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*RACS ENT Fellowship > VIVA – Anatomy – Thyroid/PTH > Flashcards

VIVA – Anatomy – Thyroid/PTH Flashcards

1
Q

Draw a cross section of the thyroid


A
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2
Q

Discuss the embryology of the thyroid


A
  • Week 2-3 thyroid develops from an endodermal bud from the floor of the pharynx between the 1st and 2nd branchial pouches (tuberculum impar and copula) - thyroid diverticulum - which may start as a single diverticulum but divides very early into two lateral lobes
  • Week 5 - duct loses its lumen and fragments and disappears by week 8
  • Remnant is the F. caecum at the junction of the ant 2/3 and post 1/3 of the tongue
  • Bud descends as the thyroglossal duct into the neck in a plane anterior to the tracheobronchial bud
  • by the end of the 7th week, has reached its final position in front of the trachea
  • It is closely related in its descent to the mesenchymal masses of the 2nd and 3rd branchial arches which become the hyoid bone
  • when the hyoid anlage rotates and fuses, the descending thyroid stalk can become adherent to its periosteum and a portion of the stalk can end up lying posterior to the hyoid bone, after first passing anterior to and then underneath the hyoid body
  • as it descends, the gland expands dorsally and laterally while losing its connection with the foramen caecum
  • Pyramidal lobe - extends superiorly from the isthmus in 50% people, and may 
be attached to the hyoid bone by fibrous tissue and/or muscle à the levator glandulae thyroideae. They represent a persistent distal end of the thyroglossal duct.
  • lateral lobes of the thyroid are thought to receive contributions from the ventral portion of the 4th and 5th branchial pouches, lateral thyroid analage
  • lobes connected by isthmus develop with the isthmus overlying the 2-4th tracheal rings
  • Parafollicular/Calcitonin/C cells – of neural crest origin à migrate ventrally into the ultimobranchial body (5th pouch) and then the thyroid gland
  • Differentiation of the thyroid into 3 stages (almost fully formed by week 11)
    • precolloid weeks 7-13
    • colloid weeks 13-14
    • follicular after 14 weeks – functions after 3 months
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3
Q

What are the embryological origins of the PTHs?


A
  • Endoderm derived from Pharyngeal pouches
    • Commences in W5
  • Superior glands from the dorsal diverticulum of the 4th pharyngeal pouch
    • Closely related to posterior midportion thyroid
    • 85% found in posterior
  • Inferior glands from the 3rd pharyngeal pouch
    • Displaced caudally by the descent of the thymus
      • Parathyroid forms from the dorsal portion / thymus from the ventral portion
      • Thymus migrates medially and inferiorly from angle of mandible to pericardium
    • 10% found within the thymus
    • Can be found anyhwere from angle of mandible to anterior mediastinum
  • once the superior parathyroids cells lose their connection to the fourth pouch, they attach to the caudally migrating thyroid gland and assume their position near the superior dorsal aspect of the gland
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4
Q

Which RLN is more likely to be injured in surgery and why?


A

The right recurrent laryngeal nerve is more susceptible to damage during thyroid surgery because it is close to the bifurcation of the right inferior thyroid artery, variably passing in front of, behind, or between the branches

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5
Q

What is the relationship of the RLN to the inf thyroid artery?


A

On left - more likely to lie behind (50%) the inferior thyroid artery (PTA)

On right - Equal chance of being in front or behind inferior thyroid artery, and passes thru branches of inferior thyroid artery in 50% (TAP)

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6
Q

What is Berry’s ligament?


A

Suspensory ligament of thyroid gland - condensation of pre-tracheal fascia – RLN passes behind

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7
Q

How does the RLN enter the larynx?


A

Always behind the pretracheal fascia (and therefore ligament of Berry) and behind the cricothyroid joint

Approaches the medial surface of the thyroid gland from below usually in front of the tracheo-oesophageal groove

At the level of the upper border of the isthmus may divide into a larger anterior motor branch and a smaller sensory posterior branch

Pass posterior to cricothyroid joint

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8
Q

If RLN branches, which branch is motor?

A

Anterior

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9
Q

What is the incidence of non recurrent RLN?

A

0.5-1%

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10
Q

What is non-recurrent laryngeal nerve associated with?

A

When (R) subclavian arises from distal aortic arch and passes posterior to the oesophagus, RLN arises from vagus at level of superior pole of thyroid and enters larynx directly (0.5-1%) – dysphagia lusoria

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11
Q

Describe the path of the SLN


A

Arises at inferior ganglion vagus near jugular foramen

Passes posterior and deep to ICA – travels medial to ICA and ECA

Divides into Internal and External laryngeal Nerves at approximately the level of the greater cornu of hyoid – may be before

  • Internal Branch of Superior Laryngeal nerve
    • Passes between thyrohyoid muscle and membrane
    • Pierces thyrohyoid membrane to reach pyriform recess with superior laryngeal artery and vein
    • Sensation to supraglottis and pyriform fossae
    • ?divides into ascending (epiglottis), transverse (AE folds, true and false VC, ventricle, parts of arytenoid) and descending (posterior and medial arytenoid, post cricoid, inter-arytenoid space) branches –
    • communicate with RLN through ansa galeni – branch of inferior division of iSLN running across dorsal surface of posterior cricoarytenoid muscle and medial wall of piriform sinus
  • External Branch of Superior Laryngeal Nerve
    • At level of superior horn of thyroid cartilage à turns medially
    • Runs posteriorly and parallel to oblique line of thyroid cartilage
    • Variable relation with inferior constrictor (superficial, piercing to deep at any point or none) à approx. at lower edge of thyroid cartilage curves anteriorly and inserts into cricothyroid
    • Lies very close to behind the superior thyroid artery medial to the upper pole
      • Typically deep
      • Anterior or between branches in up to 20%
      • Cernea classification of relationship of ESBL to a horizontal plane at the level of the upper border of the superior pole of the thyroid
        • Type 1 = 1cm or more above superior pole (60%) à Some located within fibres of inferior constrictor
        • Type 2a = Less than 1cm above superior pole (17%)
        • Type 2b = Nerve below the superior pole of the thyroid (20%)
  • Divides into 2 main branches – one to oblique and one to rectus belly of cricothyroid
  • May give branch to lat aspect of thyroarytenoid à anastomose with RLN
  • Supplies Cricothyroid muscle and possibly cricopharyngeus portion of inferior constrictor – can also innervate ipsilateral thyroarytenoid, ventricular muscl
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12
Q

What is the relationship between the SLN and the inferior constrictor?

A

ESBL has variable relation with inferior constrictor (superficial, piercing to deep at any point or none)

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13
Q

What is Zuckerkandl’s tubercle?


A

Pyramidal extension of the thyroid gland, present at the most posterior side of each lobe

RLN always deep to

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14
Q

What is dysphagia lusoria?


A

Difficulty in swallowing caused by aberrant right subclavian artery

During development of aortic arch if the proximal portion of the right fourth arch disappears instead of distal portion, the right subclavian artery will arise as the last branch of aortic arch. It then courses behind the oesophagus (or rarely in front of esophagus, or even in front of trachea) to supply blood to right arm. This causes pressure on esophagus and results in dysphagia

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15
Q

What is Ortner’s syndrome?


A

Rare cardiovocal syndrome and refers to RLN palsy from cardiovascular disease

The most common historical cause is a dilated left atrium due to mitral stenosis, but other causes, including pulmonary HTN, thoracic aortic aneurysms and aberrant subclavian artery syndrome have been reported

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16
Q

Blood supply Thyroid 


A
  • Superior thyroid artery (1st branch ECA) pierces the pretracheal fascia to reach the upper pole
    • Runs along inferior constrictor
    • Typically it is superior to the external laryngeal nerve
    • divides on the gland into anterior (to the isthmus) and posterior branches at the gland
      • Each gives off branches to the superior laryngeal artery and a cricothyroid artery
    • branch from L side may supply pyramidal lobe
  • Inferior thyroid artery (Thyrocervial trunk of the Subclavian a)
    • Runs across anterior scalene, behind CCA
    • arches upwards and medially behind the inferior pole and divides outside the pretracheal fascia into 4-5 branches – each pierce fascia separately
    • also gives of oesophageal and inferior laryngeal branches
  • Thyroid ima artery supplies the lower isthmus in 3% of patients
    • Variable presence
    • Arises from Innominate / either Subclavian / R CCA / Internal Mammary or directly from the aortic arch
17
Q

Blood supply PTH


A
  • Typically supplied by a single, dominant artery
  • Inferior thyroid artery usually supplies all glands
    • 20% superior parathyroids derive supply from superior thyroid artery
    • 10% of Inferior parathyroids derive blood supply from STA (ITA usually absent in this instance)
18
Q

What forms the inferior thyroid pedicle? 


A

?

19
Q

How do you manage the RLN in malignancy?


A

Pre-op evaluation of VC function

Aim to preserve functional nerves

Consider graft / medialisation

For well-differentiated – no survival difference between nerve sacrifice and pts treated post-o with RAI for disease left on VC

Must sacrifice if invaded in MTC

20
Q

What is the role of drains in thyroid surgery?


A

To prevent post-op haematoma and subsequent airway obstruction

Some argument to use only is selected pts i.e. large dead space, retrosternal goiter, MNG, Grave’s disease – earlier discharge

Cochrane review – no clear evidence that using drains in pts undergoing thyroid ops significantly improves pt outcomes and drains may be associated with increased length of hospital stay. Existing evidence is from trials involving pts having goiters without mediastinal extension, normal coags indices and the operation not involving any lat ND for lymphadenectomy

21
Q

What is the evidence behind intraoperative nerve monitoring in thyroid surgery?


A

Nerve monitoring

  • provide intra-op documentation of RLN fn
  • differentiate between voice changes secondary to RLN trauma and those unrelated to operation
  • anatomic variations

Majority of studies – no reduction in incidence of post-op VC paralysis

Main reason for use – if no signal at end of lobectomy à consider not doing other side

22
Q

How do you manage the calcium in the postoperative period following total thyroidectomy?

A

Check Ca 6-8hrs post-op

Check PTH

Acute Hypocalcaemia

  • If <1.9mmol/L
    • Calcium gluconate 10% (10ml over 1h)
      • 0.44mEq/ml
      • Dose 7-14mEq for hypocalcamia
      • Effective for 2-3h
    • Recheck Calcium 6-8h later and repeat as needed until >1.9mmol
    • Monitor with ionised Calcium QID aiming for 1.15-1.35
    • Commence Oral calcium supplements
      • 1-2 tablets tds (600mg tablets)
      • Up to caltrate 2 tablets tds
    • Commence calcitriol
      • 0.25-1mcg bd – generally 0.5mcg bd
      • 4X more potent than ergocalciferal
    • May need to give IV treatment for 2-3 days until oral supplements stabilise
  • Replace Magnesium
    • 2-4g IV tds until normalised then switch to oral magnesium
    • Titrate down once normal diet and normal levels achieved to avoid hypercalcaemia

Management of permanent hypoparathyroidism

  • Monitor relevant bloods weekly then decrease as able once range and dosages are stable
    • Aim for a low-normal serum calcium
  • Have a tendency towards forming renal calculi due to inadequate renal re-absorption
  • Check urinary 24h Calcium periodically (Aim <300mg/24h)
  • Consider Teriparatide
    • PTH analogue
    • Requires bd sc injection
    • Minimal ADR
23
Q

What are the U/S features of malignancy of a thyroid nodule?


A
  • Solid lesion, markedly hypoechoic
  • Increased vascularity
  • Microcalcifications
  • Irregular margins
  • Irregular, spherical, tall shape
  • Invasion adjacent structures
  • Absence of a halo - questionable
24
Q

What is the Bethesda system for reporting thyroid FNA? What are the risks of malignancy with each?

A
25
Q

Where are the PTHs?

A
  • Normally lie behind the thyroid lobes either within or outside the thyroid’s capsule of pretracheal fascia
  • Superior glands
    • More constant in position
    • Usually covered by extension of the pretracheal fascia from the thyroid
      • Able to move freely whereas thyroid nodules are fixed in position
    • Typically lying behind the superior lobe level above the inferior thyroid artery
    • 80-85% posterior to thyroid gland
      • Within 2cm radius of region centred 1cm above junction of ITA and RLN
      • Near cricothyroid joint
  • Inferior glands
    • More variable in position usually behind the lower pole below the inferior thyroid artery and lateral to the RLN
    • 50% found within 1cm of inferior pole thyroid gland
      • Typically anterior to RLN
      • 17% on or within the thyroid
      • Most common ectopic location is anterior mediastinum
        • 10% found within the thymus
        • Up to 26% within thyrothymic ligament / cervical portion thymus
      • 2.8% above the intersection of the ITA and RLN
26
Q

How many have abnormal number of PTHs?


A

13% supernumerary, 6-10% only 3 glands

27
Q

Where are the the PTHs if not in their normal position? 
Upper?

A

1% may be in retro-oesphageal or paraoesphageal space – ant to RLN

28
Q

Where are the the PTHs if not in their normal position? 
Lower?

A

1-3% intrathyroid

If lower tend to be in ant mediastinum

29
Q

What are the indications for parathyroidectomy?


A
  • Corrected Calcium >2.8
    • Begin to develop clinical consequences if above this level in a prolonged fashion
  • Suspicion of carcinoma
  • Impaired renal function believed due to hypercalcaemia ( Serial U&E)
  • 24h Urinary calcium >400mg/dl
  • <50y age with hypercalcaemia
  • Osteoporosis
30
Q

How do you confirm PTH intraoperative?


A

Can FNA parathyroid tissue and measure PTH to confirm parathyroid tissue

Frozen section

Venous sampling of PTH

31
Q

What are the intraoperative gross features of an abnormal PTH gland?

A

Parathyroid Ca

  • Fairly circumscribed
  • Single gland and can exceed 10g (other pathology usually <1g)
  • Dense fibrous capsule
  • Invasion of surrounding structures – key

Parathyroid adenoma

  • Tends to involve inf glands more than sup
  • Well circumscribed, soft, reddish brown
  • Delicate capsule
  • Other glands normal to shrunken
  • Usually a rim of compressed normal tissue is seen
32
Q

What do you do intraoperatively if you can’t find any PTHs?


A
  • Dissect into superior thymus / paratracheal tissue down to level of sternal notch
  • Mobilise pharynx and oesophagus looking into para and retro-oesophageal spaces
  • Expose length of carotid sheath
    • Often between IJV and Carotid
  • Dissect into upper neck up to level of SMG
  • Explore thyroid gland
    • Consider excision as 1-2% are intrathyroid
  • Mediastinal dissection – attention to thyrothymic ligament with thymectomy
  • Methylene blue injection
    • Localises to parathyroid tissue
    • 5mg/kg
    • Peak uptake is 30min post-injection
      • Thyroid uptake in 3-10min
    • Anticipate pseudohypoxia
    • Not helpful?
  • Radiation guided
    • Injected with Tc99 Sestamibi
    • Requires 2h to uptake
    • Intra-operative gamma probe used to locate
    • Adenoma should have ex-vivo gamma probe count >20% more than the background thyroid tissue count
    • Not helpful if patient had negative pre-operative scans
33
Q

How do you manage devascularised PTHs?

A

Chop up and re-implant in SCM

34
Q

Draw an axial diagram of the fascial layers of the neck at the level of the hyoid

A

*RACS ENT Fellowship (19 decks)

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