What is asthenozoospermia?

Asthenozoospermia is a semen finding in which the proportion of progressively motile sperm falls below the reference limit, meaning a reduced share of sperm move forward effectively.

Why are so few sperm classified as normal under strict morphology?

Strict (Tygerberg/Kruger) criteria define a normally formed sperm very narrowly, so even in fertile men only a small percentage qualifies as normal; the threshold is interpreted against reference values rather than expected to be high.

Sperm Dysfunction: Motility Disorders and Morphologic Abnormalities

Beyond the number of sperm, fertility depends on their function: their ability to move progressively (motility) and their structural form (morphology). Asthenozoospermia denotes reduced motility and teratozoospermia denotes an excess of abnormally formed sperm; these qualitative defects can impair fertilisation even when sperm numbers are adequate.

Definition

Sperm dysfunction refers to impaired motility (asthenozoospermia, a proportion of progressively motile sperm below the reference limit) or impaired morphology (teratozoospermia, a proportion of normally formed sperm below the reference limit), reflecting qualitative rather than purely quantitative deficits of the ejaculate.

Scope

The entry covers the assessment of sperm motility and morphology, the definitions of asthenozoospermia and teratozoospermia, the basis of strict morphology classification, and how functional defects relate to fertilisation potential. It is reference material on how sperm function is characterised, not clinical guidance.

Core questions

How are sperm motility and morphology measured and graded?
What distinguishes asthenozoospermia from teratozoospermia?
What is strict morphology classification and why was it introduced?
How do functional defects relate to natural and assisted fertilisation?

Key concepts

Progressive versus non-progressive motility
Asthenozoospermia
Teratozoospermia and normal forms
Strict (Tygerberg/Kruger) morphology criteria
Sperm vitality versus immotility
Flagellar and structural defects
Oligo-astheno-teratozoospermia as combined defects

Mechanisms

Progressive motility depends on an intact flagellar axoneme and the metabolic energy supply that powers it, so structural or metabolic flagellar defects reduce forward progression; distinguishing immotile but viable sperm from dead sperm requires a vitality test. Morphology is assessed by classifying the proportion of sperm with a normal head, midpiece, and tail; strict criteria, developed in the Tygerberg work reported by Kruger and colleagues, define normal form narrowly and were shown to correlate with fertilisation in vitro. Severe combined defects (oligo-astheno-teratozoospermia) reflect deficits in number, movement, and form together. Assessment is standardised against reference limits, and qualitative defects help explain impaired fertilisation when sperm numbers appear adequate.

Clinical relevance

Characterising motility and morphology refines the interpretation of a semen analysis and informs how fertilisation potential is appraised in a couple's evaluation. The entry describes how these parameters are measured and what they mean for sperm function; it is educational and does not direct individual diagnosis or treatment.

Epidemiology

Reduced motility and abnormal morphology frequently coexist with reduced counts and with each other, and many men with isolated qualitative defects are identified during infertility evaluation. As with sperm number, the distributions of motility and morphology overlap substantially between fertile and infertile populations.

Evidence & guidelines

Reference limits for progressive motility and normal morphology derive from the World Health Organization reference values (Cooper et al., 2010) and laboratory manual (2021), while strict morphology criteria trace to the work reported by Kruger et al. (1988); narrative syntheses (Agarwal et al., 2021) place these defects within male factor infertility. These are reference standards, not individual medical advice.

History

The introduction of strict morphology criteria in the 1980s, reported by Kruger and colleagues from the Tygerberg group, narrowed the definition of a normally formed sperm and linked morphology to fertilisation outcomes in vitro, influencing how the parameter was scored in subsequent World Health Organization manuals.

Debates

How clinically predictive is sperm morphology?: Strict morphology was historically tied to in vitro fertilisation outcomes, but its independent predictive value for natural conception and across assisted-reproduction settings is debated, and reference thresholds have shifted across manual editions.

Seminal works

kruger-1988
cooper-2010

Frequently asked questions

What is asthenozoospermia?: Asthenozoospermia is a semen finding in which the proportion of progressively motile sperm falls below the reference limit, meaning a reduced share of sperm move forward effectively.
Why are so few sperm classified as normal under strict morphology?: Strict (Tygerberg/Kruger) criteria define a normally formed sperm very narrowly, so even in fertile men only a small percentage qualifies as normal; the threshold is interpreted against reference values rather than expected to be high.