Minor changes in the interstitial compartment are hard to detect. But large changes are easy to detect if they are looked for. Expansion of the interstitial compartment is easier to detect than contraction.
Expansion of the interstitial compartment may be observed as oedema. Oedema depends on the volume of the interstitial compartment, gravity and the laxity of the tissues. Thus it is best seen in the feet at the end of the day and in the face in the morning. In the patient confined to bed it is best seen over the sacrum. Interstitial oedema in the lungs may be detected by auscultation of the lung bases where fine crepitations may be audible.
Shrinkage of the interstitial compartment is mainly manifest by dry mucous membranes, reduction in skin elasticity and fall in intra-ocular pressure. Dry mucous membranes may be seen by observing the tongue and the inside of the mouth. Patients who are mouth breathing will have dry tongues and mouth even if their interstitial compartment is normal. Increased body water in infants is mostly explained by a greater proportion of water in the interstitial space, see table 1 this may explain the nice soft elastic skin in babies. In contrast, the elderly have low skin elasticity and this is not a valuable sign of shrinkage of the interstitial fluid compartment in elderly people. Skin elasticity is best felt over a bony prominence such as the back of the hand where there is little fat to confuse things. Intra ocular pressure is difficult to estimate on clinical grounds.
In the infant the tension of the anterior fontanelle is a good guide to the volume of the interstitial fluid compartment.