Autopsy Manual: Measurements and Weights

The basic external measurements confirm observations about body length, limb length, pinna length, circumferences of the head, chest and abdomen, and spacing of eyes and nipples. These measures can confirm growth restriction with sparing of the head, but mostly they confirm malformations. The body weight is compared to norms for gestational age and sex of the infant.

Organ weights are measured during the dissection and can be compared to the normative values for gestation. The visceral organs tend to maintain constant ratios compared to each other. Often it is the loss of this regular relationship that needs to be determined such as small lungs or a small thymus. A simple method is to compare the visceral weight of the individual organs to the brain weight and then compare those ratios to the normative ratios.

 

A.  Introduction

 

The external measurements and body weight need to be recorded before undertaking the autopsy dissection. With practice, the measurements take only a few minutes. The measurements help confirm observed abnormalities, and prevent overlooking important features. Tables of norms are available in many textbooks of perinatal pathology, and are well presented in Wigglesworth and Singer [1]. Autopsy measurements have been used to create the best fit growth curve with 95% confidence limits [2]. An individual autopsy measurement can be compared to its position on the curve.

Tables of stillbirth or perinatal autopsy birth weight and measures only compare the infant to other autopsies, not to the normal population. They do not reflect the normal population because growth retardation, hydrops, maternal diabetes and other pathology is over-represented. The body weight, and many measurements can be plotted on a liveborn birth weight chart of normal percentiles for gestation to better compare the stillborn infant to the population. Ideally, this curve is based on the local nursery population, but published curves are available. At previable gestational ages there is no alternative live born curve. If an infant plots above the 90thor below the tenth percentile, that is a clue to either an underlying pathology or a mis-assigned gestational age.

Immediately classifying an infant as large, appropriate or small for gestation based on >10thpercentile, and < than 10thpercentile may help direct the approach to the autopsy. For example, a small infant with a proportionately small head without gross placental abnormality or a clinical history to account for utero-placental ischemia may have a genetic, or chromosomal abnormality for which tissue for testing can be obtained. An abnormally large infant with fat cheeks suggests the infant of a diabetic mother and may prompt obtaining a maternal hemoglobin A1c to resolve the role of glucose intolerance.

The autopsy weight is only a single point on a curve and cannot be used alone to exclude intrauterine growth retardation. Serial intrauterine ultrasound examinations may demonstrate a slowing of expected growth even in an infant of appropriate weight for gestation.

Abnormal measures need to be explained. Sometimes they will simply be errors of measurement or recording. Small deviations may be normal variation, but more than 1 standard deviation prompts consideration, and 2 standard deviations requires some annotation. Having only one or two measurements that are abnormal may help direct the diagnosis. Lethal osteo-chondro-dystrophies are usually apparent immediately on inspection, but in early gestation and in non-lethal disease, the measures of limbs may help confirm a suspicion, and suggest review of the post mortem radiographs. If an osteo-chrondro-dystophy is present, specific protocols for tissue sampling may help obtain the correct diagnosis.  An abnormal genital appearance can be confirmed by appropriate measurements and lead to obtaining skin fibroblast culture or other samples.

 

B.  Weight

The body weight is usually determined to the precision of grams. For small fetal autopsies an appropriate scale with tenths of a gram is helpful. In immature hydropic fetuses a surprising amount of body weight from fluid can leach to the surrounding covers. A comparison of the autopsy weight to the birth weight can demonstrate how much. On the other hand in older infants the clinical birth weight may include accessory material and elevate the true weight.

 

C.        Crown heel length

As long as the limbs are not fixed in flexion, holding the infant by the heel with the head touching the “ground” gives close to a normally extended height. A single long metal rule allows immediate measure of both the crown heel, and at the same time, the crown rump, at the level of the sacral tip. If the limbs cannot be straightened, a string can be used to outline the length. In cases with absent lower body or absent head, it is better to omit the measure, rather than have a misleading one.

A derived number is the ponderal index, which is the ratio of weight to square of the crown heel length. A high ratio alerts the prosector to distinguish among increased edema, fat, body mass or other accumulation, while a low ratio is consistent with asymmetric growth retardation. However, this ratio suffers from the need to have a very accurate crown heel length and I have not found it useful.

 

D.        Crown rump

This can be measured for the second trimester and older fetus as described above. However, in the very young, normally curled embryo, a neck rump measure is probably more comparable to the ultrasound measurement [3]. The crown rump measurement is a measure of gestational age in the first half of gestation. Later in gestation, subtracting it from the crown heel length provides a ratio of leg to trunk length.

 

E.         Foot length

The foot length is taken by setting the foot down on the rule and measuring from the tip of the toe to the heel. If the feet are normal this can provide an accurate estimate of early gestational age, and may be the only measure available in a curettage specimen.

 

F.         Head, thorax, and abdominal circumferences

These circumferences can be compared to prenatal ultrasound measurements, and used to confirm abnormalities such as ascites or macrocephaly. To obtain circumferences, tie a knot in one end of a string, wrap the string around the body with close apposition (but not indenting), and mark where the knot contacts the remaining string with your fingers. The distance from the knot to the fingers can be measured on a rule. The head is measured at the widest circumference, the chest at the level of the nipples, and the abdomen at the umbilicus.

 

G.  Inner and outer canthal measurements

These are taken with a straight non-bent ruler looking down and measuring the inner to inner and the outer to outer edges of the eyes. The measures can be used to confirm hypo or hypertelorism.

 

H.  Pinna length

The length of the pinna can be measured along the longest axis. It is often small in Down syndrome, a condition that may be overlooked in the fetal or newborn autopsy.

 

I.      Internipple distance

A direct measure is made between the nipples. At early gestational ages, the nipples must be looked for carefully in varying angles of light. This measurement is increased in early lymphatic obstruction sequence.

 

J.   Other measures

Another measure of head size is the biparietal diameter, which when done with calipers correlates well with the intrauterine ultrasound measure. An advantage of this measure is that by ultrasound, the cheek to cheek diameter ratio to the biparietal diameter was almost constant over gestation, and was normal in non-diabetic large for dates infants. The ratio for large infants of diabetic infants was higher (0.77 (SD.07) compared to 0.69 (.07) [4][5].  A useful prenatal ultrasound measure is femur length, but this does not correlate well with the autopsy measurement [6]. Other measurements that have norms and can be valuable in suspected abnormality include phallic or clitoral length (from pubic bone to tip), and extended arm length. Another proposed measure of virilization is a anogenital ratio greater than 0.5 [7]. The ratio is the anus to fourchette (point of labial fusion) distance over the anus to tip of clitoris distance.

 

Organ weights

 

During the autopsy the weights of the organs are recorded after they have been dissected free. If the adrenals and kidneys are symmetrical, I weigh them together. I weigh the lungs separately, and the right is normally larger than the left. My lung weights are slightly skewed by taking a lung culture of the right lobe, but this has not proved to be diagnostically significant.

To elucidate anomalies in the organ weights I plot the ratio of the case organ:brain weight ratios over the normative organ:brain weight ratios on a spreadsheet. If the brain was examination was not permitted, I use the expected brain weight for gestation. The use of this information will become clear during discussion of individual organs.

 

Template organ wts

 

 

K.  References

 

  1. Wigglesworth, J.S. and D.B. Singer, eds. Textbook of Fetal and Perinatal Pathology. second ed. . 1998, Blackwell Science: Malden.
  2. Barr, M., W. Blackburn, and N.J. Cooley, Human fetal somatic and visceral morphometrics.Teratol, 1994. 49: p. 487-496.
  3. Goldstein, S., Embryonic Ultrasonographic Measurements – Crown-Rump Length Revisited.American Journal of Obstetrics and Gynecology, 1991. 165: p. 497-501.
  4. Abramowicz, J., et al., The Cheek-to-Cheek Diameter in the Ultrasonographic Assessment of Fetal Growth.American Journal of Obstetrics and Gynecology, 1991. 165: p. 846-852.
  5. Abramowicz, J., D. Sherer, and J.J. Woods, Ultrasonic measurement of cheek-to-cheek diameter in fetal growth disturbances.Am J Obstet Gynecol, 1993. 169: p. 405-8.
  6. Alonso, K. and E. Portman, Fetal weights and measurements as determined by postmortem examination and their correlation with ultrasound examination.Arch Pathol Lab Med, 1995. 119: p. 179-80.
  7. Callegari, C., et al., Anogenital ratio: measure of fetal virilization in premature and full- term newborn infants.J Pediatr, 1987. 111(2): p. 240-3.
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