Fiber teasing allows assessment of the pattern of nerve disease along several internodes of the same myelinated fiber. The most sensitive method for detecting segmental myelin changes, in principle, allows workers to determine whether these changes are primary or secondary to axon disease.

Dyck and colleagues provide the most comprehensive discussion of fiber teasing methodology and interpretation (Dyck et al. 1984; Dyck and Giannini 1993). We do not routinely employ nerve teasing in the assessment of biopsy specimens. Rather, we reserve this technique for special situations or research applications; some, but by no means all, workers share this approach (Logigian et al. 1994; Oh 1990; Schaumburg et al. 1992). Teasing is a difficult and tedious work and, as Dyck indicates, even an experienced technologist can prepare only 150–300 fibers a day (Dyck and Giannini 1993). Since the information gained from teased fiber examination is nonspecific, economic considerations mandate that diligent and experienced technologists may best utilize their energy in other areas.

When teasing is not performed properly, the information derived may be unreliable. Examining too few fibers risks drawing conclusions from a nonrepresentative sample. This result is especially likely when the technologist is not highly experienced and causes excessive fiber trauma, for even the most gentle teasing produces myelin artifact (Williams and Hall 1971). Large myelinated fibers are easiest to tease free, and as a result, a biased selection may thus be obtained. Dyck and colleagues have emphasized the need for appropriate sampling, suggesting that at least 100 fibers are necessary (Dyck and Giannini 1993). Assessment of demyelination “clustering” for deciding whether demyelination is primary or secondary requires at least five internodes per teased fiber. The literature, however, abounds with papers reporting results and even “normative data” based on as few as 20–50 teased fibers. Many papers even neglect to indicate how many fibers were studied.

As reviewed in Chap.​ 1 (Table 1.​1), sural nerve biopsy may be a diagnostic procedure for many diseases, foremost among them vasculitis, amyloidosis, and leprosy. These three diagnoses, and indeed all others on the list of diseases for which sural nerve biopsy may be a diagnostic procedure, are not made on the basis of fiber teasing results. Workers most often use teased fibers to detect segmental myelin changes. In addition, teased fibers may permit distinction between primary and secondary demyelination. However, examining teased fibers does not lead to a specific diagnosis. In contrast, electron microscopy in demyelinating neuropathy can reveal macrophage-mediated myelin stripping, widened myelin lamellae, or various inclusions, all of which permit an etiologic diagnosis. Indeed, the long-standing belief based on teased fiber studies that segmental demyelination and remyelination in CMT-1 is secondary to axonal disease and the recent demonstrations that defects may be in either myelin or axonal proteins leads one to wonder how meaningful the identification of demyelination as “secondary” really is.

The degree to which teased fibers surpass semithin cross sections in revealing subtle myelin changes is unclear. In a seminal study of chronic inflammatory demyelinating polyneuropathy (CIDP, Prineas and McLeod 1976), all nerves in which 20 % or more of fibers showed segmental demyelination had evidence of the same on electron microscopy. Of 12 cases in which 1–10 % of fibers showed segmental demyelination, only 2 had EM changes. Only rarely will EM demonstrate segmental demyelination when teased fibers do not (McLeod et al. 1973). While such data suggests a considerably greater sensitivity of teased fibers, it is important to appreciate that the incidence of segmental myelin changes in controls is not zero. Research criteria for the diagnosis of CIDP require that at least 12 % of teased fibers should show segmental myelin changes (Ad Hoc Subcommittee 1991). Other authors have reported that 3.9–20 % of internodes are abnormal in control nerves on average (Behse 1990; Dyck and Giannini 1993; Tsakuda et al. 1987) with some normals showing even more frequent changes. Furthermore, the incidence of pathological alterations increases in aged controls (Dyck and Giannini 1993). By itself, paranodal myelin retraction, which teased fiber studies very easily detect, is difficult to interpret at any time, because this condition can occur in the earliest stage of segmental demyelination, in axonal degeneration (Williams and Hall 1971), and in normals (Arnold and Harriman 1970).

Evidence of axonal degeneration by demonstration of linear rows of myelin ovoids or of degeneration and regeneration by way of uniformly shortened internodes can also be seen in “normal” nerve, the incidence increasing with age (Arnold and Harriman 1970). Workers cannot regard as unequivocally abnormal up to 4 % of teased fibers showing rows of myelin ovoids (Arnold and Harriman 1970; Dyck and Giannini 1993). In addition, up to 16 % of fibers can show evidence of axonal regeneration in “normal” nerve (Behse 1990). In patients over 60 years of age, Arnold and Harriman (1970) reported that at least 6 of 24 teased fibers should show evidence of axonal degeneration or regeneration before a nerve could be considered abnormal.

Thus, the amount of “pathology” seen in normal nerves limits the usefulness of teased fiber studies in detecting subtle changes, especially with increasing age. Teased fiber abnormalities other than “wrinkled” myelin were seen in 7.5–37.5 % of 9 controls used by Behse (1990), the eldest only 54 years of age. Indeed, most normative data are based on the younger age group even though several studies (Jacobs and Love 1985; O’Sullivan and Swallow 1968; Vital et al. 1990) have shown a dramatic increase in nonspecific changes with age. Approximately half of nerves assessed in our laboratory come from patients over 60 years of age (Fig. 1.​1).

A recent study of correlation between nerve conduction studies and sural nerve biopsy reported that in 11 of 52 instances (21 %) when both cross-sectional and teased fiber (minimum of 50 fibers) examinations were employed, the histological diagnoses were discordant (Logigian et al. 1994). However, in 48 % of these biopsies, three independent observers could not agree on the interpretations of the pathology. Thus, the subjective nature of nerve biopsy interpretation represents a greater source of error than whether or not fiber teasing is performed. Notably, the discordance between electrophysiological findings and biopsy results (63 % concordant, 14 % minimally discordant, and 23 % discordant) did not change when only teased fiber results were considered (Logigian et al. 1994).

In conclusion, light microscopic examination of 20 or more semithin cross sections, supplemented by careful electron microscopic review of selected nerve fascicles, leaves little need for teased fiber preparations. In some diseases, such as tomaculous neuropathies, teased fibers provide an elegant and convincing means of illustrating the pathology; however, we cannot identify a situation where our approach would lead to the loss of diagnostically useful information. We recognize the value of fiber teasing for research in peripheral nerve disease, especially when workers can study large numbers of fibers in multiple nerves. Yet, in the daily practice of peripheral nerve pathology, this approach seems insufficiently informative to justify its cost.

Several quantitative analytic tools are available to those seeking to describe nerve pathology with greater precision than a subjective assessment of morphology allows. These “morphometric” techniques have become a sometimes useful adjunct to morphology. In our routine practice, other than an estimate of the severity of MF and UF loss, we only infrequently perform morphometric analysis, because morphology, not morphometry, gives specific diagnostic information.

Morphometry has, nevertheless, been important in the evolution of understanding of peripheral nerve function and disease. Quantitative techniques allow presentation of large amounts of data in compact graphs or scatter plots. Informed reading of the peripheral nerve literature also requires a certain familiarity with the various quantitative measures in use. Thus, the discussion below reviews some of the more important morphometric techniques but is by no means comprehensive. For more detailed information, we refer the reader to studies by Behse (1990), Dyck and co-workers (1984), Gibbels (1989), Hunter et al. (2007), and Thomas (1970).

To maximize the usefulness of fiber density measures, each laboratory should ideally use its own normals, prepared with a standard technique. The effect of tissue shrinkage resulting from use of different fixatives should be considered when comparing data from different laboratories. Workers should compare abnormal nerves only to age-matched normals. Some literature control data includes patients with stroke or ALS, on the assumption that these processes do not cause alterations in peripheral sensory nerves. Pollock and colleagues (1984) have shown that there may be alterations in the sural nerve of a hemiplegic limb. Moreover, a growing literature suggests that alterations of sensory fibers can occur in motor neuron diseases (Isaacs et al. 2007). Paid normal volunteers, where a full neurological history and physical examination can be performed along with electrophysiological studies, represent the best source of control data (Dyck and Giannini 1993). In the absence of such data, the rigorous approach described by Stoebner et al. (1989) is commendable. Their normal nerves came mostly from patients in acute (≤48 h) coma with no previous history of neuropathy, with normal electrophysiological testing of peripheral nerves, and with the specimen removed during organ harvest for transplantation. Because severely cachectic or bed-bound patients are at risk for nutritional and pressure palsies, workers should not consider them to have normal peripheral nerves.