Mitral Stenosis & Inflow Obstruction (M/A)

Mitral Stenosis & Inflow Obstruction (MA45)

-Isolated mitral valve disease is uncommon (4/1000 kids w CHD)

Embryology:

-Mitral valve leaflets and cords are mainly formed by the endocardial cushions on the left side of the AV orifice, and from the ventricular muscle; and is separated fr ventric wall via protrusion, delamination, and apoptosis

-Papillary muscle forms fr dvp of horseshoe-shaped myocardial ridge, continuous w atrial myocardium and adjacent cushion tissue. The trabecular ridge runs along ant wall and apex, to the post LV wall. The lat surface of it delaminates fr the entric wall and becomes incorp'd into apical trabeculae, contributing to the ant-lat and post-med paps.

-At the same time, endocard cushions --> leaflets and chorae

-Nl: only chordal structures remain contiguous w paps

-Leaflets and cords are fr mesenchym derived from endocardium, little neural crest/myocardial lineage.

-Mural leaflets form by protrusion and growth of a sheet of AV myocardium, into the ventricular luman and foramen of the valvar mesenchyme, on its surface (not from a cushion)

-Anterior leaflet dvp from mesenchyme of the inf and sup cushions

Anatomy:

-Mitral Annulus

-formed fr heart's fibrous skeleton

- = ring of supportive fibrous tissue, made mainly of the anteromed and laterally placed fibrous trigones and collagenous fibers that connect the trigones and encircle the ant 1/2-2/3 of the annulus.

-Rest of the annulus is defined by the post leaflet of MV

-Annulus of the mural and ant leaflet takes contribution from mesenchyme of inf/sup AV cushions and fr cardiac myocytes. --> septation at AV sulcus (which encircles the embryonic heart tube).

-Selective apoptosis of myocytes and morphogenesis of cardiac mesenchyme @ endocard surface at the sulcus --> myocyte regression & interrupt bridging muscular connections at the AV orifice, except in the region of His bundle. (If this isn't done correctly --> accessory pathway and bridging muscular tissue.)

-Leaflets-

-made of collagenous periph fibrosa and spongiosa (mucoid myxomatous central tissue)

-Ant leaflet- sail shaped, attaches w a hinge to 1/3 of the ant-med part of the annulus

-Post leaflet- C shaped, attaches w a hinge to post-lat 2/3 of the annulus

-Leaflets are separated at the ant-lat and post-med Commissures, where they attache to the fibrous trigone.

-Ant leaflet --> semicircular post border of LVOT; in direct continuity w 1/2 noncor cusp and most of L cor cusp of Ao vlv.

-If CHD w LVOTO, there can be discontinuity bn ant leaf and cor cusps (septal defect assoc w conal septal malalignment, CoAo, subAo stenosis, dTGA

-Post leaflet- attaches to rest of the annulus and extends along the post-lat freww wall of LV.

-Post mitral leaflet is longer at its base and shorter in its basal-to-apical length than the ant leaf (long and thin vs short and stout), though both have about the same area!

-Leaflets have 20% more area than the actual mitral orifice--> allows for overlap w systolic closure

-Post leaflet often subdivided by medial and lateral clefts--> scallop shape.

-Chordae Tendineae-

-network of flexible cords made of collagen, originate fr primordial endocardial derived valvar mesenchyme. It's the only remant of the valve attached to the pap muscle. They diverge w increasing # branches twd their insertion into free edges or ventric surface of the leaflets

-Get progressively thicker centrally

-distrib their forces along the valve surfaces radially

-Each leaflet attaches to BOTH pap muscles

-Papillary Muscles

-AL and PM pap muscles secure and sense the chordae/leaflets w ventric systole

-PM often bifid

-Coronary supply often varies- AL mainly by LCA and circ (usually dual supply); PM mainly by RCA (usually single supply --> incr ischemic risk)

-MV Apparatus Components:

-Annulus

-Ant & Post Leaflets

-Chordae Tendineae

-Pap Muscles

-As rapid LV filling completes, vortex currents under the ventricular surfaces of leaflets gradually move the leaflets passively twd closed position. Then the leaflets briefly reopen as the remaining 15-20% of CO enters LV during atrial systole. Then, active contraction of teh annulus starts at onset of atrial contraction and continues thru ventric systole--> reduce annular orifice significantly (20-40% decr). W contraction of intravalvar muscle fibers, leaflets become concave--> opposes leaflet eversion during systole--> better seal. (Without this--> leas leaflet support/coaptation esp at commissures). AV asynchrony--> worse MR in pt w CHF; pacemaker resynch can reduce valvar regurg bc better timing bnc annulus and intravalvar muscle fiber contraction.

-The ant leaf (sail shape) swings into/against C shaped post leaf, which engulfs and supports the ant leaflet during systole. They are also both spported by the ventricular wall and tethering cords (redistrib forces from the center to the periph of the leaflets). Leaflets are sealed by the intraluminal pressure. Durning ventric contraction, the paps/chordae dont shorten, but do maint isometric tension--> stabilize the leaflet. MV diastolic opening involves passive reversal of atrial and ventric diast P relationship, and active intravalvar and pap muscle shortening (to open valve) and annular relax'n (incr MV orifice)

-AbNl myxomatous elements of leaflets and collagen matrix of chordae-->xx; fibrous deposits or myxomatous material deposited where they shouldnt be can --> stiff valve--> MS/MR.

-S1 is mainly fr vibrations occurring w max tension on the mitral leaflets and chords after isovol contraction of LV. It gets louder w greater separation of mitral leaflets at ventric contrctn onset, bc the ant leaflet swing widely and slams against post leaflet.

-Prolonged PR interval--> leaflets are almost in apposition before onset of systole--> soft S1

-Widely opposed pliable leaflets seen w MS--> loud S1

-Redundant/floppy leaflets assoc w MR may --> loud S1

-Infiltration/tethering of mitral leaflets assoc w MS--> soft S1

-Likely get mitral valve/downstream hypoplasia bc of decr fetal Q thru it

CONGENITAL MV MALFORMATIONS ASSOCIATED WITH LV INFLOW OBSTRUCTION

Physiology:

-MV obst--> impair Pulm Vn Q fr LA to LV--> incr LA, pulm vn, pulm cap P

-Starling's concept of transvascular fluid exchange: net capillary water into interstitial/alv spaces bc incr hydrostatic P grad more than plasma oncotic P.

-&, pulm lymphangiectasia bc of incr venous/lymphatic hydrostatic P--> decr reabs of interstitial fluid & pleural effusion.

-Eventually--> hypoxic/hypercapnic pulm vasoconstriction--> incr PVR. Bronchial vein congestion--> they encroach on the small bronchiolar airways--> incr airway resistance. All this ==> decr gas exchange--> inc WOB, decr SaO2, incr CO2

-Acute incr in LAP --> incr intraluminal P--> distend vasculature --> compensatory decr PVR

-BUT, ch mV obstr can --> pulm vn/art htn, then RV dysfx

-the increased PAP and PVR --> xx

-Infants w MS: isolated pulm vn obst --> arterial/pulm vn medial muscular hypertrophy by 1yo, then --> arterial/venous intimal thickening.

-BUT, plm vn obst DOESN'T cause the irreversible intimal cell prolif/plexiform lesions seen w L to R shunts (e.g. big VSD), even if the pulm P is just as high/for just as long

-So it is a diff pathology w pulm vn obstruction vs a L to R shunt. ? exact etiology...

-L to R shunt likely due to higher Q and intimal shear forces and vessel wall stress

-THUS, after intervention for MS etc, there is better outcome if done by 2-3yo

-THOUGH if longstanding, they still can continue the phtn, so ensure proper pulm vasodilators used postop initially

-Also must c/s the effects of decr LV Q, so less CO, etc w metabolci insuffic/cachexia, circulatory maladaption to the ch low CO w excessive stim of symp n system/systemic vasoconstriction

-Renal insuffic, fluid/elec imbalance due to abNl intake, renal Q, hormones--> xx

Si/Sx:

-Little impact on fetus- Newborn w mitral atresia have Nl wt, Ln, and usually no intrauterine heart failure

-But, high freq assoc systemic congen xx (? if bc of the underlying genetic xx or bc of the heart dz)

-Usually present as infant w h/o pulm infections, FTT, irritable, tire w feeding, diaphoresis, tachypnea, ch cough

-Sx assoc w poor Px: present early infancy, low Qs Sx, RV failure

-Sev MS--> decr periph pulse/perfusion, active RV impulse if +phtn

-usually soft S1, w a Snap- mitral vlv opening sound bc of stiff leaflets (unlike the loud opening snap after S2 found in rh mitral stenosis.

-S2 varies- widely or narroly split, d/o P2 fr phtn

-RV S3 or S4 might be heard

-MURMUR: low freq, low intensity, middiastolic murmur, often w presystolic accentuation @ apex

-confirm timing w palpating periph pulse

-may be lower intensity/absent if very low Qs

-phtn may --> PR murmur

Conditions Associated w Inlet Obstruction:

-Mitral valve Annulus malalignment & Straddling of leaflets assoc w complex CHD & large inlet or AVSD

-can --> obstruction +/- incompetence, and may --> abNl diversion of systemic or pulm venous Q

-Classification -based on anatomic abNly of the valve; or use a functional approach (stresses the Sx/mechanical xx, more relevant to the surgeon)

-General types of xx:

-Hypoplasia of one of the mitral valve components causing narrowed effective valve orifice area;

-Restrictive leaflet motion bc of leaflet/chord fusion; cord fibrosis and shortening;

-Anomalous chordal attachment/insertion;

-Variable papillary muscle fusion

Cor Triatriatum

-Isolated pulm vn obstruction & cor triatriatum block pulm vn inflow

-due to abNl insertion of the common Pulm vn into the LA

-venous Q flows into an accessory chamber above teh obstructing membrane

-PFO and LAA are distal to the membrane in the native LA

-usually, MV and LV are Nl

-c/s in pt w phtn, LH inflow obstruction, pulm vns congestion/edema, low CO state

Coronary Sinus Obstruction

-persistent LSVC can--> cor sinus dilation--> obstruct LV inlet

-Nl MV leaflets, but post atrial wall is thicker--> funnel like obst in LA

-Tx w resecting roof of the dilated cor sinus

Supramitral Stenosing Ring

-Connective tissue fr atrial surface of MV leaflets accumulate and encroach the orifice

-Rarely isolated; usually w parachute MV, subAo AS, or CoAo

- --> fibrous ring or shelf fr atrial wall and extending to leaflet

-Usually abNl MV- eg commissural fusion/parachut

-here, PFO and LAA is above the membrane

-often severe obstruction

-c/s in pt w Sx of LH obstruction, esp if other mitral/ventric xx

-c/s TEE for Dx...

Hypoplasia of the MV and MV Atresia

-all the MV component parts are small, but pretty much normally formed in proportion to the valve

-Assoc w LV hypoplasia

-may be 2y restricted FO, global insult to LV (critical AS)

-often short cords, pap muscles small and close together

-symmetric chordal attachment

-Restricted valvar motion bc decr interchordal space and tethered leaflets

-80% have severe LV hypoplasia, and often w LVOTO

-often p/w LVOTO at <1wk old

-LV size <70% Nl dont survive w/o surgery beyond 1mo

-MV Atresia- assoc w complex ventriular anatomy

-DORV, single LV w TGA

-MV atresia w Nly related ventricles, Nl GA, and Nl Ao vlv is rare

-12% of pts may have Nl LV, but have large VSD or straddling TV

Congenital Mitral Stenosis

-Most common inlet obstruction

-rare (0.2% CHD)

-unlike MV hyoplasia, here there is diffuse abNly in valve components

-isolated MS is rare (1-3% of MS); 70-90% have LH lesions assoc

-Varied forms: thick, rolled leaflet margins, short and thick cordae, fibrous oblieration of interchordal spaces, abNl chordal insertions, pap muscle hypoplasia, decr interpap muscle distance/pap muscle fusion

-Classify based on whether the pap muscles are symmetric:

-Symmetric MS- (50-80%) -symmetric/Nl distrib of cords fr each leaflet to each of 2 small pap muscles

-Assoc w variable annular/leaflet hyoplasia

-Restrictive leaflet motion bc 2 or 3 valve parts are involved usually

-Valve leaflets are thick, +/-fused commissures

-Decr interpap muscle distance, and interchordal space may be oblieterated, and cord short

-Valve orifice is central ad smaller than the annular orifice--> mod-sev obtruction

-Asymmetric MS- unbalance cord attachement

-aka parachute MV

-+/- hypoplastic, fused, single pap muscle; +/-focalized cord attchmt = "classic parachute MV"

-Usually, there's 2 pap muscles, but the valve is parachute like w unbalanced chordae mainly attaching to 1 pap, mimics the look of the classic parachute MV.

MV Commissure Fusion

-fused commissure, cords poorly dvp'd, short/absent w the edge of teh leaflet adhearent to teh pap muscles

---> sev. restricted leaflet motion, central orifice very narrow

-basilar fusion/pap muscle hypertrophy can occur and cause obstr too

-abNl leaflet tethering --> MR

Excessive MV or Chordal Tissue

-Obliterated interchordal spaces bc of excess valve tissue (fibrous tissue, myxoid dysplasia)

-May have redundant mitral leaflet tissue in LVOT --> subAo AS

-Anomalous MV leaflet or chordal attachment to LV septum may --> LVOTO &rarely, late onset Ao regurg

-Assoc w AVSD

Double Orifice MV

-a bridge/limbus of tissue partly/completely divides mitral inlet into two inlets

-Might be incidental finding, but usually assoc w other CHD

-AVSD in 1/2 pts, and LH xx- CoAo, VSD in 40%

-MR in 1/2 pts

-Sev MV stenosis in 13%

-Usually the os's are unequal

-Functional MS seen if no AVSD

-Usually doesn't need surgical Tx

Mitral Arcade

-aka Hammock valve

-rare

- = MV leaflets thick, very short/absent cords, so the leaflets look like they insert directly into the pap muscles or post ventric wall

---> limit MV excursion; MS; MR

-often w abNl fibrous tissue band along free margin of the leaflet(s)--> tethers leaflet to pap muscle

Parachute MV

-Unbalanced cordal attchmts to a single pap muscle

-2 types:

-2 pap muscles but focalized chordal attachments (more common)

-1 pap muscle (classic type)

-Both have asymm cord attchmt, short/thick crds

-if single pap muscle, the commissures may be fused/absent

-Sometimes, the free margin of the leaflet attaches directly to lat pap muscle

-Annulus might be Nl, but orifice is eccentric, and over the dominant pap muscle

-usually less MS than the symmetric/balanced version

-Assoc w supramitral ring, CoAo, ASD, VSD, AS (sub and vlvr), Shones complex

-Good Px if mild-mod MS bc severity usually doesn't progress