I was interested, Kathleen to read of the premature infant described in your post denied breastmilk because of maternal intake of Amitriptyline.
I can't find any references which specifically deal with Amitriptyline exposure in the premature infant, however I would like to make several points.
Firstly the infant would have been exposed to much higher levels of Amitriptyline in utero than she would from breastmilk. There is an interesting case report in the literature describing a fullterm newborn infant with toxic effects at birth related to another TCA, Clomipramine. This baby was born with hypotonia, tremor and respiratory distress and then went on to become quite jittery at 12 hours of age. The baby was denied breastmilk for 7 days and then fully breastfed from then on. All symptoms resolved at 6 days and did not recur at all. (Schimmell M. et al, Toxic Neonatal Effects Following Maternal Clomipramine Therapy, Clinical Toxicology, 29 (4),479-484 1991). My point is that if this baby was going to have problems with Amitryptyline it should have done at birth.
A second point that the same article makes and one that I have seen discussed before is the possibility that these apparently toxic effect of this TCA may in fact be due to sudden withdrawal of the drug. If  this were the case then surely it would be more sensible to breastfeed the infant and continue to expose it to albeit small quantities of the TCA thereby possibly minimising the effect of withdrawal?
My third point is that as far as I am aware no one has ever demonstrated any long term adverse consequences for infants exposed to TCA via their mother's breastmilk. On the other hand a study by Buist looking at infants exposed to a TCA, dothiepin via their mothers milk revealed higher cognitive scores when they were later examined at 3-5 years. (Buist A, Janson J, Effect of Exposure to Dothiepin and Northiaden in Breast Milk on Child Development, British Journal of Psychiatry, 1995, 167, 370-373)
No, I am not trying to suggest that we expose all newborn infants to TCAs to improve their cognitive scores! What I am saying is that we have  absolutely no evidence that TCAs in breastmilk damage infants long term in spite of the fact that these drugs have been on the market for four decades.
On the other hand do we have some evidence that formula increases both morbidity and mortality in infants! - and most particularly premature infants. For example just look at the risk of this infant getting neonatal necrotising enterocolitis. Lucas and Cole's study found that prem infants fed formula alone were up to ten times more likely to develop NEC than those who received any breast milk at all. With a mortality rate around 25% and serious long term morbidity in survivors how can this paediatrician possibly justify denying this baby its mothers milk.(Lucas A, Cole TJ, Breast milk and neonatal necrotising enterocolitis, Lancet 1990, 336, 1519-1523)
And, I haven't even started to talk of the potential for adverse psychological consequences for the mother being forced into ceasing her Amitriptyline so that she can breastfeed her infant 30 days later!

Laureen Lawlor-Smith BMBS IBCLC
South Australia