impedance threshold valve.

intrauterine contraceptive device

Abbreviation: IUCD, IUD
See: intrauterine contraceptive device.

Kendrick extrication device

See: Kendrick extrication device

left ventricular assist device

Abbreviation: LVAD

A pump surgically implanted in patients with severe heart failure to move blood from the left ventricle to the ascending aorta. The LVAD usually augments the heart's function until it heals (following a severe myocardial infarction) or until a heart transplant becomes available, e.g., for patients with heart failure with a markedly diminished ejection fraction. The LVAD also may be used permanently for a patient who does not meet criteria for transplantation.

listening device

A speech amplifier that aids the hearing-impaired in direct person-to-person communication or telephone conversation. Such devices differ from conventional hearing aids in that they reduce interference from background noises.

medical device

Any health care product that is intended for the diagnosis, prevention, or treatment of disease and does not primarily work by effecting a chemical change in the body

mobility device

Any assistive technology that aids the movement of people with physical impairments. Examples include lift chairs, scooters, or wheelchairs.

needleless device

A device that has no exposed sharp surface, used to inject drugs and fluids. It is designed to decrease the risk of needle-stick injuries by health care professionals.

oxygen-conserving device

Abbreviation: OCR

Any device that reduces the loss of administered oxygen into the environment, e.g., one that releases oxygen to a patient only when the patient inhales.

oxygen-powered ventilation device

A multifunction ventilation devicehat uses high-flow oxygen. This device can often be triggered by negative pressure caused by an inhaling patient; it can also be operated by a button while the operator watches the patient's chest rise.

CAUTION!

During resuscitation, it is necessary to use the positive-pressure aspect of this device and manually trigger or compress the button because the patient cannot open the valve by inhaling. These devices should be fitted with an overinflation high-pressure alarm to avoid gastric distention and/or barotrauma.

personal flotation device

Abbreviation: PFD

A life vest to prevent drowning and near drowning. People engaged in water sports, such as boating or water skiing, or rescuers working on or near the water should wear PFDs at all times. The U.S. Coast Guard sets standards and establishes specifications for the manufacture and use of PFDs. Personal flotation devices may be used to provide added buoyancy for the patient during aquatic therapy.

personal assistive mobility device

Personal mobility device.

personal mobility device

Any assistive device that facilitates individual human transportation. Examples include powered wheelchairs, scooters, bicycles and unicycles. Although many such devices are used by people with activity or mobility restrictions, mobility aids can be employed generally, e.g., for urban transportation in place of automobiles.

Synonym: personal assistive mobility device

pointing device

A type of input device for sending commands to a computer. Moving the device results in movement of a cursor on the monitor or computer screen. Pointing devices range from the conventional desktop mouse, trackball, and touch-sensitive screens to infrared and ultrasound pointers mounted on the head.

See: light pointer; switch

position-indicating device

Abbreviation: PID

A device to guide the direction of the x-ray beam during the exposure of dental radiographs. This devices improves and standardizes dental radiographic imaging and reduces the patient's risk of radiation exposure.

positive beam limiting device

A collimator that automatically adjusts the size of the radiation field to match the size of the imaging device. Synonym: automatic collimator

powered mobility device

Abbreviation: PMD

Any assistive device (such as a powered wheelchair, a lift chair, or a scooter) that improves the movement of the functionally impaired.

pressure relief device

An appliance filled with air, water, gel, or foam, to reduce pressure points caused by the patient’s body weight when seated or bedridden. Examples include wheelchair cushions and air or water flotation mattresses.

prosthetic terminal device

A component of an upper extremity prosthesis that substitutes for the functions of the hand. There are many types of terminal devices, some of which are designed for use with specific tools and implements. These devices have two primary actions: voluntary opening and voluntary closing.

Synonym: hook

protective device

An external support applied to vulnerable joints or other body parts to guard against injury. Protective devices include helmets, braces, tape or wrapping, and padding.

pubovaginal device

A device fitted for use in the vagina to help prevent urinary incontinence.

See: pessary

sequential compression device

Abbreviation: SCD.

A device to reduce edema or prevent the formation of blod clots in an extremity. A chambered nylon sleeve is progressively inflated from its distal segment to the proximal segment, forcing venous and lymphatic return. Sequential compression devices are inflated with air (pneumatic compression) or, less commonly, chilled water (cryocompression). SCDs are used frequently in the perioperative period. See: intermittent compression

single-use device

A medical device used once for the care of a single patient and then immediately discarded.

spine arthroplasty device

A prosthesis to replace a damaged intervertebral disk.

superconductive quantum interference device

Abbreviation: SQUID

A biomagnetometer used to measure magnetic fields in the body or the presence of magnetically active elements or minerals, such as body stores of iron.

telecommunication device for the deaf

Abbreviation: TDD

A device that allows the hearing-impaired to use the telephone even if they cannot comprehend speech. A keyboard and display screen are used.

VENOUS ACCESS DEVICES: A. An over-the-needle catheter; B. An inside-the-needle catheter.

venous access device

A specially designed catheter for gaining and maintaining access to the venous system. This device provides access for patients who require intravenous fluids or medications for several days or more, e.g., those having a bone marrow transplant or who are receiving long-term total parenteral nutrition. See: venous port

See: illustration

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ventricular assist device

Abbreviation: VAD

A pump to treat heart failure. It helps the ventricles to contract and move blood to the lungs and/or the aorta.

See: left ventricular assist device

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powered mobility device

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Any assistive device (such as a powered wheelchair, a lift chair, or a scooter) that improves the movement of the functionally impaired.

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